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Can biostimulants enhance plant resilience to heat and water stress in the Mediterranean hotspot?

March 2025
Plant Stress 16(100802):21

DOI:10.1016/j.stress.2025.100802

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CC BY-NC-ND 4.0

Authors:

University of Campania "Luigi Vanvitelli"

The study examines how biostimulants can support plant adaptation to drought and high temperatures, focusing on key Mediterranean crops. Rather than offering a miraculous solution, biostimulants enhance the plant’s natural resilience by modulating key physiological and biochemical pathways. This underscores the importance of continued research into the endogenous defense strategies of plants, as well as the need to design tailored biostimulants that specifically amplify these existing protective mechanisms. Advancing this knowledge will be essential for optimizing sustainable agricultural practices in the face of climate change. This is the link to the article: https://www.sciencedirect.com/science/article/pii/S2667064X25000673?via%3Dihub

Conceptual graphic model illustrating how biostimulants enhance plant-intrinsic stress-response mechanisms to mitigate drought and heat stress. The diagram highlights the cascade of stress-induced cellular dysfunction, retrograde signaling pathways, and key physiological responses strengthened by biostimulants, including osmoregulation, oxidative defense, and hormonal regulation. It also presents application strategies (e.g., foliar spraying, seed priming, root drenching), as well as limitations and future directions for optimizing their role in sustainable agriculture.

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Review

Can biostimulants enhance plant resilience to heat and water stress in the

Mediterranean hotspot?

Petronia Carillo

Department of Environmental, Biological and Pharmaceutical Sciences and Technologies, University of Campania “Luigi Vanvitelli”, 81100 Caserta, Italy

ARTICLE INFO

Keywords:

Osmotic stress

Oxidative stress

Retrograde signalling

Osmolytes

heat shock factors (HSFs)

Heat shock proteins (HSPs)

Stress tolerance

ABSTRACT

Heat and water stress are imposing signicant constraints on agricultural systems, particularly in Mediterranean

regions experiencing prolonged droughts, rising temperatures, and increasing aridity. These abiotic stresses

trigger secondary effects, including osmotic and oxidative stress, simultaneously inuencing multiple plant traits.

Under drought conditions, stomatal closure limits CO₂ uptake, interfering with photosynthetic electron transport

and increasing the production of reactive oxygen species (ROS). Elevated ROS determine oxidative stress,

damaging cell membranes, causing genotoxicity, and disrupting key metabolic processes like nutrient transport,

cell division, and expansion. Plants activate natural defence mechanisms to counter these stresses, but these

responses are energetically costly. The diversion of carbon skeletons and energy from growth and biomass

accumulation to stress responses results in reduced yields, especially in key Mediterranean crops such as wheat,

tomato, grapevine, and olive trees, which are highly vulnerable to extreme climatic events. Biostimulants hold

signicant potential as an innovative approach to strengthening plants’ natural defences and enhancing their

capacity to endure heat and drought stress. By modulating stress-related pathways, enhancing antioxidant

defence mechanisms, and promoting the accumulation of osmolytes, these products help maintain water use

efciency (WUE), sustain photosynthetic activity, and reduce stress-induced yield losses. In areas where water

scarcity is a major limiting factor for agriculture, biostimulants offer a promising strategy to enhance plant

adaptation to increasingly unpredictable precipitation patterns and higher temperatures. Beyond their imme-

diate benets, biostimulants offer a sustainable solution for supporting crop productivity amidst climate change.

Further research into their biochemical, physiological, and metabolic impacts, specically focusing on Medi-

terranean cropping systems, will be essential to optimise their application and integrate them effectively into

modern, sustainable farming strategies.

1. Introduction

Plants are sessile organisms incapable of escaping from environ-

mental constraints. To cope with this limitation, they rely on

physiological responses that drive their developmental plasticity and

enable adaptation to varying abiotic factors, such as chemical and

physical conditions. These mechanisms support plant survival and are

not indicative of disease. However, when environmental changes are too

Abbreviations: Abscisic Acid, (ABA); Advanced Glycation End Products, (AGEs); Aminocyclopropane-1-Carboxylic Acid, (ACC); Aquaporins, (AQP); Arbuscular

Mycorrhizal, (AM); Ascophyllum nodosum Extract, (ANE); Ascorbate Peroxidase, (APX); Betaine Aldehyde Dehydrogenase, (BADH); Basic Helix-Loop-Helix, (bHLH);

Brassinosteroids, (BRs); Calcineurin B-Like Proteins, (CBLs); Calcium-Dependent Protein Kinases, (CDPKs); Carbon-Nitrogen Ratio, (C/N Ratio); Catalase, (CAT);

Chlorophyll, (Chl); Choline Monooxygenase, (CMO); Compatible Solutes, (CS); Cyclic Nucleotide-Dependent Calcium Channels, (CNGCs); Cyclin-Dependent Kinases,

(CDKs); Cyclin P2;1, (CYCP2;1); Cytokinins, (CK); Deg Proteases, (Deg); Dehydrins, (DHNs); Drought-Responsive Element Binding, (DREB); Electron Transport Chain,

(ETC); Ethylene, (ET); Field Capacity, (FC); Fucoidan, (Fu); Gamma-Aminobutyric Acid, (GABA); Gibberellins, (GA); Glutamine, (Gln); Glutathione, (GSH); Gluta-

thione Reductase, (GR); Glutathione S-Transferase, (GST); Heat Shock Factors, (HSFs); Heat Shock Proteins, (HSPs); Hydrogen Peroxide, (H₂O₂); Indole-3-Acetic Acid,

(IAA); Jasmonic Acid, (JA); Karrikins, (KAR); Late Embryogenesis Abundant, (LEA); Malondialdehyde, (MDA); Methylglyoxal, (MG); Methyl Jasmonate, (MeJA);

Monosilicic Acid, (MSA); Net photosynthesis, (Pn); Nitric Oxide, (NO); Phenylalanine Ammonia-Lyase, (PAL); Photosystem II, (PSII); Phytochrome Interacting

Factors, (PIF); Polyethylene Glycol, (PEG); Proline Dehydrogenase, (PDH); Reactive Carbonyl Species, (RCS); Reactive Oxygen Species, (ROS); Relative Water

Content, (RWC); Retrograde Signaling, (RS); Silicon, (Si); Stay-Green, (SGR); Stomatal Conductance, (gₛ); Strigolactones, (SLs); Superoxide, (O₂•⁻); Superoxide

Dismutase, (SOD); Transcription Factors, (TFs); Tricarboxylic Acid Cycle, (TCA); Ulva lactuca Extract, (ULE); Water Use Efciency, (WUE); Zinc, (Zn).

E-mail address: petronia.carillo@unicampania.it.

Contents lists available at ScienceDirect

Plant Stress

journal homepage: www.sciencedirect.com/journal/plant-stress

https://doi.org/10.1016/j.stress.2025.100802

Received 9 January 2025; Received in revised form 19 February 2025; Accepted 10 March 2025

Plant Stress 16 (2025) 100802

Available online 11 March 2025

http://creativecommons.org/licenses/by-

nc-nd/4.0/ ).

rapid or intense, plants perceive them as stressors. In recent years,

exacerbating environmental conditions and meteorological phenomena

have led to heightened intensity and frequency of both single and

multiple abiotic stress phenomena for plants, signicantly reducing

global agricultural production (Benitez-Alfonso et al., 2023). High

temperatures and drought are the primary environmental stresses that

affect plant growth and productivity by directly impairing physiological

functions, altering metabolic pathways, and reducing resource avail-

ability. These stressors also damage natural resources, crops, livestock,

and livelihoods. Drought stress limits water availability, restricting cell

expansion, reducing stomatal conductance, and compromising photo-

synthetic efciency, thus leading to lower biomass accumulation. Pro-

longed water decits cause oxidative stress, induce hormonal

imbalances, and alter root architecture, affecting plant development at

all growth stages (Osakabe et al., 2014). Conversely, heat stress accel-

erates metabolic processes, increases transpiration rates, and disrupts

cellular homeostasis. Extreme temperatures denature proteins, impair

membrane stability, and inhibit enzymatic functions essential for carbon

xation and energy production (dos Santos et al., 2022). When drought

and heat stress co-occur, their combined impact is often more severe,

exacerbating physiological and biochemical disruptions beyond the ef-

fects of each stress alone (Sato et al., 2024). In the Horn of Africa, an

extreme drought, the most severe in forty years, has persisted, with

nearly four years of no rainfall (UNOCHA, 2023; WEF, 2022). This crisis

has resulted in widespread food shortages, malnutrition, and displace-

ment, severely impacting livelihoods and stability across the affected

areas. Here, the control of water could lead to conicts as nations and

groups compete for stable water supplies amid shortages, pollution, and

climate change (Syed, 2020). Similarly, the Mediterranean basin faces a

progressive decline in precipitation and increasing temperatures,

intensifying water scarcity and placing agricultural systems under se-

vere pressure. This area has been identied as one of the most vulnerable

regions to climate change, with annual temperatures rising approxi-

mately 20% faster than the global average, with rainfall patterns

becoming increasingly erratic (Lazoglou et al., 2024; Le Monde, 2023).

Between January and September 2023, global temperatures rose by 1.45

◦C above pre-industrial levels, making it the hottest year recorded at the

time (WMO, 2023). Summer 2023 was the warmest globally, with

Europe experiencing its fth hottest summer (Copernicus, 2025). This

warming trend intensied in 2024, with temperatures rising 1.55 ◦C

above pre-industrial levels, marking another record-breaking summer

(WMO, 2025). These climatic shifts are already severely impacting

Mediterranean agriculture, where rising temperatures and prolonged

droughts have led to signicant declines in key crop yields. By the end of

2023, the situation in Spain appeared so critical that both the New

Zealand Ministry of Foreign Affairs and Trade (MFAT, 2023) and the

Dutch Ministry of Agriculture, Nature and Food Quality (Agroberichten

Buitenland, 2024) published reports highlighting the severe conse-

quences of climate change on Spanish agriculture. These analyses

highlighted the persistent drought, record-high temperatures, and water

shortages that caused an unprecedented decline in key crops

(Copernicus, 2025). Particularly Spain cereals production fell to 11.9 Mt

in 2023, 38% and decreased from 19.3 Mt in 2022 and 25.5 Mt in 2021,

making it one of the most severe drought-driven declines in recent de-

cades. The impact was particularly acute in Castilla y Le´

on, the country’s

primary wheat-producing region, where yields dropped 40% below the

ve-year average. Common wheat and spelt production declined to 3.6

Mt in 2023, compared to 5.8 Mt in 2022 and 7.8 Mt in 2021, under-

scoring the severe water shortages and extreme heat stress affecting key

growth stages (EC, 2025). The sharp decline in Spain’s grain production

and disruptions in Ukrainian grain exports worsened global food

shortages and added to market instability. Despite these alarming g-

ures, early 2024 data suggest a partial recovery for most sectors, except

for durum wheat and common wheat, which remain signicantly below

pre-drought levels. However, concerns persist regarding the long-term

impacts of climate change, as water scarcity continues to challenge

Spain’s agricultural resilience. Indeed the impact of climate change on

Mediterranean agriculture is further exacerbated by socio-economic

factors, including a population that has more than tripled in the last

70 years, causing a rise of atmospheric CO

levels from below 300 ppm

to 426.9 ppm as of May 2024 (NOAA, 2024). Recent reports indicate

that without signicant mitigation efforts, global temperatures are

projected to increase by about 3.1 ◦C by the end of the century,

threatening ecosystems, human health, and economies on a global scale

(Carbon Brief, 2024). In addition to temperature increases, reduced

water availability is becoming a major constraint on Mediterranean crop

production. Projections indicate a 4% to 22% reduction in annual pre-

cipitation, with southern Europe potentially seeing up to a 30% decrease

in winter precipitation (Kostopoulou and Giannakopoulos, 2023; Phi-

landras et al., 2011; IPCC 2022). In European Mediterranean countries,

the intensication and prolonged duration of this water scarcity have

elevated water to a critical resource. Water scarcity is primarily driven

by demand and consumption, with agriculture accounting for about

70%, industry 19%, and municipal use 11%. Italy is among the European

countries experiencing medium to extremely high water stress, with the

Emilia-Romagna region acting as a climatic demarcation line on the

peninsula. The ood in May 2023 was a pivotal event that marked the

end of the drought in Northern Italy and shifted the focus of water

scarcity towards the southern regions and major islands (Copernicus,

2023; Euronews, 2023). However, by late 2024, water scarcity had

intensied in southern regions and major islands. Data from Greenpeace

and the National Research Council (CNR) indicate that approximately

69% of Sicily’s territory and 47% of Calabria’s were affected by extreme

drought conditions (Icona Clima, 2023). These conditions have severely

impacted water resources and local communities, underscoring the ur-

gent need for comprehensive water management strategies in these

areas (Blue Community, 2024; The Independent, 2023). Crop yields are

expected to decrease by roughly 3–5% with every 1 ◦C rise in temper-

ature. This reduction is primarily due to the shortening of the crop

growing season and increased heat and water stress during critical

growth stages. These impacts are expected to be more severe in southern

Mediterranean areas, where higher temperatures and reduced rainfall

will likely exacerbate water scarcity and crop stress (Grasso and Feola,

2012; IPCC, 2022). Consequently, the Mediterranean agricultural sector

is under pressure, requiring innovative strategies to reduce the harmful

effects of climate change on food security and crop yields. One prom-

ising approach is using biostimulants, which have been shown to

enhance plant resilience to abiotic stresses by modulating key physio-

logical and biochemical pathways. Derived from natural substances or

microorganisms, these products stimulate plant physiological processes,

improving nutrient utilisation, stress tolerance under adverse conditions

such as drought and salinity, and overall crop productivity and quality

(Carillo et al., 2025). The European Regulation (EU) 2019/1009 denes

biostimulants as products that “stimulate plant nutrition processes

independently of the product’s nutrient content with the sole aim of

improving one or more of the following characteristics of the plant or the

plant rhizosphere: (a) nutrient use efciency; (b) tolerance to abiotic

stress; (c) quality traits; or (d) availability of conned nutrients in the

soil or rhizosphere.” (EU, 2019). Biostimulants improve water use ef-

ciency, protect photosynthetic processes, and mitigate oxidative stress,

thereby supporting plant growth under adverse conditions (Carillo,

Kyratzis, et al., 2020; Fernandes et al., 2022; Gedeon et al., 2022; Iacuzzi

et al., 2024; Rouphael et al., 2023; Spada et al., 2024). However, despite

increasing research on their benets, further studies are required to

optimise their application and understand their long-term effects in

Mediterranean cropping systems (Fusco et al., 2022; Jim´

enez-Arias

et al., 2022). This review aims to explore the impact of heat and drought

stress on plant physiology, with a specic focus on Mediterranean

agriculture, and to assess the potential of biostimulants as a sustainable

tool to mitigate climate-induced yield losses.

P. Carillo

Plant Stress 16 (2025) 100802

2. Effects of water and heat stress on plant growth and

development

The challenges for plants under drought conditions begin as early as

the seed stage. Seed hydration, a critical step for activating key enzymes

essential for germination, is signicantly hindered under water stress.

Drought-induced water shortages reduce these enzymes’ activity,

resulting in poor germination, difculty in seedling establishment, and a

loss of seedling strength (Biju et al., 2017). Gibberellic acid (GA), a

primary hormone promoting seed germination, plays a crucial role in

activating hydrolytic enzymes that break down seed reserves to support

growth. However, its function is antagonised by abscisic acid (ABA),

which inhibits germination by suppressing GA activity (Saha et al.,

2022; Tong et al., 2017). Accordingly, water stress induced by drought

or polyethylene glycol (PEG-6000), can signicantly delay or inhibit

seed germination by reducing water potential, thereby limiting the

essential water uptake required for seed hydration and emergence. This

stress elevates ROS levels, triggering calcium ion (Ca²⁺) cascades and

ABA accumulation, prolonging seed dormancy and hindering seedling

establishment(Oracz and Karpi´

nski, 2016). For instance, the expression

of OsNCED3, a key gene in the biosynthesis of ABA in rice, is signi-

cantly upregulated under drought stress, resulting in increased ABA

accumulation (Liu et al., 2019). Additionally, drought stress lowers

indole-3-acetic acid (IAA) and cytokinins (CK) levels, further hindering

seedling establishment (Prerostova et al., 2018). Under moderate water

stress, seedlings may not exhibit signicant changes in overall size;

however, an increase in root growth often occurs as a compensatory

response to optimise water uptake. This adaptive mechanism enhances

the plant’s ability to access deeper soil moisture, thereby mitigating the

effects of limited water availability (Thomas et al., 2024). Conversely,

severe or prolonged water stress adversely affects shoot and root growth.

This intense stress impairs photosynthesis, reducing the production and

transport of photosynthates, and consequently diminishes root biomass.

The decrease in root mass further limits water uptake, compromising the

development of shoots and roots (Osakabe et al., 2014). Accordingly, a

study on basil (Ocimum basilicum cv. Genovese) demonstrated that under

severe drought conditions (30% soil water capacity), plants exhibited a

50% reduction in both fresh and dry yield, along with shorter stature

and narrower canopies (Mulugeta and Rad´

acsi, 2022). Similarly,

research on tomato (Solanum lycopersicum cv. Marmande) under drought

stress (50% eld capacity, FC) revealed signicant reductions in growth

and physiological parameters, including a 43% decrease in shoot weight

and 17% in total chlorophyll content, along with declines in key hor-

mones like GA (93%) and salicylic acid (SA) (82%) (Turan et al., 2023).

Similar effects have been observed in other Mediterranean staple crops.

In durum wheat (Triticum durum), drought stress reduces grain yield and

quality by limiting carbon assimilation and disrupting grain lling

(Kakanur Jagadeesha et al., 2024; KiliÇ and Tacettin, 2010; Moral et al.,

2003). In grapevine (Vitis vinifera), water decit affects berry develop-

ment, modulating secondary metabolism and altering polyphenol con-

tent, with signicant implications for wine production (Cataldo et al.,

2023; Chac´

on-Vozmediano et al., 2021; Lovisolo et al., 2010; Savoi

et al., 2020). Additionally, olive trees (Olea europaea) exhibit high

drought tolerance, mainly through stomatal regulation and osmopro-

tectant accumulation (Sofo et al., 2004), yet prolonged stress can impair

fruit yield and oil quality (Brito et al., 2019; Gholami et al., 2022; Parri

et al., 2023). These ndings emphasise the necessity of implementing

water-saving strategies, such as decit irrigation and mulching, to sus-

tain productivity in drought-prone regions while optimising water use

efciency (Bekele and Asefa, 2023). High temperatures further impede

seed germination through cellular damage, reduced water availability,

increased respiration rates, protein denaturation, and desiccation, ulti-

mately leading to germination failure. For instance, seed germination

rates can decrease by approximately 40% at 32 ◦C compared to optimal

conditions at 24 ◦C (Ghaleb et al., 2022). Seed problems can also stem

from the parent plant’s exposure to heat stress. Moderately high

temperatures impair grain lling and reduce grain size, signicantly

decreasing rice yield and quality. Prolonged exposure to severe heat

stress (39 ◦C) amplies these impacts, causing grain malformation and

further compromising crop productivity (Begcy et al., 2018).

At moderately high temperatures (27–32 ◦C), newly germinated

seedlings undergo thermomorphogenesis, displaying elongation of the

hypocotyl or epicotyl and upward leaf bending (hyponasty or thermot-

ropism). These adaptations help seedlings distance themselves from the

heat-retaining ground and enhance cooling by increasing exposure to air

currents. Such moderate heat stress accelerates developmental transi-

tions and structural changes without causing plant death (Vu, Gevaert,

et al., 2019). The stem elongation and leaf hyponasty associated with

heat stress depend on the basic helix-loop-helix (bHLH) transcription

regulators Phytochrome Interacting Factor 4 (PIF4) and PIF7 (Koini

et al., 2009). These regulators act through IAA, which is transported

from the leaves to the petiole and then reaches the hypocotyl or epicotyl

under high-temperature conditions. In conjunction with brassinoste-

roids (BRs), IAA promotes growth by elongation and/or expansion. This

effect is enhanced by stabilising hormone receptors and increased hor-

mone transport at elevated temperatures (Nemhauser et al., 2004; Vu,

Xu, et al., 2019). In addition, the temperature-sensitive PIN-LIKES (PILS)

proteins in the endoplasmic reticulum (ER) play a role in intracellular

auxin distribution. PILS6, in particular, decreases under heat shock

conditions, enabling subcellular auxin redistribution and enhancing its

signalling capacity, which promotes thermomorphogenic growth (Li

et al., 2021). In contrast, high temperatures signicantly impact the

germination of tomato seeds and the development of plants. Toki´

c et al.

(2023) investigated the effect of elevated temperatures on tomato

seedlings under conditions mimicking frequent summer heat in Medi-

terranean semi-arid and arid regions. Seedlings were subjected to tem-

peratures of 37 ◦C and extreme heat conditions of 45 ◦C. Primary root

length was affected by both heat stresses, whereas lateral root devel-

opment was notably suppressed only at 37 ◦C. The 37 ◦C treatment also

led to a notable increase in the ethylene precursor 1-aminocyclopropa-

ne-1-carboxylic acid (ACC), likely in modifying root architecture. In

contrast, the treatment at 45 ◦C triggered pronounced physiological and

morphological damage, including chlorosis, leaf wilting, and stem

bending, in both young and mature plants. These severe phenotypic

changes were accompanied by accumulated stress markers such as

proline, malondialdehyde (MDA), and heat shock protein (HSP) 90.

Additionally, heat stress markedly impacted the expression of tran-

scription factors (TFs) related to stress responses, with DREB1 emerging

as a consistent heat stress marker, highlighting its potential for devel-

oping heat-tolerant crops (Toki´

c et al., 2023). In chickpeas (Cicer arie-

tinum), high temperatures during owering impair pod set and seed

development, leading to signicant yield losses (Jumrani and Bhatia,

2014). Lentil (Lens culinaris) shows a similar sensitivity, with heat stress

reducing seed viability and germination rates (Sita et al., 2017). In

durum wheat, elevated temperatures accelerate grain lling but

compromise starch accumulation, affecting pasta-making quality

(Labuschagne et al., 2009; Sabella et al., 2020). Therefore, extreme heat

stress (temperatures above 40 ◦C) can trigger rapid damage to cells and

result in plant death. However, prior exposure to sub-lethal high tem-

peratures (below 37 ◦C) can trigger acquired thermotolerance,

enhancing a plant’s ability to survive subsequent extreme heat events

(Vu, Xu, et al., 2019). This emphasises immediate and acclimatory heat

response mechanisms in plant resilience, including thermomorpho-

genesis and heat stress responses (HSR). Recent studies reveal that

plants signicantly increase transpiration rates under high-temperature

stress, even in dry conditions. This response is driven by physical

changes, such as reduced water viscosity and the melting of cuticular

waxes, alongside physiological adjustments, including enhanced aqua-

porin activity, potassium-mediated transmembrane water transport, and

altered membrane uidity. These mechanisms enable evaporative

cooling, reducing leaf temperatures by up to 14 ◦C, helping maintain

optimal photosynthesis conditions. However, the increased water loss

P. Carillo

Plant Stress 16 (2025) 100802

can lead to decits that adversely impact critical stages of seed devel-

opment and lling, revealing a compromise between cooling and water

conservation under heat stress (Sadok et al., 2021).

3. Understanding plant stress resistance mechanisms as a key to

adaptation

High temperature and drought stress induce two major secondary

stresses: osmotic and oxidative stress. These stresses cause pleiotropic

effects, simultaneously impacting multiple plant traits. Stomatal closure

reduces CO₂ uptake, disrupting photosynthetic electron transport and

elevating ROS levels. This oxidative stress causes genotoxicity, damages

cell membranes, and disrupts metabolic processes, hindering nutrient

transport, cell division, and expansion. These disruptions culminate in

reduced plant growth and yield, emphasising the complex interplay of

stress factors that affect crop productivity (Cruz de Carvalho, 2008;

Pandey et al., 2015).

Increased membrane uidity at high temperatures and impaired

electron transport under drought generate retrograde signals that

modulate nuclear gene expression via calcium uxes. Dysfunctions in

these transport chains lead to ROS accumulation, mainly hydrogen

peroxide (H₂O₂), which acts as both a stress marker and a regulatory

signal for adaptive responses (Foyer and Hanke, 2022), While excessive

ROS cause cellular damage, moderate concentrations integrate redox

uctuations with environmental cues, linking oxidative stress with

stress-induced senescence (Carillo and Ferrante, 2025; Savchenko and

Tikhonov, 2021). Similarly, ROS generated through ABA-mediated sig-

nalling pathways play a vital role in stress adaptationby inducing H₂O₂

production via NADPH oxidase, positioning it as a key effector in

regulating stress responses. This signaling cascade activates genes

involved in osmolytes synthesis, such as proline and glycine betaine

(GB), vital for osmotic adjustment, water retention, and oxidative

damage mitigation under drought and/or heat stress. Osmotic and ionic

stresses also trigger the generation of reactive carbonyl species (RCS),

such as methylglyoxal (MG). MG plays a dual role: at high concentra-

tions, it is cytotoxic and leads to the formation of Advanced Glycation

End Products (AGEs), which impair protein and DNA functions, while at

low concentrations, it acts as a signalling molecule ne-tuning retro-

grade responses (Sun et al., 2024). MG detoxication is managed by the

glyoxalase (GLO) system, which primarily involves

glutathione-dependent glyoxalase I and II, converting MG into less toxic

products like d-lactate, thereby preventing AGE accumulation and pre-

serving cellular homeostasis(Hoque et al., 2016). In an experiment by

Wang et al. (2020), Triticum aestivum plants subjected to simulated

drought (soil water potential of −0.8 to −1 MPa for 5 days) and heat

stress (daytime temperatures >35 ◦C) during the grain-lling stage

exhibited impaired stress responses when ABA biosynthesis was

inhibited using uridone (1 µM). This inhibition reduced ABA and H₂O₂

levels, disrupting mechanisms essential for stress resilience, ultimately

causing signicant declines in leaf water potential and grain yield.

Conversely, scavenging H₂O₂ with dimethylthiourea (5 mM) modied

downstream responses without affecting ABA synthesis, emphasizing

the indispensable role of H₂O₂ as a downstream mediator in ABA-driven

adaptation to stress. ROS also activate calcium signalling and interact

with other secondary messengers, such as nitric oxide (NO), enhancing

the plant’s capacity to sense and react to environmental stress (Sandalio

et al., 2023). Calcium concentrations vary signicantly across cellular

compartments: approximately 10⁻³ M outside the cell, around 100

micromolar in the endoplasmic reticulum, and 0.5–200 nM in the

cytosol under resting conditions. However, cytosolic calcium levels rise

approximately tenfold upon stimulation, reaching 1–2 micromolar. High

temperatures further increase plasma membrane uidity, activating

heat sensors that facilitate transient Ca²⁺ inux from the apoplast into

the cytosol, initiating signalling events. Takahashi et al. (2020) noted

that during drought stress conditions, ROS and Ca²⁺ waves serve as

mobile signals enabling communication between tissues and over long

distances, vital for whole-plant drought stress resistance. In particular,

root-to-shoot signallings, such as hydraulic pressure and ROS/Ca²⁺

waves, in addition to peptide signals, are critical under dehydration

stress. Shoot tissues mediate stomatal closure, ensuring water conser-

vation during stress. Locally, these signals, peptides, and ABA regulate

stomatal function to optimise plant WUE (Takahashi et al., 2020).

Beyond local responses, they also trigger systemic thermotolerance and

drought resistance pathways by stabilizing membrane-enhancing anti-

oxidant defences, promoting the accumulation of osmolytes, and upre-

gulating heat shock proteins (HSPs). This interplay ne-tunes stress

responses, ensuring rapid response to heat and drought stress

Exposure to high temperatures (up to 37 ◦C) and/or mild water stress

can trigger plant stress tolerance responses. One response is the heat

shock response (HSR), a signalling cascade that activates heat shock

factors (HSFs). These TFs regulate the gene expression prole associated

with heat stress, initiating protective responses such as synthesising

HSPs. The primary temperature sensor in plants appears to be an in-

crease in plasma membrane uidity, which activates cyclic nucleotide-

dependent calcium channels. This Ca²⁺ inux during heat shock trig-

gers intracellular signalling cascades, synergistic and/or interrelated

with redox changes in chloroplasts, mediated by the H₂O₂ molecules,

that activate HSFA1 (Saidi et al., 2011). This latter is a key regulator that

drives the expression of HSP70 and HSP90, addressing protein mis-

folding and aggregation caused by heat stress (Dickinson et al., 2018).

HSPs have highly conserved sequences and act as molecular chaperones

to stabilise proteins and avoid misfolding or aggregation, particularly

under thermal and drought stress conditions. Correct protein folding

(secondary and tertiary structures) and the assembly of multisubunit

protein complexes (quaternary structures) are essential for protein

function (Maytin, 1995; Wang et al., 2004). In Arabidopsis thaliana,

diurnal variations in thermotolerance align with increased HSP70

expression during the day, inuenced by chloroplast-derived redox

signals. HSFA1 further promotes HSF2 transcription, forming an

HSFA1-HSFA2 superactivator complex that amplies HSPs production

and establishes acquired thermotolerance. This intricate network of HSF

activation, redox signalling, and HSP synthesis allows plants to with-

stand high temperatures and adapt to diverse environmental stresses

effectively. Recent research highlights the functions of hormones like

BRs and ABA in enhancing heat stress tolerance by modulating the ac-

tivity of HSFs. Furthermore, recent studies reveal that various cellular

compartments detect heat stress, including the nucleus, where mecha-

nisms such as nuclear condensate formation through liquid-liquid phase

separation are crucial for coordinating stress responses (Dündar et al.,

2024).

High temperatures and drought often result in excessive evapo-

transpiration, inducing osmotic stress in plants. To address this, the

accumulation of osmolytes is essential for fostering tolerance to these

abiotic stresses. In cereals, osmolytes such as proline and GB function as

osmoregulators, maintaining cellular water balance under stress. These

molecules, along with antioxidants like ascorbic acid (AA), glutathione

(GSH), and various antioxidant enzymes, stabilise membranes and

proteins, maintain redox balance, and strengthen the plant’s capacity to

withstand stress. Protecting cellular structures and functions makes

these compounds essential for plants to thrive under adverse conditions.

However, osmoregulation and antioxidant defences come at a high en-

ergetic cost, requiring plants to use approximately 50–70 moles of ATP

to synthesise each mole of osmolytes or protective metabolites (Raven,

1985). This high energy demand diverts carbon skeletons and energy

from growth-related pathways, reducing biomass accumulation and

lower yields. This trade-off underscores the substantial impact of stress

responses on plant productivity, as resources are reallocated to prioritise

survival over growth during abiotic stress conditions.

4. Integrating agronomic approaches for climate adaptation

Given the intrinsic vulnerability of agricultural productivity to

P. Carillo

Plant Stress 16 (2025) 100802

climate change, implementing adaptive agronomic strategies is essential

to ensure long-term yield stability and resource-use efciency. Among

the most effective approaches, sowing date modications, decit irri-

gation techniques, shading systems, and soil conservation practices have

been identied as key measures to mitigate the adverse effects of rising

temperatures and irregular water availability (Agho et al., 2024).

However, these strategies must be carefully tailored to specic envi-

ronmental conditions, crop physiology, and local agronomic constraints

to maximise their effectiveness and minimise trade-offs (Minoli et al.,

2022).

Optimising sowing dates is one of the primary adaptation strategies

available to farmers in semi-arid Mediterranean environments, where

temperature and rainfall patterns dictate the success of crop establish-

ment, development, and yield formation. Bread wheat (Triticum aestivum

L.), phenological responses to thermal regimes are particularly signi-

cant, as elevated temperatures accelerate growth cycles, shortening the

grain-lling period and compromising both yield and quality (Hossain

and Teixeira Da Silva, 2012). Adjusting sowing schedules can help

synchronise critical developmental phases with favourable climatic

conditions, mitigating heat stress during anthesis and ensuring adequate

water availability during grain lling (Agho et al., 2024). However, the

success of this approach is closely tied to cultivar selection, as different

wheat varieties exhibit distinct vernalisation requirements and thermal

thresholds that determine their adaptation potential under shifting

climate conditions (Gouache et al., 2012). Recent advances in precision

agriculture have further rened the ability to optimise sowing dates,

with remote sensing tools such as the Normalized Difference Vegetation

Index (NDVI) and the Normalized Difference Red Edge (NDRE) enabling

real-time assessments of plant water status and nutrient uptake

(Allende-Montalban et al., 2024). By integrating these technologies with

predictive climate models, farmers can dynamically adjust management

practices, minimising exposure to climatic risks while enhancing

resource-use efciency. Nevertheless, modifying sowing dates alone

does not fully counteract the challenges posed by increasing tempera-

tures and erratic precipitation. Extreme weather events, unexpected

heat waves, and prolonged dry spells continue to threaten crop pro-

ductivity, requiring additional agronomic measures to bolster resilience

(Georgopoulou et al., 2024).

Decit irrigation (DI) is another widely adopted agronomic strategy

for optimising WUE without excessive yield penalties in Mediterranean

areas where water scarcity represents a fundamental constraint to yield

stability (Fereres and Soriano, 2006). Among the most prominent DI

techniques, sustained decit irrigation (SDI) involves a uniform reduc-

tion in water supply throughout the growing season, whereas regulated

decit irrigation (RDI) strategically withholds water during non-critical

phenological stages to maintain productivity while conserving resources

(Burato et al., 2024; Galindo et al., 2018; Zarrouk et al., 2016). Partial

root-zone drying (PRD) alternates irrigation between root zones, stim-

ulating ABA-mediated or independent stomatal regulation to enhance

drought tolerance without entirely restricting water uptake (Jovanovic

and Stikic, 2018; Kang and Zhang, 2004; P´

erez-P´

erez et al., 2020). These

methods have demonstrated considerable potential in Mediterranean

crops, such as tomato, olive, lemon, grapevine, and pomegranate,

improving fruit composition while reducing overall water consumption

(Gil et al., 2018; Giuliani et al., 2017; P´

erez-P´

erez et al., 2012; Wahbi

et al., 2005). However, the practical implementation of DI strategies is

often hindered by site-specic constraints, including soil heterogeneity,

unpredictable rainfall patterns, and the difculty of precisely moni-

toring soil moisture levels (O’Donnell et al., 2023). Furthermore, con-

cerns regarding long-term soil salinisation and nutrient leaching remain

insufcient, emphasising the need for a more integrated approach

combining water management with complementary agronomic prac-

tices (Aragü´

es et al., 2015).

Soil conservation strategies are also crucial in enhancing crop resil-

ience to climate change, particularly in rainfed Mediterranean systems

where water retention is a limiting factor. Among these, straw mulching

and reduced tillage have proven highly effective in mitigating soil

evaporation, stabilising temperature uctuations, and improving soil

structure. By forming a protective layer over the soil surface, mulching

minimises direct exposure to solar radiation, reducing moisture loss

while fostering microbial activity and nutrient cycling (Chen et al.,

2007). In parallel, reduced tillage maintains soil integrity, enhancing

aggregate stability and organic matter content while limiting mechani-

cal disturbance to microbial communities (Zhang et al., 2016). Inte-

grating no-tillage (NT) with low-emission fertilisation further improved

durum wheat yield (+15%) and grain quality in dry years, enhancing

nutrient use efciency. This combination also maintained crop perfor-

mance and increased economic returns, highlighting its potential for

sustainable wheat production under water-limited conditions (De Santis

et al., 2024). However, the impact of NT on crop yield in semi-arid re-

gions remains variable. A study in southern Italy found that NT

improved soil water retention and boosted yields in drier years, partic-

ularly in Foggia. At the same time, in wetter conditions, conventional

tillage (CT) led to higher grain protein content, likely due to different

water dynamics (De Vita et al., 2007). This means their effectiveness is

highly dependent on site-specic conditions, with some studies report-

ing initial declines in productivity due to transitional shifts in soil

structure and microbial dynamics. Beyond soil and irrigation manage-

ment, adopting shading systems has gained increasing attention as an

effective means of mitigating heat and radiation stress, particularly in

high-temperature environments where excessive solar exposure can

compromise physiological performance. Shading nets, which vary in

mechanical and optical properties, modulate microclimatic conditions

by reducing radiation intensity, stabilising temperature uctuations,

and altering the spectral composition of transmitted light (Boini et al.,

2021; Caruso et al., 2020; Statuto et al., 2019). Traditional shading

materials, such as black or grey nets, primarily function by lowering

light intensity, while photo-selective nets have been designed to

enhance diffuse radiation, optimising photosynthesis while minimising

photoinhibition (Formisano, 2022). Shading nets have been widely

studied in heat-sensitive horticultural crops, including bell pepper, let-

tuce, and rocket, where they have been shown to increase leaf expan-

sion, regulate stomatal activity, and improve nutrient uptake (Caruso

et al., 2020; Díaz-P´

erez and St. John, 2019; Formisano et al., 2021).

Additionally, shading contributes to post-harvest quality by maintaining

higher levels of bioactive compounds, such as avonoids and glucosi-

nolates, in leafy vegetables (Caruso et al., 2020; Jim´

enez-Viveros and

Valiente-Banuet, 2023). However, shading can also reduce carbohydrate

accumulation due to lower photosynthetic rates under diminished light

availability. Therefore, the balance between heat stress mitigation and

optimal light interception must be carefully managed to ensure that

shading does not inadvertently suppress yield potential (Gallo et al.,

2024).

While these strategies provide substantial benets, their effective-

ness depends on environmental conditions, crop physiology, and farm-

level management, requiring integration into a broader framework of

sustainable intensication. The combined implementation of these

adaptive practices enhances resilience to climate-induced stressors.

However, no single approach can fully mitigate the multifaceted chal-

lenges of climate change, suggesting that further integration and

renement may be necessary to maximize their impact. Emerging

research recognizes biostimulants as a promising complement to agro-

nomic strategies, with potential for enhancing crop resilience and sus-

tainability. Their effectiveness may further improve through integration

with existing practices and adaptive approaches (Franzoni et al., 2022;

Rouphael and Colla, 2020).

5. Biostimulants as a strategy to alleviate heat and drought

stress in plants

Biostimulants are increasingly acknowledged as a crucial asset in

modern agriculture, promoting crop growth, resilience, and quality

P. Carillo

Plant Stress 16 (2025) 100802

while supporting sustainable farming practices. Derived from natural

substances or microorganisms, these products enhance plant natural

processes, improving nutrient use efciency (NUE), stress tolerance (e.

g., drought and salinity), and overall plant health and productivity. The

European Biostimulants Industry Council (EBIC, https://biostimulants.

eu/), representing the plant biostimulants industry in policy and prac-

tice, has been instrumental in advocating for their commercial potential

and economic value. At the same time, research into biostimulants over

the past 15 years has played a crucial role in demonstrating their ef-

cacy, mode and mechanisms of action. Numerous European research

programs have provided signicant funding and resources to advance

the understanding and application of biostimulants. In collaboration

with industry stakeholders, these initiatives have driven innovation and

expanded the adoption of biostimulants across diverse agricultural

systems. On the policy front, the European Union’s Regulation 2019/

1009 has established a global benchmark by dening biostimulants as

products that enhance plant growth by supporting natural processes

without directly supplying nutrients. This regulation has claried their

classication and use while ensuring compliance with strict safety and

environmental standards. Furthermore, the European Innovation Part-

nership for Agricultural Productivity and Sustainability (EIP-AGRI),

launched by the European Commission in 2012, emphasizes the stra-

tegic importance of biostimulants in addressing sustainability challenges

and fostering innovation within agriculture (EC, 2023). Together, these

efforts reect a unied momentum from research, industry, and policy

to position biostimulants as a cornerstone of sustainable farming. The

following sections will examine specic biostimulant products that have

demonstrated efcacy against heat and drought stress, showcasing their

potential to enhance plant resilience under challenging environmental

conditions. As research continues to deepen our understanding of bio-

stimulants, their application can be further optimized to address climate

change and resource scarcity while supporting agricultural productivity.

5.1. Priming strategies with biostimulants against water and thermal

stresses

Priming plants with biostimulants has emerged as an effective

strategy to enhance their resilience to abiotic stresses. Seed priming is a

pre-sowing strategy to improve seed germination, early seedling estab-

lishment, and resilience to abiotic stress. Exposing seeds to controlled

stimuli before sowing triggers metabolic and physiological adjustments

that enhance water uptake, boost enzymatic activity, and strengthen

Table 1

A summary of the mechanisms through which seed priming biostimulants enhance plant molecular and physiological responses to heat and drought stress.

Type of Biostimulant Plant Species Application method and dose Biostimulatory Mechanisms Targeted

stress

References

Ascophyllum nodosum extract Spinacia oleracea Seed priming: 1.5–12 mL l

-1

solution

Improved chlorophyll content, antioxidant

activity, and overall plant growth

Drought (Pereira dos Anjos

Neto et al., 2020)

Azospirillum brasilense, Bacillus lentus,

and Pseudomonas sp. mixture

Ocimum basilicum Seed inoculation: 10⁸ CFU

-1

Improved root growth, enhanced nutrient

absorption under reduced water stress

conditions

Drought (Heidari and

Golpayegani,

2012)

Azotobacter Triticum aestivum Seed inoculation: 10⁸ CFU

-1

Enhanced chlorophyll retention, improved

WUE, increased antioxidant defenses and

resilience to drought and salinity

Drought,

salinity

(Chaudhary et al.,

2013)

Bacillus cereus Solanum

lycopersicum

Seed priming: bacterial

suspension, applied before

sowing

Activation of antioxidant pathways,

reduced lipid peroxidation, improved

growth

Heat,

drought

(Khan et al., 2020)

Bacillus polymyxa Lycopersicon

esculentum

Seed priming: 5 mL l

-1

solution (10

CFU mL

-1

)

Enhanced photosynthetic efciency,

reduced oxidative damage, improved

biomass

Heat (Shintu & Jayaram,

2015)

Commercial Bacillus strains

(B. amyloliquefaciens, B. licheniformis,

and Brevibacillus laterosporus)

Zea mays Soil application: 2 L ha

-1

during planting

Enhanced drought resilience, proline

biosynthesis, phenylpropanoid pathway

activation

Drought (Lephatsi et al.,

2022)

γ-Aminobutyric Acid (GABA) Helianthus annuus Seed priming: 2

mol l

-1

for

24 h

Improved osmotic adjustment, reduced

oxidative stress, enhanced recovery

Heat,

drought

(Abdel Razik et al.,

2021)

γ-Aminobutyric Acid (GABA) Trifolium repens Seed priming: 2

mol l

-1

for

24 h

Enhanced germination, root length,

antioxidant enzyme activity, and reduced

oxidative damage

Heat,

drought

(Zhou et al., 2021)

Chlamydomonas reinhardtii (cc124) and

Chlorella spp. (MACC-360, MACC-38)

Medicago

truncatula

Seed priming: 1 g l

-1

for 20

min; Root drenching: 0.05 g

-1

weekly for 35 days

Promotion of auxin-cytokinin homeostasis,

enhanced growth and resilience

Heat,

drought

(Gitau et al., 2021)

Enterobacter spp. Abelmoschus

esculentus

Seed treatment: 10⁵ CFU per

seed

Improved seed germination and early

development under stress

Drought (Roslan et al.,

2020)

Karrikins (KAR) Sapium sebiferum Seed priming: 1 nmol l

-1

solution for 24 h

Improved drought tolerance, antioxidant

defenses, delayed senescence

Drought (Shah et al., 2020)

Klebsiella aerogenes Triticum aestivum Seed treatment: 10⁵ CFU per

seed

Activated stress pathways, improved

proline levels, reduced ROS damage

Drought (Shafque et al.,

2023)

Rhizophagus irregularis AMF Lactuca sativa,

Solanum

lycopersicum

Soil inoculation: 10 g of

inoculum per plant

Improved nutrient uptake, upregulation of

ABA-responsive genes, reduced oxidative

damage

Drought (Ruiz-Lozano et al.,

2016)

Silicon (Si) Oryza sativa Seed priming: 7 mg l

-1

/ Soil

application: 300 kg ha

-1

Improved seed germination, enhanced

fruit yield, and water productivity

Drought (Chakma et al.,

2021)

Spirulina maxima extract Oryza sativa Seed priming: 2.5–5 g l

-1

aqueous extract

Enhanced germination, chlorophyll levels,

root elongation, and antioxidant activity

Heat,

salinity

(Sornchai et al.,

2014)

Spirulina platensis extract Triticum aestivum Seed priming: 100 mL l

-1

aqueous extract for 24 h

Enhanced photosynthesis, increased

photosynthetic pigments, improved WUE

Drought (Elnajar et al.,

2024)

Trichoderma harzianum Cuminum cyminum Root inoculation: 10 spores

-1

Enhanced root biomass, improved nutrient

uptake, increased drought resilience

Drought (Piri et al., 2019)

Ulva lactuca extract Allium cepa Seed priming: 50 mL l

-1

aqueous extract

Enhanced chlorophyll levels, leaf area, and

root development under water stress

Drought (Muhie et al.,

2020)

Citrullus lanatus Seed priming: 30–80 mL l

-1

aqueous extract for 24 h

Improved germination rates, reduced

oxidative stress, enhanced root

development

Drought (Radwan et al.,

2023)

P. Carillo

Plant Stress 16 (2025) 100802

antioxidant defences (Paparella et al., 2015). As a result, primed seeds

show faster and more uniform germination, better-developed root and

shoot systems, and improved tolerance to drought and heat stress

(Savvides et al., 2016). Additionally, seed priming can induce stress

memory, allowing seedlings and young plants to enact a quicker and

more efcient defence response when exposed to environmental

stressors (Liu et al., 2022). The use of biostimulants in priming has

gained increasing attention also for due to their ability to further

enhance stress tolerance and plant performance through multiple

physiological and biochemical pathways The process integrates

biochemical, physiological, and molecular adaptations, ensuring plants

can efciently manage oxidative stress, osmotic imbalance, and

heat-induced cellular damage (Cocetta et al., 2022). Different types of

biostimulants offer distinct benets for priming (Table 1). All mea-

surement units have been standardised across different biostimulant

applications to ensure consistency and facilitate direct comparison of

treatment effects.

5.1.1. Seed priming with algal extracts

Algal extracts have demonstrated signicant effectiveness in pro-

moting seed germination and improving plant resilience to abiotic

stresses such as water scarcity, salinity and elevated temperatures. In

wheat, the grains priming with Spirulina platensis aqueous extract at a

100 ml l

-1

concentration signicantly enhanced plants drought toler-

ance. Treated plants exhibited up to a 53.4% increase in photosynthetic

efciency, enabling better carbon assimilation under water-limited

conditions. Additionally, the treatment improved plant vigour, with

drought-stressed cultivars showing a 153% rise in economic yield

compared to untreated controls. Growth parameters such as spike

number increased by 76.5%, while physiological traits like gas exchange

efciency and carbohydrate content improved signicantly (Elnajar

et al., 2024). Research shows that algal-based extracts enhance the

accumulation of osmolytes, including proline and GB, which are vital for

osmotic regulation during stressful conditions. Additionally, they

enhance the activity of key antioxidant enzymes, including ascorbate

peroxidase (APX), catalase (CAT), glutathione reductase (GR), peroxi-

dase (POD), and superoxide dismutase (SOD), helping to reduce the

oxidative damage in plant cells (Mughunth et al., 2024). This dual action

signicantly reduces oxidative stress, safeguarding cellular integrity

under adverse conditions (Carillo, Ciarmiello, et al., 2020). They also

increase photosynthetic pigments like chlorophylls and carotenoids and

promote larger leaf areas, longer shoots, and deeper root systems

(Parmar et al., 2023). In Medicago truncatula, treatments with

microalgae-based biostimulants, including Chlamydomonas reinhardtii

(cc124) and Chlorella spp. (MACC-360 and MACC-38), applied through

seed priming (1 g/L for 20 min) and weekly root drenching (0.05 g/L for

35 days), demonstrated strain-specic effects on auxin and cytokinin

homeostasis, signicantly inuencing plant growth and development.

Among the strains, Chlorella MACC-360 stood out for its ability to

enhance axillary shoot development, increase leaf size, and boost overall

biomass. This was attributed to its production of growth-promoting

phytohormones and exopolysaccharides (EPS), which improved

root-microbe interactions and enriched soil properties, further ampli-

fying its growth-promoting effects. In contrast, Chlorella MACC-38 and

C. reinhardtii exhibited more variable outcomes. While C. reinhardtii

enhanced micronutrient content in roots and leaves, it also induced

developmental irregularities, such as delayed owering and bifurcation,

likely due to disruptions in auxin-regulated gene expression (Gitau et al.,

2021). These ndings highlight the species or strain-specic ability of

(micro)algae to modulate plant growth dynamics, enhancing develop-

ment and improving plant resilience to environmental stresses. For

instance, Spirulina maxima extract (2.5–5 g L⁻¹) substantially enhanced

germination and early seedling development in Oryza sativa under

salinity and heat stress by enhancing chlorophyll levels, root elongation,

and antioxidant activity (Sornchai et al., 2014). In T. aestivum, seed

priming with Ulva lactuca extract (10–100 mL l

-1

10%) for 12 h boosted

drought and salinity resilience by raising proline levels, reinforcing

antioxidant systems, and promoting WUE as well as root development

(Ibrahim et al., 2014). Other studies demonstrated similar benets in

different crops. In Allium cepa, Ulva lactuca extract (5%) promoted the

germination of seeds and seedling vitality under water stress, leading to

increased chlorophyll levels, leaf area and root development (Muhie

et al., 2020). A study evaluating the effect of ve Ascophyllum nodosum

extract concentrations (0.0, 0.15, 0.30, 0.60, and 1.2%) on Spinacia

oleracea seeds found that 0.3% extract priming determined the highest

germination percentage (+18%), faster germination speed (+22%), and

improved seedling vigor (+20%) compared to non-primed seeds under

heat stress (30 ◦C). Additionally, the priming reduced H₂O₂ and MDA

levels by 25% and 30%, respectively, under heat stress, indicating

enhanced stress tolerance mechanisms (Pereira dos Anjos Neto et al.,

2020). An aqueous extract of Ulva lactuca (3–8%) applied to Citrullus

lanatus (Gurum) improved germination rates and seedling vigor under

drought conditions by enhancing proline content and antioxidant

enzyme activity, reducing oxidative damage (Radwan et al., 2023). Xu

et al. (2018) discovered a CGTCA motif located in the promoter region of

choline monooxygenase (CMO) and betaine aldehyde dehydrogenase

(BADH) genes in Citrullus lanatus, which is responsive to methyl jasm-

onate (MeJA). This nding demonstrated that C. lanatus cells treated

with MeJA could synthesize GB even without environmental stress

(Annunziata et al., 2019). The activation of this MeJA-GB synthesis

cascade may offer a valuable perspective on the potential modes of ac-

tion through which algal biostimulants may enhance plant stress toler-

ance. This mechanism also highlights the role of GB as a particularly

intriguing compound in stress adaptation. It is notably synthesised and

accumulated in halophytes and some cultivated plants like Poaceae. Its

amphoteric nature allows it to remain electrically neutral across a wide

physiological pH range and is highly water-soluble. Under stress con-

ditions, GB acts as an osmoregulator, stabilizes macromolecules and

their functions, maintains membrane integrity, and mitigate cellular

damage, playing many essential roles (Annunziata et al., 2019). GB

typically accumulates in young tissues during prolonged stress. How-

ever, plants synthesize it sparingly and only when necessary since it is

non-recyclable and gradually diluted (Carillo et al., 2008). A delay in GB

synthesis under drought and/or salinity can lead to structural disrup-

tions, particularly at the root apex, with signicant vacuolization,

disorganized apical tissue, and slight plasmolysis due to reduced cell

adhesion. These changes can inhibit growth and differentiation, severely

affecting plant development (Annunziata et al., 2017; Duan et al., 2010).

The supply of GB (5 mM) to the culture medium has been shown to

effectively counteract osmotic stress in barley roots by preserving root

apex structure and mitigating growth inhibition, as demonstrated by

Cuin and Shabala (2005). Similarly, in Citrullus lanatus, the activation of

the MeJA-GB pathway by algal extracts likely prevents such structural

damage by promoting early GB synthesis and maintaining cellular

integrity under stress conditions (Radwan et al., 2023; Xu et al., 2018).

5.1.2. Seed priming with microbial biostimulants

Microbial biostimulants, including arbuscular mycorrhizal fungi

(AMF) and plant growth-promoting bacteria (PGPB), boost nutrient

acquisition through processes such as nitrogen xation and the solubi-

lization of phosphorus. These microorganisms form mutually benecial

relationships with plants, establishing dense hyphal networks that in-

crease root nutrient absorption. They also reinforce cell walls and create

protective biolms on root surfaces, improving water retention and

salinity resilience under thermal and water stress conditions (Fusco

et al., 2022). These interactions activate crucial physiological and

metabolic pathways, promoting the accumulation of protective com-

pounds like proline, AA, and GSH while boosting the activity of anti-

oxidant enzymes (SOD, POD, CAT). Microbial biostimulants also

enhance the uptake of essential major nutrients (N, P, K, Ca, Mg) and

trace elements (Fe, Zn, Cu, Mn, B), improving chlorophyll synthesis,

photosynthetic efciency, carbon assimilation, and biomass production

P. Carillo

Plant Stress 16 (2025) 100802

(Papa et al., 2022). This supports root development, leaf expansion, and

overall plant growth, ultimately increasing resilience to environmental

stresses like drought and heat, ensuring improved survival and pro-

ductivity under challenging conditions (Fadiji et al., 2021). For example,

Rhizophagus irregularis (strain EEZ 58) enhanced drought tolerance in

Lactuca sativa, cv. Romanain and S. lycopersicum cv. Reimlams Rhums

through arbuscular mycorrhizal (AM) symbiosis . At sowing, soil was

supplied with 10 grams of inoculum or autoclaved inoculum, respec-

tively, eachcontaining approximately 60 infective propagules per gram,

ensuring the establishment of the symbiosis. Under drought stress, root

colonization increased signicantly, reaching 63% in lettuce and 54% in

tomato after 8 weeks. This enhanced colonization improved shoot dry

weight (+26% in lettuce and +17% higher in tomato) improved

photosystem II efciency and stomatal conductance, supporting photo-

synthetic activity under drought. AMF treated plants also showed

increased ABA levels, upregulation of ABA biosynthesis genes such as

LsNCED2, and activation of drought-responsive genes, including dehy-

drins (DHNs) from the late embryogenesis abundant (LEA) family,

which stabilized membranes mitigated oxidative stress damage. Simi-

larly, strigolactones (SLs), essential for root-fungus communication,

were signicantly upregulated in mycorrhized plants under drought.

Tomato plants in symbiosis with R. irregularis showed higher levels of

SLs like solanacol and didehydro-orobanchol, and upregulation of the SL

biosynthesis gene SlCCD7. By stimulating the ABA and strigolactone

signalling pathways and enhancing critical stress-response mechanisms,

AM symbiosis emerges as an effective and sustainable approach to boost

crop resilience in water-limited conditions (Ruiz-Lozano et al., 2016).

Klebsiella aerogenes SH-8 (10

CFU/Petri) signicantly improved

drought tolerance in T. aestivum by enhancing proline accumulation,

boosting antioxidant enzyme activity, and increasing nutrient uptake,

resulting in better root development and overall plant growth under

drought conditions (Shafque et al., 2023). Similarly, Enterobacter spp.

(10

CFU seed

-1

) enhanced seed germination and initial seedling

development in Abelmoschus esculentus by increasing chlorophyll con-

tent and leaf area, thereby supporting plant vigor during water decit

(Roslan et al., 2020). In Cuminum cyminum, a combination of Pseudo-

monas uorescence (10

CFU mL

-1

) and Trichoderma harzianum (10

spores mL

-1

) strains effectively enhanced root biomass and improved

nutrient uptake, signicantly increasing the plant’s drought resilience

(Piri et al., 2019). In Hordeum vulgare, the application of Hartmannibacter

diazotrophicus (10⁸ CFU mL⁻¹ seed⁻¹) resulted in improved chlorophyll

content, greater root development, and enhanced antioxidant activity,

supporting better growth under water scarcity (Suarez et al., 2015).

Bacillus polymyxa (5 mL l

-1

of a solution of 10

CFU mL

-1

) enhanced the

stress tolerance of Lycopersicon esculentum through seed priming. Primed

seeds were sown in garden pots, and after 21 days of vegetative growth,

plants underwent a 3-day water stress treatment, followed by regular

irrigation This treatment led to higher proline levels, improved photo-

synthetic efciency, and increaed antioxidant defences, which mitigated

the effects of heat stress (Shintu, 2015). Additionally, Bacillus cereus

priming improved the tolerance to elevated temperatures in

S. lycopersicum by enhancing the transcription of SlWRKY33b and

autophagy-related genes (SlATG5), and upregulating genes encoding

heat-responsive TFs (SlHsfA1a) and potassium transporters (SlHKT1).

This mechanism supported root development and reduced oxidative

damage under elevated temperatures (Khan et al., 2020). The inocula-

tion of Ocimum basilicum seeds with a bacterial suspension of Azospir-

illum brasilense, Bacillus lentus, and Pseudomonas sp. (10

CFU mL

-1

)

mixed with perlite enhanced drought tolerance by promoting shoot and

root growth and supporting nutrient absorption, even under reduced

water availability (60% and 40% of the FC vs 80% control) (Heidari and

Golpayegani, 2012). Additionally, inoculation of T. aestivum seeds with

Azotobacter (10

CFU mL

-1

) improved WUE and chlorophyll retention,

boosting plant resilience to drought and salinity (Chaudhary et al.,

2013). A commercial biostimulant formulation, consisting of ve Ba-

cillus strains (one strain of Bacillus amyloliquefaciens, two strains of

Bacillus licheniformis, and two strains of Brevibacillus laterosporus) was

supplied at a dose of 2 L per hectare, delivering directly into the soil

ensuring direct contact with the seed of Zea mays at planting. The con-

sortia signicantly enhanced drought resilience by activating

stress-responsive pathways. This priming led to upregulating ZmDREB2

and ZmP5CS, essential for osmotic adjustment via proline biosynthesis.

The consequently elevated proline levels, coupled with a 6.3-fold in-

crease of the expression of phenylalanine ammonia-lyase (PAL), and a

0.6-fold decrease of avone synthase (FSNII), further enhanced drought

tolerance. PAL acts as a pivotal enzyme in linking primary and sec-

ondary metabolism through the phenylpropanoid pathway, which con-

verts phenylalanine into cinnamic acid, a fundamental precursor for

important phenylpropanoids. Conversely, FSNII facilitates the direct

conversion of avanones into avones, contributing to the production of

secondary metabolites that support growth and defence mechanisms

under water stress (Lephatsi et al., 2022). In Z. mays, Bacillus sp. MGW9

(10

CFU mL

-1

) demonstrated similar benets by supporting root system

development and antioxidant activity, improving resilience to water

stress (Li et al., 2021).

5.1.3. Seed priming with plant extracts, protein hydrolysates and

metabolites

Seed priming with plant extracts, protein hydrolysates (PHs), or

metabolites is an established approach to enhance germination and

improve water stress tolerance. This technique activates key physio-

logical mechanisms, such as DNA repair, cell division, and antioxidant

enzyme activity, fostering uniform germination, robust shoot develop-

ment, and improved relative water content (RWC). By reducing ROS and

MDA accumulation, it mitigates oxidative stress, stabilizes cellular

membranes, and enhances drought resilience, enabling faster recovery

after rehydration (Ahmed et al., 2019; Wang et al., 2022). For example,

γ-aminobutyric acid (GABA) pretreatment at 2

mol l

-1

signicantly

enhanced drought tolerance in Trifolium repens (white clover) seeds,

increasing germination percentage by over 20% and root length by 15%

under water-limited conditions. This treatment also enhanced soluble

sugars and antioxidant metabolites’ levels (e.g., AA and GSH) while

enhancing antioxidant enzyme activities (e.g., CAT, POD, and SOD) by

25–40%. Furthermore, GABA priming increased DHN proteins and

DREB TFs levels, improving osmotic adjustment and reducing oxidative

damage during germination (Zhou et al., 2021). Similarly, O. sativa

seeds exposed to MeJA under osmotic stress induced by polyethylene

glycol exhibited increased metabolic regulation and resilience, high-

lighting the effectiveness of priming strategies in alleviating abiotic

stress (Sheteiwy et al., 2018). These ndings underscore the effective-

ness of seed priming in enhancing stress tolerance through metabolic

and antioxidant modulation. Also priming seeds of O. sativa with 0.5

mmol l

-1

GABA markedly enhanced germination and seedling develop-

ment under osmotic stress caused by polyethylene glycol (30 g l

-1

PEG

6000). GABA priming enhanced water relations and photosynthetic ef-

ciency while increasing the levels of GABA, soluble sugars, starch and

proteins. Additionally, antioxidant enzyme activities (e.g., GR) and

detoxication-related enzymes were signicantly enhanced, thus

reducing oxidative stress. Notably, GABA-treated seeds exhibited lower

proline levels and reduced free radicals and MDA under stress.

Furthermore, the transcription of stress-responsive genes, particularly

OsCIPK02, OsCIPK07, and OsCIPK09, was upregulated under osmotic

stress, highlighting the molecular basis of GABA-mediated stress toler-

ance. The OsCIPK (Calcineurin B-like Protein-Interacting Protein Ki-

nases) are genes in rice that respond to stress and function in calcium

signalling pathways. These kinases interact with Calcineurin B-like

proteins (CBLs) to regulate ion homeostasis, and to activate antioxidant

defences under osmotic stress conditions. They enable cellular adapta-

tion and improve plant resilience during water-decit and salinity stress

(Xiang et al., 2007). Karrikins (KARs), active components of smoke, have

been recognized as valuable tools for improving tolerance to abiotic

stresses. KAR1 at a concentration of 1 nM signicantly improved

P. Carillo

Plant Stress 16 (2025) 100802

germination of Sapium sebiferum seeds and seedling vitality under os-

motic stress (100–300 mmol l

-1

mannitol) conditions. The primed seeds

showed higher activities of antioxidative enzymes including SOD, POX,

CAT, and APX, lower levels of H

, MDA, and electrolyte leakage (EL),

together with improved germination rates (Shah et al., 2020).

Silicon-based treatments, including seed priming and soil application,

were evaluated for their effectiveness in improving drought resilience in

grape tomato (S. lycopersicon L. var. cerasiforme). Monosilicic acid

(MSA) proved ineffective under severe drought conditions (50% FC)

irrespective of the application method or dose. However, MSA applied as

seed priming at a concentration of 0.25 mM or applied to the soil at a

rate of 300 kg/ha boosted fruit yield and irrigation water productivity

(IWP) under moderate (75% FC) and optimal (100% FC) moisture levels

(Chakma et al., 2021).

In Pisum sativum, seed priming with 50 mg L⁻¹ SA improved seedling

vigor and physiological stability under heat stress conditions (43 ◦C).

SA-treated seeds exhibited higher germination rates, improved seedling

elongation, higher fresh and dry biomass, and increased relative water

content. At the physiological level, SA enhanced chlorophyll retention

and membrane stability, supporting photosynthetic efciency and water

balance (Tamindˇ

zi´

c et al., 2023). In Hordeum vulgare cv. Haider-93, seed

priming with 20 mM GB signicantly improved germination, shoot

biomass, and water retention under high temperatures. Seedlings from

GB-treated seeds exhibited higher shoot dry weight and net photosyn-

thesis (Pn) while maintaining lower membrane permeability than un-

treated plants. Additionally, GB reduced ion leakage, particularly Ca²⁺,

Table 2

An overview of the mechanisms by which foliar spray and root drenching biostimulants enhance plant molecular and physiological adaptations to heat and drought

stress. All concentrations have been standardized to ensure comparability across treatments.

Type of biostimulant Plant species Application method and dose Biostimulatory mechanisms Targeted

stress

References

Ascophyllum nodosum

extract

Arabidopsis thaliana Foliar spraying: 2 mL l

-1

on days 25 and 27

after germination

Enhanced RWC, reduced oxidative

stress, regulated ROS, improved cell

cycle activity

Drought (Rasul et al., 2021)

Ascophyllum nodosum

and Laminaria digitata

extracts

Solanum

lycopersicum

Foliar spraying: 0, 2.5, and 5 mL L⁻¹ during

critical growth stages

Improved fresh/dry weight, enhanced

RWC, antioxidative enzyme activity,

stabilized chlorophyll

Drought (Campobenedetto

et al., 2021)

Ascophyllum nodosum

extract and Silicon (Si)

Solanum

lycopersicum

Foliar: 0, 1.25, 2.5, 3.75, 5 mL L⁻¹; Silicon: 60

kg ha⁻¹ applied to soil

Improved water productivity, reduced

ion leakage, stabilized biomass,

enhanced soluble solids and fruit

quality

Drought (Ahmed et al., 2023)

Bacillus amyloliquefaciens

SN13

Oryza sativa Root inoculation: 10 ml l

-1

solution Increased osmoprotectants, reduced

oxidative damage, upregulated stress-

responsive genes

Heat,

drought

(Tiwari et al., 2017)

Bacillus mycoides strain

Arabidopsis thaliana Foliar spraying: 1 ×10⁸ CFU mL

-1

applied

twice

Improved root architecture, increased

antioxidant enzyme activity, reduced

EL and MDA

Heat,

drought

(Kurniawan and

Chuang, 2022)

Bacillus zanthoxyli HS1 Brassica rapa spp.

pekinensis, Cucumis

sativus

Soil inoculation: 1.3 ×10⁷ CFU mL

-1

or VOC

exposure

Enhanced shoot/root growth, improved

antioxidant activity, modulated

stomatal conductance, reduced stress

sensitivity

Heat,

drought

(Barghi and Jung,

2024)

β-Aminobutyric Acid

(BABA)

Linum usitatissimum

(linseed)

Seed priming: 50–100

mol l

-1

; Foliar

spraying: not specied

Enhanced CAT/POX activity, increased

proline levels, reduced oxidative

damage, improved yield

Drought (Yasir et al., 2023)

γ-Aminobutyric Acid

(GABA)

Helianthus annuus Foliar spraying: 1 mmol l

-1

applied 20 days

after treatment

Improved gas exchange, enhanced PSII

efciency, reduced oxidative stress,

increased seed yield

Heat,

drought

(Abdel Razik et al.,

2021)

Glutamate Brassica napus Root drenching: daily application, 2 mL of 20

mmol l

-1

glutamate

Enhanced proline levels, improved

RWC, boosted drought resilience

Drought (La et al., 2020)

Karrikins (KAR1) Agrostis stolonifera Foliar spraying: 100 nmol l

-1

Increased RWC, delayed senescence,

enhanced antioxidant enzyme activity

Drought (Tan et al., 2023)

Mannitol Triticum aestivum Root drenching: 100 mmol l

-1

applied for 24

Enhanced antioxidant enzyme activity,

improved tolerance to osmotic and

salinity stress

Drought,

salinity

(Seckin et al., 2009)

Nannochloris sp.

biostimulant

Lycopersicon

esculentum

Foliar spraying: 0.5% solution (0.5 kg 100 l

-1

) applied twice (2nd and 29th days post-

repotting)

Enhanced root/leaf development,

reduced ROS, improved growth under

drought stress

Drought (Oancea et al., 2013)

Protein hydrolysates

(PHs)

Capsicum annuum Fertigation: 1.5 mL L⁻¹ Maintained chlorophyll content,

improved biomass, enhanced

antioxidant defences

Drought (Agliassa et al.,

2021)

Satureja hortensis Foliar spraying: 2 mL l

-1

applied twice Increased proline, improved pigment

levels, enhanced yield, supported

growth under drought

Drought (Rezaei-Chiyaneh

et al., 2023)

Solanum

lycopersicum

Foliar spraying: 3 mL L⁻¹ weekly Activated defence mechanisms,

improved WUE, reduced stress-related

agents

Drought (Leporino et al.,

2024)

Septoglomus constrictum

AM fungi

Solanum

lycopersicum

Root inoculation: 30 g inoculum/pot (27–32

spores g

-1

)

Improved biomass, enhanced

antioxidant enzyme activity, reduced

MDA, upregulated stress-responsive

genes

Heat,

drought

(Duc et al., 2018)

Thuricin 17 (Th17) Brassica napus Root drenching: 1 nmol l

-1

(severe drought)

or 10 pmol l

-1

(heat +drought)

Enhanced root length, increased CAT/

POD/SOD activity, reduced oxidative

stress, improved water uptake

Heat,

drought

(Nazari and Smith,

2023)

Urea, zinc, iron, silicon Triticum aestivum Seed priming: urea (20 g l

-1

), FeSO₄ (50 mg l

), ZnSO₄ (50 mg l

-1

), silicon (20 mg l

-1

);

Foliar spray: urea (40 g l

-1

), silicon (112 mg l

), FeSO₄ (6 g l

-1

), ZnSO₄ (4 g l

-1

)

Increased grain yield, improved

nutrient biofortication (Zn and Fe),

reduced oxidative damage

Drought (Moradi and

Siosemardeh, 2023)

P. Carillo

Plant Stress 16 (2025) 100802

K⁺, and NO₃⁻, helping to stabilise cell membranes under heat stress.

These ndings indicate that GB priming enhances seedling vigour by

preserving water balance, photosynthetic efciency, and cellular sta-

bility, potentially improving crop establishment in high-temperature

environments (Wahid and Shabbir, 2005). Pre-sowing seed treatment

with a commercial biostimulant based on lignin derivatives, amino

acids, and molybdenum, improved germination and early growth in

Cucumis sativus L., particularly under heat stress (35 ◦C). After 48 h,

treated seeds showed higher germination (+6.54%) and fresh biomass

(+13%). The treatment upregulated the expression of antioxidant

enzyme genes (CuZnSOD +1.78, MnSOD +1.75, CAT +3.39) and

increased non-protein thiols (+20%), reduced oxidative damage

(decrease of H₂O₂ levels by 70% at 28 ◦C and 80% at 35 ◦C) and stim-

ulated metabolic activity (37% increase of isocitrate lyase activity), thus

improving seedling establishment under heat stress (Campobenedetto

et al., 2020).

5.2. Foliar spraying and root drenching for managing water and thermal

stresses

Foliar spraying and root drenching, which involve applying bio-

stimulant solutions directly to plant leaves or roots, have proven effec-

tive in alleviating heat and drought stress. Such methods improve plant

physiological and biochemical functions by enhancing WUE, increasing

nutrient absorption, and triggering stress-response pathways. Under

elevated temperatures, foliar applications can boost cellular defences by

increasing HSPs and antioxidants, whereas root drenching supports root

health and water uptake. Under drought stress, these approaches

improve stomatal control and increase the accumulation of compatible

osmolytes such as proline and GB, thereby stabilising cell structures

during water scarcity and minimising oxidative damage. Delivering

active substances directly to key plant tissues through foliar and root

applications mitigates immediate stress impacts and enhances long-term

resilience, promoting sustainable yields even under adverse climatic

conditions (Table 2).

5.2.1. Algal extracts for foliar and root applications

Algal extracts and biostimulants derived from microalgae can

enhance plant resilience against drought and heat stress when applied

via foliar spraying or root drenching. These biostimulants demonstrate a

range of benecial effects, including an increase of the osmoprotectants

proline and GB, stimulation of the activity of antioxidant enzymes (e.g.,

APX, CAT, GR, POD and SOD), and promotion of photosynthetic

pigment synthesis (chlorophylls and carotenoids). These biochemical

changes enhance photosynthesis, light protection, and water retention

while supporting growth metrics like leaf expansion, stem elongation,

and root growth. Together, these effects facilitate nutrient and water

uptake in stress-prone conditions (Carillo, Ciarmiello, et al., 2020).

An extract from Ascophyllum nodosum, foliar sprayed at a 2 mL l

-1

concentration on days 25 and 27 after germination, effectively allevi-

ated drought stress in A. thaliana, induced by withholding irrigation

between days 31 and 42 post-germination. Plants treated with the

extract exhibited reduced oxidative stress, as evidenced by lower ROS

accumulation, alongside improved water retention and stomatal regu-

lation compared to untreated controls. Enhanced RWC and decreased

ion leakage in leaves indicated greater cellular stability and reduced

damage. The presence of specic components like laminarin, fucoidan,

alginate, and ulvan in algal extracts enhances water stress tolerance

through the priming of antioxidant defences and regulation of ROS

levels. These compounds mitigate oxidative damage, stabilize cellular

structures, and promote photosynthetic efciency under stress condi-

tions. Molecular analyses also revealed that the extract facilitated cell

cycle activity by sustaining the expression of genes such as HISTONE H4

and Cyclin P2;1, a key regulator of cell division at the shoot apical

meristem (SAM) under drought conditions. Additionally, the extract

downregulated RD26, a stress-responsive growth suppressor, in the

SAM, ensuring its functionality and promoting sustained growth (Rasul

et al., 2021).

In S. lycopersicum, foliar applications of seaweed extracts derived

from A. nodosum and Laminaria digitata under water stress conditions

demonstrated signicant growth, yield, and stress tolerance improve-

ments. The seaweed extract was applied at concentrations of 0, 2.5, and

5 mL L⁻¹ during critical growth stages, with evaluations performed

under both optimal irrigation and drought conditions. The biostimulant-

treated plants showed increased fresh weight and dry matter content

and improved stem water potential, suggesting better water retention

and transport mechanisms. Biochemical adjustments included enhanced

activity of antioxidative enzymes (e.g., SOD and CAT), increased proline

levels, and stabilised chlorophyll concentrations. The treatments

improved RWC, mitigated oxidative damage, and sustained photosyn-

thetic efciency, ultimately enhancing fruit yield under drought stress

(Campobenedetto et al., 2021). Moreover, always in S. lycopersicum, the

simultaneous use of A. nodosum extract and silicon (Si) across different

soil moisture conditions (50%, 75%, and 100% FC) demonstrated

additional benets in mitigating water stress. The algal extract was used

as a foliar spray at concentrations of 0, 1.25, 2.5, 3.75, and 5 mL L⁻¹,

whereas silicon, as mono silicic acid, was applied to the soil at a dosage

of 60 kg ha⁻¹. This combination improved water productivity and fruit

quality traits, including higher soluble solids, rmness, and pH. Under

severe drought stress (50% FC), the treatments reduced yield losses,

oxidative damage, and ion leakage, while maintaining higher RWC and

promoting shoot and root biomass. Algal and Si applications resulted in

comparable yields at moderate (75%) and sufcient (100%) soil mois-

ture levels, highlighting their potential to sustain productivity across

varying water availability conditions (Ahmed et al., 2023).

In a greenhouse study, a microalgae-based biostimulant derived from

Nannochloris sp. 424–1 was tested on tomato plants (Lycopersicon escu-

lentum cv. Cristal F1) under water-stressed and non-stressed conditions.

The experiment included six treatments, with foliar sprays applied on

the 2nd and 29th days post-repotting. Plants were irrigated either every

ve days (non-stressed) or every two weeks (stressed) at 100% eld

capacity. The biostimulant, composed on a dry weight basis of proteins

(88.26%), proline (5.63%), carbohydrates (9.41%), and betaines

(0.023%), was applied as a 5 g l

-1

solution (0.5 kg 100 l

-1

) in

comparison with a commercial A. nodosum based extract. After eight

weeks, the Nannochloris sp.-based biostimulant effectively alleviated

water stress effects, promoting growth and productivity in a way com-

parable to or exceeding the commercial product. When tested on non-

water-stressed tomato plants, the microalgae-based biostimulant out-

performed the commercial seaweed extract in stimulating root length

(+2%) and leaf number (+13). Under water stress conditions, untreated

tomato plants experienced reductions of nearly 20% in height and over

25% in root length. However, the application of the microalgae-based

biostimulant mitigated these effects, reducing the negative impact on

height by nearly 50% and supporting root development. The microalgae

biostimulant and the commercial macroalgae product demonstrated

similar results during the 30-day owering and fruiting cycle (Oancea

et al., 2013). Therefore algal and microalgae-derived biostimulants

represent a sustainable and effective strategy to mitigate the effects of

abiotic stresses such as drought and heat. These biostimulants enhance

plant resilience by activating specic physiological and molecular

pathways, providing practical solutions to maintain agricultural pro-

ductivity in increasingly challenging environmental conditions.

5.2.2. Root inoculation with microbial biostimulants

Microbial biostimulants, including arbuscular mycorrhizal (AM)

fungi, may also improve plant tolerance to drought and heat stresses

through diverse biochemical and physiological mechanisms. In a

controlled experiment on S. lycopersicum, inoculation with Septoglomus

deserticola and Septoglomus constrictum at transplant was assessed for its

ability to alleviate combined drought (50% eld capacity) and heat

stress (42 ◦C for 6 h). AM inoculation was achieved by applying 30

P. Carillo

Plant Stress 16 (2025) 100802

grams of inoculum per pot containing 27–32 spores per gram.

S. constrictum signicantly improved plant biomass, increasing root and

shoot dry weight by 14% and 18% under drought and by 22% and 24%

under combined stress, respectively, compared to non-inoculated con-

trols. The maximum quantum efciency of photosystem II (Fv/Fm)

improved by 9% under combined stress, while stomatal conductance

and relative water content also increased. Leaf water potential showed a

21% improvement under drought, reecting better water retention.

Oxidative stress markers, including MDA and H

, were notably

reduced in AM plants: in particular, under combined stress,

S. constrictum inoculation lowered MDA by 18% and H

by 37%.

Enhanced activities of the antioxidant enzymes CAT, SOD, and POD

contributed to reduced oxidative damage; CAT activity, for instance,

increased by 87% in roots under drought and by 28% under combined

stress. At the molecular level, S. constrictum inuenced the expression of

stress-responsive genes. SlLOXD, involved in MeJA biosynthesis, was

upregulated by 32% under combined stress, supporting stress mitigation

pathways. Conversely, SlNCED, related to ABA synthesis, was down-

regulated, correlating with improved WUE (Duc et al., 2018) . Beyond

AM fungi, also plant growth promoting rhizobacteria (PGPR) may

modulate key biochemical and molecular pathways to increase plant

resilience to enviromental stresses. In particular, inoculation with Ba-

cillus amyloliquefaciens SN13 (SN13) at 1% signicantly enhanced the

resilience of O. sativa seedlings to combined heat and drought stress.

SN13-treated plants demonstrated higher levels of osmoprotectants,

reduced oxidative damage, and enhanced stress-responsive gene

expression, collectively improving their ability to withstand stress.

Proline levels increased by 20% in SN13-inoculated seedlings compared

to untreated controls, especially within the rst 3 h of stress exposure.

Total soluble sugars also showed a remarkable rise, with levels

increasing by 328% at 3 h and maintaining 200–400% higher concen-

trations at 24 h. SOD and CAT activities were signicantly enhanced,

protecting cells from ROS, and reducing oxidative damage as also evi-

denced by the lower MDA levels, indicating reduced lipid peroxidation

and improved membrane integrity. On a molecular level, SN13 activated

key stress-responsive genes, including those encoding for DHN and LEA

proteins, and glutathione S-transferases (GSTs), which were upregulated

by 7–12-fold during stress. These genes played a crucial role in stabi-

lizing cellular structures, mitigate oxidative damage, and maintaining

cellular functions under adverse conditions (Tiwari et al., 2017).

Notably, GSTs play a crucial role in plant defence mechanisms by

detoxifying ROS through conjugation with glutathione. This process

protects cellular structures, maintains redox balance, and supports stress

signalling, thereby enhancing plant resilience under abiotic stress con-

ditions (Edwards et al., 2000). Two-week-old soil-grown seedlings of

A. thaliana Col-0 were treated twice with Bacillus mycoides strain A3

(BmA3) at 10

CFU mL

-1

before withholding water for 7 days or

exposing them to 45 ◦C for 20 min, followed by recovery at 23 ◦C for 24

h. BmA3-treated plants under water or thermal stresses showed lower

H₂O₂, MDA, and EL, and greater fresh weights and higher survival rates

after rewatering or 7 days post-recovery from thermal stress. BmA3

enhanced root architecture, increasing lateral roots and root hairs, and

maintained high chlorophyll levels thus boosting photosynthetic ef-

ciency and starch accumulation. Antioxidant defences were strength-

ened via higher activities of CAT, GPX, APX, and PAL, while phenolics,

avonoids, and glucosinolates increased signicantly. Stress-responsive

genes, including DREB2A and HsFA2, were upregulated alongside those

involved in antioxidant enzyme (APX, AOX), JA (MYC2, LOX1), and SA

(SARD1, CBP60 G) pathways, thereby improving adaptation and re-

covery under abiotic stress (Kurniawan and Chuang, 2022).

Root drenching with Bacillus cereus, applied to S. lycopersicum one

week after transplanting and repeated twice at ve-day intervals,

enhanced heat tolerance by stimulating the transcription of SlWRKY33b

and autophagy-related genes (SlATG5), as well as upregulating heat-

responsive transcription factors (SlHsfA1a) and potassium transporters

(SlHKT1). The activation of these pathways supported root development

and mitigated oxidative damage when plants were later exposed to

elevated temperatures (Khan et al., 2020). The application of Bacillus

zanthoxyli HS1, a strain isolated from cucumber rhizosphere soils, and its

volatile organic compounds (VOCs), enhanced heat stress tolerance in

Brassica rapa spp. pekinensis cv. Ryeong-gwang and Cucumis sativus cv.

Jo-eun-baeg-log-da-da-gi. Seedlings were transplanted into soil pre-

treated with B. zanthoxyli HS1 at 1.3 ×10

CFU mL-1 or exposed to its

VOCs before being subjected to 36 ±2 ◦C for two days, followed by

recovery. Bacillus HS1 treatment improved shoot and root growth,

increased chlorophyll content, and reduced proline levels, indicating

reduced stress sensitivity and more efcient stress response. The efcacy

of HS1 was also compared with azelaic acid (AzA), a well-known primer

for systemic acquired resistance (SRA). HS1 proved more effective in

mitigating abiotic stress, while AzA disrupted HS1

′

s protective effects

when co-applied. HS1 also upregulated the expression of key antioxi-

dant genes (APX, CAT, and SOD), promoted callose deposition thus

improving structural defence, modulated stomatal conductance for

water conservation, and reinforced systemic tolerance through

VOC-mediated signalling (Barghi and Jung, 2024). Some microbes, such

as Pseudomonas spp. and Achromobacter piechaudii, identied in

semi-arid regions, have demonstrated remarkable potential in confer-

ring drought and heat tolerance to different crops like T. aestivum

(Shakir et al., 2012), S. lycopersicum (Chen et al., 2014), and Capsicum

annuum (Maxton et al., 2018), Vitis vinifera (Duan et al., 2021) These

PGPR reduce ethylene levels in plants by hydrolyzing its precursor ACC

into ammonia and

-ketobutyrate through the action of ACC deaminase

(ACCD). This enzymatic process not only mitigates the accumulation of

stress-induced ethylene, which can inhibit plant growth and lead to

premature senescence and abscission, but also enables PGPR to utilize

ACC as a carbon and nitrogen source. By reducing ethylene synthesis,

these PGPR improve RWC, photosynthetic pigment concentrations, and

leaf area while lowering proline levels. The reduction in proline reects

alleviated stress and enhanced physiological adaptation. The dual role of

microbial ACCD in reducing the synthesis of ethylene and using its

precursor as a nutrient source makes it a cost-effective and sustainable

strategy for improving plant resilience under abiotic stress. Moreover,

the inducibility of the bacterial AcdS gene, which encodes ACCD, allows

for adaptive regulation under diverse environmental conditions, making

it a valuable tool for enhancing crop performance in stress-prone envi-

ronments. This synergy of microbial and genetic strategies underscores

the pivotal role of ACCD in fostering resilient agricultural systems (Singh

et al., 2022). These ndings align with Induced Systemic Tolerance

(IST), where PGPR triggers physical and biochemical adjustments to

enhance resilience against abiotic stresses. Accordingly, in Sorghum

bicolor Moench, the priming with 77 non-pathogenic rhizobacterial

isolates, applied three weeks after planting as 1 mL of a 10

CFU mL

−1

cell suspension per plant, resulted in signicant metabolomic reprog-

ramming (Carlson et al., 2020). UHPLC

–

HDMS analysis identied key

biomarkers linked to drought stress tolerance, including enhanced

antioxidant capacity, modied root architecture driven by upregulated

synthesis of GA, ABA, and CK, and the early activation of stress signal-

ling pathways involving brassinolides, SA, JA, sphingosine, and psy-

chosine (two lipid molecules involved in cellular signalling and stress

responses). Moreover, osmolytes and epicuticular wax (docosanoic acid)

further contributed to drought tolerance. ACC deaminase activity

reduced ethylene levels, mitigating stress-induced growth inhibition

(Carlson et al., 2020).

5.2.3. Foliar and root treatments with metabolites and protein hydrolysates

Foliar treatments with 50 µmol L⁻¹ of IAA, GA, or BR, and 10 µmol L⁻¹

of CK were applied to Oryza sativa cultivars Fedearroz 67 (F67) and

Fedearroz 2000 (F2000) twice, 5 days before and 5 days after the onset

of heat stress, using 20 mL per plant with 1 mL l

-1

agricultural adjuvant

to ensure leaf coverage. Heat stress (40 ◦C day/30 ◦C night) was imposed

for 8 consecutive days in growth chambers, while control plants were

maintained under optimal conditions (30 ◦C day/25 ◦C night). CK

P. Carillo

Plant Stress 16 (2025) 100802

treatments signicantly enhanced chlorophyll content in heat-stressed

plants F67 (+38%) and F2000 (+43%) compared to untreated stressed

plants, while stomatal conductance in F2000 increased by 232%, indi-

cating better WUE. BR and CK applications reduced canopy tempera-

tures by 2–3 ◦C and oxidative damage, as evidenced by lower MDA

levels. Relative tolerance indices showed that CK (98%) and BR (61%)

were highly effective in maintaining water content, chlorophyll levels,

and overall plant vitality, demonstrating their potential as agronomic

strategies to mitigate heat stress in the two rice cultivars

(Pantoja-Benavides et al., 2021). Also, SA enhanced heat resilience in

grapevine (Vitis vinifera) by stabilising photosynthetic efciency and

accelerating post-stress recovery. Seedlings were foliar-sprayed with SA

0.1 mmol l

-1

24 h before heat stress and exposed to 43 ◦C for 5 h, followed

by a recovery phase at 25 ◦C. Though SA did not enhance Pn under

normal conditions, it attenuated heat-induced Pn and Rubisco activation

declines, accelerating post-stress recovery. It also preserved PSII func-

tion and sustained HSP21 accumulation, reinforcing PSII stability and

thermotolerance (Wang et al., 2010).

In Agrostis stolonifera (creeping bentgrass), exogenous application of

100 nM KAR1 signicantly enhanced drought resilience under

controlled drought conditions (soil water potential maintained at −0.6

MPa for 10 days) by increasing leaf RWC, chlorophyll retention,

photosystem II efciency, proline accumulation and higher membrane

stability. Antioxidant enzyme activities, including APX2, CAT1, POD2,

and Cu/Zn-SOD, were markedly increased, with associated upregulation

of KAR-responsive genes (AFL1, KAI2, and MAX2), TFs (ABF3, DREB2A,

bHLH148, and MYB13), and antioxidant-related genes. Meanwhile,

chlorophyll degradation genes (PPH and Chl-PRX) were downregulated,

delaying senescence under drought conditions (Tan et al., 2023).

Overexpression of the KARRIKINS INSENSITIVE2 (KAI2) gene in

A. thaliana signicantly enhanced drought tolerance by promoting

anthocyanin biosynthesis, improving stomatal regulation, maintaining

membrane integrity, and supporting cuticle formation. Under drought

conditions, KAI2 mutants showed increased water loss, cuticular

permeability, and membrane damage, coupled with reduced anthocy-

anin levels and lower sensitivity to ABA, a key regulator of drought

response. Transcriptomic analyses revealed KAI2 involvement in path-

ways that integrate ABA signalling to optimize plant adaptation. These

insights highlight the potential of targeting KAI2-dependent pathways to

develop crops with improved drought resilience (Li et al., 2017).

Additionally, 1 nM KAR1 supplementation in growth medium to

S. sebiferum seedlings determined improved biomass, longer taproots,

and a higher number of lateral roots, indicating better adaptation to

stress. Metabolomic analysis revealed elevated levels of organic acids

and amino acids, contributing to redox homeostasis. Interestingly, KAR1

modulated the expression of ABA signalling genes, such as those coding

for SNF1-RELATED PROTEIN KINASE2.3 and ABI5, without increasing

endogenous ABA levels, demonstrating its potential role in stress alle-

viation via ABA-independent pathways (Shah et al., 2020).

In Helianthus annuus, 20 days after spraying leaves with 1 mmol l

-1

GABA, plant height and fresh/dry biomass signicantly improved,

counteracting the sharp declines observed in these parameters under

drought and heat stress conditions. GABA alleviated the negative im-

pacts of the two stresses by improving gas exchange parameters such as

stomatal function and transpiration, maintaining chlorophyll levels, and

enhancing PSII efciency (Fv/Fm), and, therefore, photosynthetic per-

formance. GABA enhanced water relations by signicantly increasing

RWC (+25%) and reducing EL, MDA and H₂O₂ content under both stress

conditions, improving membrane stability and oxidative stress toler-

ance. On the molecular level, GABA-treated plants showed upregulation

of genes responsive to stress, including HSP70, DHNs and LEA proteins,

and aquaporins (AQP), which are crucial for osmotic adjustment, water

transport, and cellular protection. SOD, APX and POD activities

increased signicantly in GABA-treated plants, effectively scavenging

ROS and mitigating oxidative damage under stress. Furthermore, GABA

enhanced the accumulation of proline, total soluble sugars, and

-tocopherol (vitamin E), contributing to a 12% increase in seed yield

under drought and heat stress conditions. In chloroplasts,

-tocopherol

functions as a critical antioxidant, mitigating lipid peroxidation and

promoting the stability of cellular components during high-temperature

conditions. It also facilitates the accumulation of 3

′

-phosphoadenosine

′

-phosphate (PAP), which stabilizes microRNAs (miRNAs) involved in

gene regulation under thermal stress (Foyer and Hanke, 2022;

Munn´

e-Bosch, 2019; Rimbach et al., 2010). This highlights GABA’s ef-

cacy as a potent biostimulant for enhancing biochemical resilience and

productivity under combined drought and heat stresses (Abdel Razik

et al., 2021). Indeed, under drought and heat stress conditions, where

stomatal closure restricts CO₂ uptake, GABA may offer signicant

physiological benets by supporting plant metabolism. Carillo (2018)

proposed that GABA can reactivate the Calvin cycle by supplying CO₂

through the decarboxylation of glutamate, serving as an alternative CO₂

source when gas exchange is limited. This mechanism helps plants

dissipate excess energy, preventing photooxidative damage caused by

high light intensity. Also, GABA is a highly efcient osmolyte, protecting

the function and structure of membranes and proteins. Its potent anti-

oxidant activity, exceeding that of proline and GB, further enhances

stress tolerance by mitigating oxidative damage by eliminating ROS like

, singlet oxygen and superoxide radicals. The GABA shunt con-

tributes to stress recovery by supplying NADH and succinate to the

tricarboxylic acid (TCA) cycle, which supports cellular respiration and

energy production. In conjunction with calcium, GABA regulates the

expression of genes encoding 14–3–3 proteins, which are crucial for

nitrogen and carbon metabolism. Additionally, endogenous GABA syn-

thesis through the decarboxylation of glutamate, catalyzed by glutamate

decarboxylase (GAD), helps maintain cellular pH under salinity stress,

consuming protons and buffering cytosolic acidosis (Carillo, 2018).

Collectively, these properties enable GABA to alleviate drought and heat

stress, promoting osmotic balance, metabolic stability, and photosyn-

thetic recovery even under prolonged stress conditions (Carillo, 2018).

Seckin et al. (2009) investigated the impact of exogenous mannitol

application on wheat roots under osmotic and ionic stress. They found

that after applying 100 mmol l

-1

mannitol for 24 h, antioxidant enzyme

activities in the roots were signicantly enhanced, thereby improving

tolerance to drought and salinity stress. Application of 1 nmol l

-1

Thuricin 17 (Th17) through root drenching, a bioactive compound

produced by Bacillus thuringiensis NEB17, showed signicant potential in

enhancing plant resilience to drought in Brassica napus (canola). In fact,

the Th17 treatment increased root length, surface area, and volume by

over 20% under severe drought, while also boosting leaf area and

biomass. under combined heat and drought stress, 10 pmol l

-1

Th17 was

even more effective, reecting stress-specic responses. The application

of Th17 at both concentrations enhanced CAT, POD and SOD antioxi-

dant activities, improved water uptake, and strengthened root systems.

Root drenching, especially when paired with seed treatment, emerged as

a sustainable strategy for mitigating stress-related growth declines,

supporting eco-friendly agricultural resilience under drought and/or

heat stresses (Nazari and Smith, 2023). In Brassica napus, daily appli-

cation of 2 mL of 20 mmol l

-1

glutamate signicantly boosted proline

levels, improving RWC and drought resilience under limited irrigation

(La et al., 2020). Glutamate modulates proline metabolism by down-

regulating proline dehydrogenase (PDH) expression and upregulating

Δ¹-pyrroline-5-carboxylate synthetase (P5CS), leading to elevated pro-

line concentrations in the phloem and xylem, thereby enhancing

drought tolerance (La et al., 2020). Indeed, proline is the most wide-

spread osmolyte, serving as a key component in plant stress responses. It

is amphiphilic, neutral, and capable of detoxifying ROS. It can preserve

the integrity and functionality of proteins and membranes, stabilise the

redox potential of cells, and stimulate specic sequences within the

ProDH promoter region, particularly the Proline Responsive Element

(ACTCAT), thereby enhancing the expression of genes associated with

stress responses (Carillo, 2018). Unlike GB, which accumulates pri-

marily in young tissues, proline is synthesised rapidly in mature tissues

P. Carillo

Plant Stress 16 (2025) 100802

at the onset of stress and degraded after the stress subsides, supplying

energy, carbon, and nitrogen for recovery. This spatial and temporal

discrepancy underscores the unique adaptive strategies associated with

the accumulation of proline under stress (Carillo et al., 2008). In Satureja

hortensis, the foliar application of a PH, containing 43.7% total protein,

50% amino acids, and 20% total nitrogen, was carried out twice at the

onset of the shooting stage, and four weeks later, using a concentration

of 2 mL l

-1

of water. It enhanced proline content, pigment levels, and

yield under drought stress, supporting better growth and quality. The

application of PH signicantly improved dry matter yield, essential oil

content, and essential oil yield, with respective increases of 22%, 31%,

and 57%. The highest oil yields were achieved under conditions of

moderate (75% FC) and severe (50% FC) water decit and PH treat-

ments (Rezaei-Chiyaneh et al., 2023). In Zea mays (maize), hydroponic

applications of plant PHs containing 12% amino acids signicantly

enhanced root and shoot development and mitigated heat and drought

stress effects (Vaseva et al., 2022). Similarly, C. annuum treated with PHs

through fertigation (1.5 ml L⁻¹) demonstrated improved growth and

resilience by maintaining chlorophyll content, enhancing antioxidant

defences, and stabilizing biomass under stress conditions (Agliassa et al.,

2021). In S. lycopersicum, weekly foliar sprays of PHs derived from

Malvaceae (PH1) or Fabaceae (PH2) species (3 ml L⁻¹) increased levels of

dipeptides such as Arg-Leu and PyroGlu-Val, which played crucial roles

in activating defence mechanisms, improving WUE, and maintaining

growth under heat and drought stress (Leporino et al., 2024). Func-

tioning as metabolic regulators, these dipeptides inuence enzymatic

activities crucial for carbon ow and nitrogen metabolism during pe-

riods of stress. Arg-Leu may play a signicant role in nitrogen redistri-

bution and osmolyte production, fortifying the plant’s resilience to

abiotic challenges. Similarly, PyroGlu-Val is can be associated with

enhanced oxidative stress management through its ability to neutralize

ROS. Under stress conditions, these dipeptides support alternative car-

bon metabolism pathways, a vital mechanism when photosynthetic ef-

ciency is impaired, while also activating adaptive signalling processes

that sustain cellular stability and metabolic function. Additionally,

biostimulant treatments with PH1 and PH2 appear to induce metabolic

adjustments, evidenced by a reduction in the production of certain

stress-related detoxifying agents such as (R)-S-lactoylglutathione, which

is involved in methylglyoxal detoxication (Hoque et al., 2016). This

shift suggests that dipeptides, together with other antioxidant molecules

like phenols, may take over detoxication roles, enabling the plant to

allocate resources more effectively towards growth and antioxidant

defence systems (Leporino et al., 2024).

Foliar spraying of biostimulant preparations has been proven even

more effective when combined with seed priming across various crops.

Together, these two treatments signicantly boost physiological resil-

ience, nutrient biofortication, and yield quality, offering scalable

strategies for managing drought. In rainfed wheat, seed priming with a

nutrient solution containing urea (20 g L

_−1

), FeSO₄ (50 mg l

-1

), ZnSO₄

(50 mg l

-1

), and silicon (20 mg l

-1

) signicantly improved root length,

volume, and dry mass. Foliar applications at the anthesis stage with urea

(40 g l

-1

), silicon (112 mg l

-1

), FeSO₄ (6 g l

-1

), and ZnSO₄ (4 g l

-1

) further

enhanced physiological traits, including chlorophyll content and carot-

enoids, while reducing MDA and oxidative damage. Zn and Fe bio-

fortication increased grain yield by 29.17% and 19.51%, respectively,

and protein content from 11.14% to 12.46% (Moradi and Siosemardeh,

2023). In linseed, priming with β-aminobutyric acid (BABA) at doses of

50–100 µmol l

-1

and foliar spraying under severe drought (25% soil

moisture) enhanced antioxidant enzyme activity (e.g., CAT and POX)

and proline accumulation while reducing oxidative stress. Therefore,

BABA treatment maintained antioxidant defence and drought tolerance,

allowing an increase in root and shoot dry weights and improving yield

components such as capsule numbers, seeds per capsule, and seed

weight, ensuring productivity even under severe water stress (Yasir

et al., 2023).

5.3. Comparative effectiveness, application challenges, and risks

Biostimulants have been widely recognised for their role in

enhancing plant resilience to heat and drought stress, as extensively

discussed in the previous sections. However, while their efcacy is well-

established, a deeper understanding of their common mechanisms of

action, limitations, and potential risks is necessary for optimising their

application in real-world agricultural settings. This section synthesises

the comparative effectiveness of different biostimulants, highlights

shared physiological and molecular mechanisms and critically examines

their use’s challenges and potential risks.

5.3.1. Comparative effectiveness of biostimulants in mitigating heat and

drought stress

Different classes of biostimulants (e.g., algal extracts, microbial in-

oculants, PHs, and plant-derived metabolites) have been extensively

described for their role in mitigating heat and drought stress. Despite

differences in composition and mode of action, biostimulants share

several key physiological and molecular mechanisms that enhance plant

tolerance to heat and drought stress (Table 3). One of the most universal

mechanisms is regulating osmotic balance by accumulating compatible

solutes, such as proline, GB, and soluble sugars, which maintain cell

turgor and protect macromolecules under water decit conditions. Algal

extracts and protein hydrolysates (PHs) are particularly effective in this

regard, as they promote osmolyte synthesis, improve RWC, and stabilise

photosynthetic activity during stress. For instance, wheat seeds primed

with Spirulina platensis extract exhibited increased photosynthetic ef-

ciency and higher spike number under drought stress (Elnajar et al.,

2024). Similarly, PHs derived from legumes can improve stomatal

regulation and enhance leaf RWC in tomato and spinach (Leporino et al.,

2024). Another widely observed mechanism is enhancing antioxidant

defence systems to mitigate oxidative stress caused by heat and drought.

ROS accumulate rapidly under these conditions, leading to cellular

damage and impaired physiological functions. Microbial inoculants,

especially AMF and PGPR, have demonstrated the strongest effect in

enhancing ROS detoxication through increased activity of antioxidant

enzymes. Inoculation with Rhizophagus irregularis improved drought

tolerance in lettuce and tomato by enhancing shoot biomass (in lettuce

and tomato) and upregulating drought-responsive genes involved in

antioxidant defence (Ruiz-Lozano et al., 2016). Algal extracts, such as

Ulva lactuca, have also been reported to increase antioxidant enzyme

activities in Allium cepa, contributing to higher chlorophyll content and

root development under water stress (Muhie et al., 2020). Hormonal

regulation also plays a central role in biostimulant-mediated stress

tolerance. AMF and PGPR signicantly inuence ABA and SL biosyn-

thesis, which, in turn, regulate stomatal closure, root growth, and

drought-adaptive metabolic shifts. For example, in tomato plants treated

with R. irregularis, ABA biosynthesis genes (SlNCED2) and SL biosyn-

thesis genes (SlCCD7) were upregulated, promoting improved drought

resilience (Ruiz-Lozano et al., 2016). Conversely, PHs and plant extracts

primarily modulate JA and SA pathways, supporting cell membrane

stabilisation and osmolyte accumulation (Han et al., 2024). These

hormone-mediated responses contribute to better WUE, increased

thermotolerance, and overall stress adaptation. At the molecular level,

biostimulants inuence the expression of stress-responsive genes,

though their efcacy varies. PHs and algal extracts have been shown to

upregulate HSPs, DHNs, and LEA proteins, which protect cellular

structures from heat-induced damage (Gonz´

alez-Morales et al., 2021).

Treatments with Bacillus cereus in tomato plants induced the transcrip-

tion of heat-responsive TFs (SlHsfA1a) and autophagy-related genes

(SlATG5), leading to enhanced thermotolerance and root development

(Khan et al., 2020).

5.3.2.Application. challenges and practical limitations

While biostimulants demonstrate promising stress-mitigating effects

in controlled studies, several factors limit their effectiveness in eld

P. Carillo

Plant Stress 16 (2025) 100802

conditions. The Application method is critical in determining efcacy,

yet each technique has inherent constraints.

Seed priming is widely recognised for enhancing germination and

early stress tolerance by preconditioning seeds to activate protective

physiological mechanisms. However, its long-term effectiveness is

inconsistent, as bioactive compounds may degrade over time or be

insufciently retained in plant tissues, limiting sustained stress protec-

tion (Herrmann et al., 2024). The efcacy of priming is highly

species-dependent, requiring tailored protocols to optimise benets and

minimise potential adverse effects. For instance, variations in response

have been observed even among different cultivars of the same species,

like Brassica napus, emphasising the need for genotype-specic optimi-

zation of priming treatments (Zhu et al., 2021). Additionally, storage

conditions play a crucial role in maintaining priming-induced benets.

Studies highlight that primed seeds stored at 25 ◦C show reduced

viability and seedling vigour over time, underscoring the importance of

controlled storage environments to preserve priming efcacy (Hussain

et al., 2015). Another critical factor is the potential phytotoxicity of

some priming agents, particularly at high concentrations. For example,

the application of silver nanoparticles for seed priming has been re-

ported to inhibit germination and seedling growth in some plant species,

highlighting the need for careful concentration adjustments to prevent

toxicity (Budhani et al., 2019). Emerging technologies, such as

nano-priming, offer promising alternatives by improving the bioavail-

ability of priming agents while maintaining seed viability and stress

resilience. These approaches integrate nanotechnology-based carriers to

enhance the retention and controlled release of active compounds,

potentially overcoming the limitations of traditional priming methods

(Gohari et al., 2024). Despite these advancements, further research is

needed to rene priming techniques for large-scale agricultural appli-

cations, ensuring consistency across different environmental conditions

and crop species. Standardizing priming protocols, optimizing storage

conditions, and evaluating long-term eld performance remain crucial

steps for effectively integrating seed priming into climate-resilient

cropping systems.

As previously seen, foliar spraying is another widely adopted tech-

nique for delivering biostimulants directly to plant tissues, providing

rapid physiological benets by modulating stomatal regulation, anti-

oxidant defences, and hormonal signalling. However, its efcacy de-

pends on environmental conditions, formulation properties, and plant

surface characteristics, leading to variable outcomes in eld applica-

tions. Temperature, humidity, and wind speed signicantly inuence

foliar absorption. Under high temperatures and low humidity, rapid

evaporation reduces the retention time of sprayed solutions, limiting

their uptake by leaf tissues. At the same time, excessive rainfall may

wash off biostimulants before absorption occurs, resulting in inconsis-

tent effectiveness in open-eld conditions (Fern´

andez et al., 2021). The

physicochemical properties of foliar-applied biostimulants, including

molecular weight, polarity, and solubility, also play a key role in

determining absorption efciency. Low-molecular-weight compounds

like amino acids and phytohormones penetrate the cuticle more ef-

ciently than larger macromolecules like polysaccharides and proteins

(Sch¨

onherr, 2006). Additionally, leaf surface characteristics, including

cuticle thickness, trichome density, and wax composition, further in-

uence uptake rates. Younger leaves, with a thinner cuticle and higher

metabolic activity, generally absorb foliar-applied compounds more

effectively than mature leaves with a well-developed wax layer

(Fern´

andez et al., 2017). These factors contribute to variability in bio-

stimulant efcacy across different plant species and growth stages.

Advanced formulation strategies have been explored to address these

limitations to enhance foliar uptake and retention. Using nanocarriers,

such as lipid-based or polymeric nanoparticles, improves adhesion and

Table 3

Comparative effects of different classes of biostimulants in enhancing plant resilience to heat and drought stress.

Biostimulant Type Crop Species Mechanism of Action Physiological and Molecular Adaptations References

Algal Extracts (Ascophyllum

nodosum, Ulva lactuca, Spirulina

spp.)

Triticum aestivum (bread

wheat)

Enhanced osmotic adjustment via

increased GB and proline

accumulation

Improved photosynthetic efciency, increased

spike number and economic yield under

drought stress

Elnajar et al., 2024

Oryza sativa (rice) Increased antioxidant defense and

root elongation

Improved germination and root growth under

salinity and heat stress

Sornchai et al., 2014

Medicago truncatula

(alfalfa)

Modulation of auxin and cytokinin

homeostasis

Increased biomass, enhanced root-microbe

interactions, better drought resilience

Gitau et al., 2021

Microbial Biostimulants

(Rhizophagus irregularis, Bacillus

spp., Enterobacter spp.)

Lactuca sativa (lettuce),

Solanum lycopersicum

(tomato)

Arbuscular mycorrhizal (AM)

symbiosis, upregulation of ABA

biosynthesis genes

Increased AM colonization and improved

shoot dry weight (both in lettuce and tomato)

Ruiz-Lozano et al.,

2016

Triticum aestivum (bread

wheat)

Induction of ROS detoxication via

antioxidant enzyme activity

Increased proline levels, improved

antioxidant defenses, enhanced drought

tolerance

Shafque et al., 2023

Hordeum vulgare (barley) Enhanced root development,

nitrogen assimilation

Increased chlorophyll content, improved root

growth under drought

Suarez et al., 2015

Protein Hydrolysates (PHs) Zea mays (maize) Activation of osmoprotectant

pathways, improved nutrient

uptake

Increased root and shoot development,

improved drought resilience

Vaseva et al., 2022

Capsicum annuum

(pepper)

Stimulation of antioxidant enzyme

activity

Increased chlorophyll content, improved

biomass retention under drought stress

Agliassa et al., 2021

Solanum lycopersicum

(tomato)

Regulation of stomatal

conductance, upregulation of WUE-

related genes

Activation of defense-related dipeptides,

enhanced resilience to heat and drought

stress

Leporino et al., 2024

γ-Aminobutyric Acid (GABA) Helianthus annuus

(sunower)

Enhanced membrane stability,

reduced oxidative stress

Increased RWC, decreased electrolyte leakage,

MDA, and H₂O₂ levels, improved drought and

heat stress tolerance

Abdel Razik et al.,

2021

Silicon-Based Biostimulants Solanum lycopersicum

(tomato)

Improved water-use efciency,

stabilization of photosynthesis

Increased fruit yield and irrigation water

productivity under moderate drought stress

Chakma et al., 2021

Karrikins (KARs) Agrostis stolonifera

(creeping bentgrass)

Regulation of drought-induced

stress genes

Increased chlorophyll retention, improved

PSII efciency

Tan et al., 2023

Oryza sativa (rice) Enhancement of ABA-independent

pathways, improved stomatal

regulation

Delayed senescence, better water

conservation under heat stress

Pantoja-Benavides

et al., 2021

Polyamine-Based Biostimulants Triticum aestivum (bread

wheat)

Increased osmolyte accumulation,

regulation of stress-responsive

genes

Improved photosynthesis, reduced oxidative

damage, enhanced drought tolerance

Chaudhary et al., 2013

P. Carillo

Plant Stress 16 (2025) 100802

enables the controlled release of biostimulants, reducing losses due to

wash-off and evaporation (Kisvarga et al., 2022; Singh et al., 2024).

Surfactants and adjuvants are commonly incorporated to lower surface

tension, promote uniform droplet spreading, and increase contact time

on the leaf surface, thus enhancing penetration into the cuticle. How-

ever, the compatibility of these additives with different biostimulant

compounds must be carefully assessed to prevent potential phytotoxicity

or adverse chemical interactions (Silva et al., 2024). Moreover, despite

the advantages of foliar application, certain limitations must be

considered, particularly regarding dosage optimisation and systemic

translocation. Unlike soil-applied biostimulants, which can be absorbed

through roots and transported via the xylem, foliar-applied compounds

primarily exert localized effects, with limited movement to untreated

plant organs. Consequently, repeated applications throughout the

growth cycle are often required to sustain benecial effects, increasing

labour and operational costs, particularly in large-scale agricultural

systems (Niu et al., 2021; ´

Swierczy´

nski and Bosiacki, 2022). Future

research should optimise biostimulant formulations to enhance foliar

absorption while minimizing environmental losses. In addition, stan-

dardized protocols for foliar application, considering environmental

variability and plant-specic responses, will be essential for maximizing

the effectiveness of this biostimulant delivery approach.

Organic matter plays a crucial role in enhancing microbial activity,

improving soil structure, and increasing nutrient retention, thereby

promoting the sustained release and efcacy of biostimulants (du Jar-

din, 2015). Soil drenching is widely used to deliver microbial inoculants.

However, it is also an effective method for applying non-microbial

biostimulants, including algal extracts, protein hydrolysates, and

plant-derived metabolites, ensuring direct root uptake and systemic ef-

fects. However, its success depends on multiple soil properties, such as

pH, organic matter content, and moisture availability, inuencing bio-

stimulant stability and bioavailability (Rouphael and Colla, 2020). Mi-

crobial inoculants applied through soil drenching often face competition

from native soil microbiota, which can reduce colonisation success and

persistence. Additionally, abiotic factors such as temperature uctua-

tions, desiccation, and soil salinity can negatively impact microbial

viability and activity (O’Callaghan et al., 2022). For non-microbial

biostimulants, soil composition plays a critical role in determining ef-

cacy. Sandy soils with low water retention may lead to rapid leaching

of applied compounds, reducing their residence time in the rhizosphere

(Swartz and Motis, 2021). Conversely, clay-rich soils with high cation

exchange capacity can retain biostimulants for extended periods, but

this may limit their immediate bioavailability to plant roots (Fageria and

Moreira, 2011). Various formulation and application strategies have

been explored to overcome these challenges and improve biostimulant

efcacy in soil-drenching applications. For microbial inoculants, using

protective carriers, co-inoculation with compatible microbial consortia,

and optimising application timing can enhance survival and activity in

eld conditions (Aloo et al., 2022; Basu et al., 2021). For non-microbial

biostimulants, the incorporation of organic amendments such as bio-

char, compost, and humic substances has been shown to improve

retention, bioavailability, and nutrient-use efciency, supporting plant

growth under stress conditions (Canellas et al., 2015). These approaches

contribute to developing more sustainable and effective soil bio-

stimulant application strategies in both controlled and open-eld envi-

ronments.An additional signicant limitation to the widespread

adoption of biostimulants is the evaluation of their cost-effectiveness.

While these products offer agronomic benets, such as enhancing

plant growth and resilience to abiotic stresses, their economic justi-

cation is often more apparent in high-value crops like vegetables, fruits,

and vineyards, where higher prot margins can offset the investment.

For instance, a meta-analysis reported an average yield increase of

17.9% across various biostimulant categories, with the highest impact

observed in vegetable cultivation (e.g., greenhouse horticulture) (Li

et al., 2022). However, for extensive crops typical of Mediterranean

regions, such as cereals and legumes, the economic sustainability of

biostimulant use is less clear. The cost of these products and the actual

increase in productivity are central elements in adoption decisions.

Moreover, the variability of results in open-eld conditions, inuenced

by factors such as soil quality and climatic conditions, makes it chal-

lenging to quantify the direct benets derived from biostimulant use

precisely (Meena et al., 2025). Therefore, it is essential for farmers to

carefully assess the cost-benet ratio of biostimulants within their spe-

cic operational contexts. Further research and eld trials are necessary

to provide concrete data on the effectiveness and protability of these

products across diverse Mediterranean agricultural settings (Li et al.,

2022). In addition, exploring the interactive effects and potential syn-

ergies of biostimulants and biopesticides could offer a more integrated

approach to productivity and sustainability, particularly for organic

farming. A recent study currently underway at the University of Naples

(https://www.bbhort.unina.it/) is investigating these synergistic ef-

fects, aiming to enhance crop resilience, reduce chemical input de-

pendency, and promote long-term agricultural sustainability.

5.3.3.Biostimulants’. potential risks and regulatory considerations

Despite their largely benecial effects, the widespread adoption of

biostimulants raises important ecological and regulatory concerns,

particularly regarding microbial inoculants. One of the primary risks is

the potential for unintended environmental consequences, such as dis-

ruptions in native microbial communities and alterations in soil nutrient

dynamics. Introducing non-native microbial strains may outcompete

indigenous populations, affecting plant-microbe interactions and

potentially leading to imbalances in ecosystem function

(Barros-Rodríguez et al., 2020). Another key concern is contamination

with opportunistic pathogens, which could pose risks to plant health

and, in some cases, human safety. The large-scale application of mi-

crobial biostimulants necessitates rigorous quality control measures to

prevent contamination and ensure strain purity (Kumari et al., 2022).

The European Regulation (EU) 2019/1009 (EU, 2019) has established

safety guidelines for microbial biostimulants, but enforcement and

standardization remain challenging across different agricultural regions.

Additionally, concerns have been raised regarding horizontal gene

transfer between introduced and native microbial populations, which

could lead to unforeseen genetic modications with potential ecological

implications (Moore et al., 2022). Beyond microbial biostimulants,

biochemical-based products such as protein hydrolysates (PHs), algal

extracts, and plant-derived metabolites are generally considered safe,

with minimal direct toxicity to humans and the environment (Gupta and

van Staden, 2021). These biostimulants are primarily composed of

naturally occurring compounds, including amino acids, peptides, phy-

tohormones, and polysaccharides, which are biodegradable and typi-

cally do not persist in the environment at harmful levels (du Jardin,

2015). However, their safety prole depends on formulation properties,

concentration, and application methods. Biochemical-based bio-

stimulants, such as PHs and algal extracts, are generally considered safe

for plants, humans, and the environment. However, concerns regarding

potential contamination and unintended toxic effects have been raised.

One major concern is the overaccumulation of specic bioactive com-

pounds, such as auxins, cytokinins, or abscisic acid analogues, which at

high concentrations, may interfere with endogenous hormonal regula-

tion in plants. For example, excessive foliar application of auxin-rich

seaweed extracts has been associated with abnormal root elongation,

altered leaf morphology, and delayed owering in certain crop species

(Khan et al., 2009). Similarly, PHs rich in free amino acids can disrupt

nitrogen metabolism if applied at inappropriate doses, leading to

excessive vegetative growth at the expense of reproductive development

(Colla et al., 2017). Algal-based biostimulants may contain bioactive

secondary metabolites that could inuence non-target organisms,

including benecial insects and soil microfauna, though these effects

remain poorly characterised (Kapoore et al., 2021). Certain marine algae

can bioaccumulate heavy metals from their environment, which can

introduce contaminants into agricultural systems if not properly sourced

P. Carillo

Plant Stress 16 (2025) 100802

and processed. For instance, macroalgae such as Macrocystis pyrifera,

commonly used in commercial biostimulants, can accumulate metals

depending on the water quality of their habitat. Therefore, ensuring that

algal biomass is harvested from non-contaminated waters and un-

dergoes rigorous quality control is essential to prevent the introduction

of heavy metals into the soil and plant tissues (L¨

ahteenm¨

aki-Uutela

et al., 2021). In addition, the extraction process is critical in determining

the purity and safety of biostimulant formulations. Cold extraction

methods, such as Cold State Cell Disruption (CSCD), which do not

involve chemical solvents, can preserve the integrity of bioactive mol-

ecules while minimising the risk of contamination with heavy metals or

toxic by-products (Szparaga et al., 2023). Environmental conditions also

inuence the potential for phytotoxicity of biostimulants. Under high

temperatures or drought stress, foliar-applied biostimulants may cause

cuticular damage, leaf scorching, or osmotic imbalances, mainly if sur-

factants or adjuvants are included to enhance penetration (Silva et al.,

2024). Additionally, biostimulants that modify soil microbial activity,

such as polysaccharide-based formulations or humic substances, could

alter soil pH or disrupt benecial microbial communities if applied in

excess (Povero et al., 2016). Repeated applications of carbon-rich bio-

stimulants may further shift microbial populations, sometimes creating

favorable conditions for opportunistic or pathogenic microorganisms

(Pascale et al., 2019). To minimize potential risks, future research

should prioritise optimizing formulations, dening crop- and

environment-specic dosage guidelines, and strengthening regulatory

frameworks. Comprehensive toxicity assessments, including ecotoxico-

logical evaluations of soil and aquatic ecosystems, will ensure the safe

and sustainable integration of non-microbial biostimulants into agri-

cultural systems.

6. Conclusions

Biostimulants are increasingly recognized as a valuable strategy for

enhancing plant resilience to heat and drought stress, particularly in

Mediterranean cropping systems where climate variability poses sig-

nicant challenges. Their ability to improve water use efciency, miti-

gate oxidative damage, and support key physiological and biochemical

processes highlights their potential in sustainable agriculture. However,

their efcacy depends on multiple factors, including crop species,

environmental conditions, and application techniques, making their

optimization essential for broader adoption. Further research is needed

to rene evaluation methods, ensuring a better understanding of their

long-term effects on plant metabolism, soil microbial communities, and

overall ecosystem balance. Investigating the combined impact of bio-

stimulants under multiple stress conditions could reveal synergistic

benets while improving efciency and reducing input requirements.

Economic considerations remain central, requiring the development of

cost-effective solutions tailored to different agricultural systems. Addi-

tionally, strengthening regulatory frameworks will be key to ensuring

product reliability and environmental safety, fostering condence in

their practical use. By addressing these challenges, biostimulants can be

more effectively integrated into climate-resilient farming strategies,

contributing to sustainable food production while mitigating the

adverse effects of an increasingly unpredictable climate.

Funding

This work was supported by the projects BBHORT (PRIN 2022 PNRR

– grant P2022P52XK) and GREENHORT (PRIN 2022 - grant

2022WHTNZT).

CRediT authorship contribution statement

Petronia Carillo: Writing – review & editing, Writing – original

draft, Visualization, Validation, Supervision, Funding acquisition,

Conceptualization.

Declaration of competing interest

The author declares that she has no known competing nancial in-

terests or personal relationships that could have appeared to inuence

the work reported in this paper.

Data availability

No data was used for the research described in the article.

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Jan 2025
ANN BOT-LONDON

Background Plant senescence is a genetically controlled process that results in the programmed death of plant cells, organs, or the entire plant. This process is essential for nutrient recycling and supports the production of plant offspring. Environmental stresses such as drought and heat can hasten senescence, reducing photosynthetic efficiency and significantly affecting crop quality and yield. Scope This mini-review seeks to clarify the complex metabolic and biochemical transformations involved in plant senescence by explaining the mechanisms in a straightforward and connected manner. It focuses on key cellular processes such as genetically programmed or stress-induced senescence, chlorophyll metabolism and nutrient recycling, while also exploring the roles of signaling molecules and pathways. Conclusions Understanding the complexities of plant senescence may help manage crop aging, address climate change, and cut post-harvest losses. Enhancing crop resilience to stress and decelerating aging can reduce the need for overproduction, thereby decreasing pollution and conserving resources. Tackling food waste, which constitutes about one-third of global supplies, is crucial for ensuring food security and fostering environmental sustainability.

Biostimulants in Agriculture: Editorial

Article

Full-text available

Jan 2025
PHYSIOL PLANTARUM

Biostimulants are becoming an important tool in agriculture, helping to improve crop growth, resilience, and quality while supporting sustainable farming practices. These products, made from natural substances or microorganisms, enhance plants' natural processes. They can improve how plants use nutrients, handle stressful conditions like drought or salinity, and boost overall crop productivity and quality. The European Regulation (EU) 2019/1009 defines biostimulants as products “the function of which is to stimulate plant nutrition processes independently of the product's nutrient content with the sole aim of improving one or more of the following characteristics of the plant or the plant rhizosphere: (a) nutrient use efficiency; (b) tolerance to abiotic stress; (c) quality traits; or (d) availability of confined nutrients in the soil or rhizosphere”. This regulation has helped clarify what biostimulants are while encouraging their disciplined use in agriculture (EU 2019). Although biostimulants have shown great potential, there is still much to learn about how they work and how to maximise their benefits. Their effectiveness depends on crop type, dose, and timing, with a particular focus on crop responses under suboptimal conditions. Biostimulants have great potential to address cropping challenges such as climate change, resource scarcity, and the need for more sustainable farming methods. They offer a practical approach to increasing food production despite abiotic constraints. This Special Issue of Physiologia Plantarum “Biostimulants in Agriculture” includes seventeen studies that explore how biostimulants can help solve some of agriculture's biggest challenges. These studies investigate how biostimulants improve nutrient use efficiency, enhance plant stress tolerance, boost growth and increase quality traits. They also explore new formulations, like nanoparticles, and innovative concepts, such as glycostimulation, which uses various glycomolecules to enhance plant growth.

Biostimulants for sustainable development of agriculture: a bibliometric content analysis

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Full-text available

Jan 2025

This paper presents a bibliometric content analysis of the research on biostimulants to examine their roles in improving the sustainability of agriculture. The research on biostimulants is developing and has been attracting significant attention. It has focused on understanding the functional mechanisms of biostimulants and their impact on various indicators of sustainability. It suggests that biostimulants improve crop yield and product quality, reduce external application of fertilizers, enhance water-use efficiency, and improve crops’ capability to tolerate abiotic stresses. Given the growing negative externalities of intensive agriculture to natural resources, human health, and the environment and the predictions of climate becoming more severe, there is a need to focus more on basic and applied research on biostimulants.

Integrative approaches to enhance reproductive resilience of crops for climate-proof agriculture

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Dec 2024

A Two‐Year Evaluation of Biostimulant Effects on Yield and Quality Parameters of Tomato Landrace ‘Pizzutello Delle Valli Ericine’ Cultivated Without Irrigation

Article

Full-text available

Dec 2024

The use of biostimulants in agriculture provides a sustainable and efficient technology to improve resource‐use efficiency. Biostimulants may boost vegetative growth, enhancing plant tolerance to biotic and abiotic stress. The tomato (Solanum lycopersicum L.) is sensitive to drought stress, particularly during fruit setting and fruit development stages. In Italy, long‐storage tomato genotypes characterised by drought resistance were selected. In this 2‐year study, the foliar application of different biostimulants (betaine, seaweed extracts, vegetal protein hydrolysate and animal protein hydrolysate) was evaluated to determine effects on yield and quality of a local tomato landrace (Pizzutello) cultivated in Sicily without irrigation. The highest dry matter (9.9%) and solid soluble content (6.9° Brix) were observed in plants treated with betaine. Plants treated with A. nodosum or animal protein hydrolysate showed the highest potassium concentrations, whereas those supplied with vegetal protein hydrolysate had the highest calcium concentrations. Tomato treated with betaine were found to have the highest nitrate concentrations. The highest marketable yield (13.8 t ha⁻¹) was recorded in plants treated with vegetal protein hydrolysate, with an increase of 17.4% compared to the control plants. The highest unmarketable yield was observed in control plants and in those treated with betaine (1.1 t ha‐1). In conclusion, we can say that each biostimulant had a different effect on the different parameters analysed. Overall, the application of biostimulants has improved tomato growth, productivity and quality in limited water conditions. Our results highlight the potential of biostimulant applications to optimise both the yield and fruit quality of renowned local varieties. This study demonstrated the improvement in the agronomic performance of the Pizzutello tomato, which is particularly significant not only in response to the growing consumer demand for high‐quality traditional tomatoes, but also for the enhancement of the technological traits valued by the food industry.

Identification of climate change hotspots in the Mediterranean

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Full-text available

Nov 2024

The Mediterranean region has long been identified as a climate change hotspot. However, within the Mediterranean, there are smaller sub-areas that exhibit a higher risk of climate change and extremes. Previous research has often focused on indices based on mean climate values, yet extremes are typically more impactful on humans and ecosystems. This study aims to identify the most vulnerable sub-areas of the Mediterranean as climate change hotspots using two indices: the newly introduced Mediterranean Hotspot Index (MED-HOT) and the well-defined Regional Climate Change Index (RCCI). The MED-HOT focuses on extreme high maximum and minimum temperatures, rainfall, and drought, while RCCI assesses changes in mean climate conditions. By combining these indices, we provide an identification of Mediterranean hotspots, capturing both mean climate shifts and extremes. The spatiotemporal variation of both indices across the Mediterranean region is presented and the 20 subregions are categorized into distinct groups. The results reveal that the southeastern Mediterranean is at high risk according to both indices. Additionally, southern Italy is identified as high risk due to changes in mean climate (RCCI), while the northern part is at risk due to extreme events (MED-HOT). The Iberian Peninsula and Greece are also highlighted as vulnerable areas requiring extra attention.

Impact of No Tillage and Low Emission N Fertilization on Durum Wheat Sustainability, Profitability and Quality

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Full-text available

Nov 2024

Mitigation practices for cereal systems, including conservation agriculture and low emission fertilization, are required to face global challenges of food security and climate change. The combination of these climate-smart approaches was investigated for durum wheat in a dry region of the Mediterranean basin in two crop seasons. The experimental design consisted in two different genotypes, Marco Aurelio (high protein content) and Saragolla (higher adaptability), subjected to no tillage (NT) vs. conventional tillage (CT) and to two fertilization strategies (standard vs. low emission plus an unfertilized control). Different environmental and economic sustainability parameters as well as two different technological and nutritional quality traits were evaluated. Saragolla showed a better environmental adaptability and a higher nitrogen use efficiency, evaluated as partial nutrient balance (+27%), and was associated with a lower protein content (14.5% vs. 15.6%). NT was associated with an improvement in yield (+15%) and quality, i.e., micronutrients (Fe, Zn) and antioxidant capacity (+15%), in the drier crop year. Low emission fertilization did not reduce crop performance and its combination with NT showed a higher economic net return. The combination of the two mitigation practices improved not only environmental and economic sustainability but also the health quality of durum wheat under water limited conditions.

Limitations of soil-applied non-microbial and microbial biostimulants in enhancing soil P turnover and recycled P fertilizer utilization - a study with and without plants

Article

Full-text available

Nov 2024

Introduction Phosphorus recovery from waste streams is a global concern due to open nutrient cycles. However, the reliability and efficiency of recycled P fertilizers are often low. Biostimulants (BS), as a potential enhancer of P availability in soil, could help to overcome current barriers using recycled P fertilizers. For this, a deeper understanding of the influence of BSs on soil P turnover and the interaction of BSs with plants is needed. Methods We conducted an incubation and a pot trial with maize in which we testednon-microbial (humic acids and plant extracts) and microbial BSs (microbial consortia) in combination with two recycled fertilizers for their impact on soil P turnover, plant available P, and plant growth. Results and discussion BSs could not stimulate P turnover processes (phosphatase activity, microbial biomass P) and had a minor impact on calcium acetate-lactate extractable P (CAL-P) in the incubation trial. Even though stimulation of microbial P turnover by the microbial consortium and humic acids in combination with the sewage sludge ash could be identified in the plant trial with maize, this was not reflected in the plant performance and soil P turnover processes. Concerning the recycled P fertilizers, the CAL-P content in soil was not a reliable predictor of plant performance with both products resulting in competitive plant growth and P uptake. While this study questions the reliability of BSs, it also highlights the necessity toimprove our understanding and distinguish the mechanisms of P mobilization in soil and the stimulation of plant P acquisition to optimize future usage.

Effects of acclimation to long-term shading on photosynthesis in grapevines: roles of non-structural carbohydrates and stomatal conductance

Article

Nov 2024
PHYSIOL PLANTARUM

Plant acclimation to varying light environments can enhance productivity. However, limited research exists on how plants re‐acclimate to sunlight after shading, despite the increasing use of transient crop shading techniques to mitigate climate change impacts. This study focused on grapevine, a species highly responsive to shade, to explore the effects of prolonged shading on photosynthetic re‐acclimation of plants upon sun re‐exposure. An experiment was conducted using pot‐grown plants subjected to different shading durations and then transferred to sunlight conditions. Results varied depending on whether leaves developed under sun or shade and on the duration of shading. Under shade, stomatal conductance decreased by up to 50%, while photosynthesis was reduced by 30–60%, with longer shading periods causing a greater decrease. Upon sun re‐exposure of shaded plants, sun‐developed leaves gradually reached 100% of the stomatal conductance of sun‐control plants, but their photosynthesis remained at 70%. Despite reduced photosynthesis, analyses of non‐structural carbohydrate (NSC) contents revealed that carbohydrate pools were maintained in shaded leaves, likely due to reduced carbon demand by plant sinks under shade. The maintenance of NSC limited optimal photosynthetic recovery upon sunlight re‐exposure. In contrast, shade‐developed leaves did not increase photosynthesis upon transfer to sunlight due to irreversible morphological changes. This study provides new insights into the impact of shade acclimation on biochemical, physiological and morphological processes when plants are subjected to changing light environments.

The dual role of methylglyoxal in plant stress response and regulation of DJ-1 protein

Article

Nov 2024
PHYSIOL PLANTARUM

Methylglyoxal (MG) is a highly reactive metabolic intermediate that plays important roles in plant salt stress response. This review explores the sources of MG in plants, how salt stress promotes MG production, and the dual role of MG under salt stress conditions. Both the positive role of low concentrations of MG as a signalling molecule and the toxic effects of high concentrations of MG in plant response to salt stress are discussed. The MG detoxification pathways, especially the glyoxalase system, are described in detail. Special attention is given to the novel role of the DJ‐1 protein in the glyoxalase system as glyoxalase III to remove MG, and as a deglycase to decrease glycation damage caused by MG on DNA, proteins, and other biomolecules. This review aims to provide readers with comprehensive perspectives on the functions of MG in plant salt stress response, the roles of the DJ‐1 protein in MG detoxification and repair of glycation‐damaged molecules, as well as the broader functional implications of MG in plant salt stress tolerance. New perspectives on maintaining plant genome stability, breeding for salt‐tolerant crop varieties, and improving crop quality are discussed.

Can biostimulants enhance plant resilience to heat and water stress in the Mediterranean hotspot?

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