ArticlePDF AvailableLiterature Review

Abstract

Environmental pressure poses a major challenge to the agricultural sector, which requires the development of cultivation techniques that can effectively reduce the impact of abiotic stress affecting crop yield and quality (e.g., thermal stress, wind, and hail) and of biotic factors, such as insect pests. The increased consumer interest in premium-quality vegetables requires the implementation of sustainable integrated pest management (IPM) strategies towards an ever-increasing insect pressure, also boosted by cultivation under protected structures. In this respect, insect nets represent an excellent, eco-friendly solution. This review aims to provide an integrative investigation of the effects of the insect screens in agriculture. Attention is dedicated to the impact on growth, yield, and quality of vegetables, focusing on the physiological and biochemical mechanisms of response to heat stress induced by insect screens. The performance of insect nets depends on many factors—foremost, on the screen mesh, with finer mesh being more effective as a barrier. However, finer mesh nets impose high-pressure drops and restrict airflow by reducing ventilation, which can result in a detrimental effect on crop growth and yield due to high temperatures. The predicted outcomes are wide ranging, because heat stress can impact (i) plant morpho-physiological attributes; (ii) biochemical and molecular properties through changes in the primary and secondary metabolisms; (iii) enzymatic activity, chloroplast proteins, and photosynthetic and respiratory processes; (iv) flowering and fruit settings; (v) the accumulation of reactive oxygen species (ROSs); and (vi) the biosynthesis of secondary biomolecules endowed with antioxidant capacity.
biology
Review
Biochemical, Physiological, and Productive Response
of Greenhouse Vegetables to Suboptimal Growth
Environment Induced by Insect Nets
Luigi Formisano , Christophe El-Nakhel , Giandomenico Corrado , Stefania De Pascale
and Youssef Rouphael *
Department of Agricultural Sciences, University of Naples Federico II, 80055 Portici, Italy;
luigi.formisano3@unina.it (L.F.); christophe.elnakhel@unina.it (C.E.-N.); giandomenico.corrado@unina.it (G.C.);
depascal@unina.it (S.D.P.)
*Correspondence: youssef.rouphael@unina.it
Received: 4 November 2020; Accepted: 28 November 2020; Published: 30 November 2020

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Simple Summary:
Global warming jeopardizes agriculture, which must satisfy the demands
of the world’s expanding population for both staple and high-quality products while ensuring
increased sustainability. Environmental and regulatory pressure has prompted farmers to convert
their production strategies towards sustainable agriculture systems, by introducing for instance,
integrated pest management strategies. Insect nets are a suitable tool for pest control but require
careful assessment of their eects on the generated microclimate. The low porosity, mandatory for
proper exclusion, results in suboptimal airflow and in temperature rise with detrimental eects
on crop production and quality. The biochemical and morpho-physiological changes induced by
high-temperature impact vegetable crop performance and product quality in advanced growing
systems, and also represent a challenge for the most impoverished developing countries of the world,
which rely on local horticultural products as a key source of dietary diversity.
Abstract: Environmental pressure poses a major challenge to the agricultural sector, which requires
the development of cultivation techniques that can eectively reduce the impact of abiotic stress
aecting crop yield and quality (e.g., thermal stress, wind, and hail) and of biotic factors, such as insect
pests. The increased consumer interest in premium-quality vegetables requires the implementation of
sustainable integrated pest management (IPM) strategies towards an ever-increasing insect pressure,
also boosted by cultivation under protected structures. In this respect, insect nets represent an
excellent, eco-friendly solution. This review aims to provide an integrative investigation of the eects
of the insect screens in agriculture. Attention is dedicated to the impact on growth, yield, and quality
of vegetables, focusing on the physiological and biochemical mechanisms of response to heat stress
induced by insect screens. The performance of insect nets depends on many factors—foremost,
on the screen mesh, with finer mesh being more eective as a barrier. However, finer mesh nets
impose high-pressure drops and restrict airflow by reducing ventilation, which can result in a
detrimental eect on crop growth and yield due to high temperatures. The predicted outcomes are
wide ranging, because heat stress can impact (i) plant morpho-physiological attributes; (ii) biochemical
and molecular properties through changes in the primary and secondary metabolisms; (iii) enzymatic
activity, chloroplast proteins, and photosynthetic and respiratory processes; (iv) flowering and fruit
settings; (v) the accumulation of reactive oxygen species (ROSs); and (vi) the biosynthesis of secondary
biomolecules endowed with antioxidant capacity.
Keywords:
protected cultivation; insect-proof screen; airflow; heat stress; biochemical and
physiological responses; functional quality
Biology 2020,9, 432; doi:10.3390/biology9120432 www.mdpi.com/journal/biology
Biology 2020,9, 432 2 of 21
1. Introduction
The concept of quality has radically evolved driven by a “consumer-oriented” revolution.
Nowadays, consumers are more and more sensitive about the nutritional aspects of food and demand
attracting high-quality products. According to the consumers’ perception, the functional quality is
mainly related to the bioactive phytochemical content. The novel quality concept is supported by
consumer interest in the health aspects of food and culinary satisfaction [
1
]. A product with a high
sensory profile and nutritional value is safe, appealing, and sustainable. Interestingly, vegetables are
highly rich in water and macronutrients, low in protein and lipids, and are an excellent source of
vitamins and minerals, conveying significant benefits such as compounds with antioxidant potential
(vitamin C, carotenoids, and phenolics) when included in daily diets [
2
]. Phenols and polyphenols
are natural compounds endowed with reinforcing health repercussions. Recent studies revealed
that phenolic compounds safeguard cells during early cancer development (skin, lung, stomach,
esophagus, duodenum, pancreas, liver, breast, and colon) [
3
]. They also exert considerable antioxidant
activity with beneficial eects on the vascular and nervous systems, thus reducing the impact of
dementia and Alzheimer and Parkinson’s diseases [
4
]. They are also delineated by having antibacterial,
hypocholesterolemic, and hypotriglyceridemic activities [
5
,
6
]. Nonetheless, the accumulation of
antioxidant molecules is aected by preharvest factors such as genotype; cultivation technique;
maturation stage; and climate (e.g., heat, drought, and salinity) [7,8].
On average, farmers worldwide harvest about 50% of their potential yield (i.e., the yield they
would achieve under optimal growth conditions) [
9
]. Of this loss, abiotic factors induce about 60–70%,
while the other 30–40% is due to biotic stress. These are a challenge to the agricultural sector and
require the development of cultivation techniques that reduce the impact of environmental factors, like
wind, hail, excessive radiation, and especially, insect damage and thermal stress [
10
]. The climatic
conditions in protected environments foster insect development, such as whiteflies, thrips, and aphids,
which cause direct crop damage and transmit phytopathogenic organisms (bacteria, viruses, or fungi),
jeopardizing vegetable yield and quality, unless adequately managed [
11
]. Farmers rely widely on
synthetic insecticides for insect control, and researchers have developed more ecient and selective
insecticides with reduced environmental impacts. Moreover, we have also witnessed a consistent
diusion of biological pest management methods.
On the other hand, the consumer demand for pesticide-free vegetables and the increased insect
resistance to pesticides make insect control always challenging. One of the most important tasks
for agriculture remains to contain insect attacks by implementing economically and ecologically
sustainable integrated pest management (IPM) strategies. From this perspective, physical barriers are
an eective and greener method for reducing chemical insecticides in protected environments [
12
].
Increasing consumer interest in organic foods and the stricter regulation of chemicals have increased
the marketability of anti-insect nets for agriculture. Their performance depends on many factors, like
screen mesh and small-hole nets being more ecient [
13
]. However, small-hole nets are characterized
by a high-pressure drop [
14
], resulting in high airflow resistance, decreased ventilation, and a possible
detrimental increase in temperature [15].
The sessile state of plants forces them to adapt to a range of environmental stresses. The eect of
thermal stress depends on plant tolerance and its ability to adapt quickly to suboptimal conditions,
duration, and intensity. Genotype- and environment-dependent adaptive mechanisms ensure their
ability to survive and produce under extreme conditions [
16
]. Plants have a complex set of sensors in
dierent cellular compartments to activate their defense mechanisms as response to thermal stress.
These sensors regulate responses to tolerance development. Thermal stimulus-induced response
activation is enabled by the interaction of cofactors and signaling molecules capable of activating thermal
stress-sensitive genes such as phytohormones, nitric oxide (NO), sugars (as signaling molecules),
and Ca-dependent protein kinases (CDPKs) and mitogen-activated protein kinases (MAPK/MPKs) [
17
].
For example, the increase in membrane fluidity is associated with the activation of signaling cascades
Biology 2020,9, 432 3 of 21
coupled to an increase in Ca
2+
influx, with consequent cytoskeletal reorganization leading to osmolytes
and antioxidants production in response to thermal stress [18].
Although stress-induced responses are usually multifaceted, life-cycle modification, protective
morpho-physiological barriers activation (avoidance or acclimation mechanisms), and the molecular
response (tolerance mechanisms) are typical plant reactions to heat stress. Common examples of
avoidance and acclimation mechanisms include reducing the absorption of solar radiation by
changing leaf orientation (paraheliotropism), reducing water loss by controlling stomatal density,
reducing leaf size or abscission, and altering membrane phospholipids [
16
]. Plants exposed to
high thermal stress activate their adaptive response by modifying their morpho-physiological,
biochemical, and molecular properties [
15
,
18
]. Such stress alters photosynthetic and respiratory
processes [
19
21
], impairs flowering and fructification [
22
,
23
], reduces enzymatic and chloroplastic
activity [
24
,
25
], and promotes reactive oxygen species (ROSs) accumulation [
26
]. As illustrated by
Hasanuzzaman et al. [
16
], high temperatures activate the transcription of heat stress-responsive
genes, resulting in the synthesis of signaling molecules; osmoprotectants; nonenzymatic antioxidant
compounds such as ascorbate (AsA), glutathione (GHS), tocopherol, and carotene; and enzymatic
antioxidant compounds such as catalase (CAT), ascorbate peroxidase (APX), superoxide dismutase
(SOD), peroxidase (POX), and glutathione reductase (GR).
Research demonstrated the eectiveness of fine-meshed screens in excluding harmful insects,
in addition to the detrimental reduction in airflow due to their use. To date, the main aim of research was
to increase airflow by enhancing the intrinsic netting characteristics and to improve growth conditions
without aecting exclusion eciency. However, due to the “antioxidant response” to oxidative stress,
high temperatures can alter the intrinsic and extrinsic quality of vegetables, both positively and
negatively. A recent study showed the eectiveness of insect nets in enhancing the quality of zucchini
squash without aecting yield and, at the same time, ensuring early production [
27
]. To the best of our
knowledge, despite relevant available research papers on the improved airflow of insect nets and their
high-temperature eects on the production and quality of horticultural crops, the reviewed literature
showed a gap of information in this field of research. The few available contributions suggest that
further studies are required to relate the suboptimal growth environment of insect nets to the quality
of the produced vegetables, regardless of their exclusion eciency.
The aim of this review is to investigate and critically analyze the eects of the insect screens
from the plant point of view. The following topics are discussed: (i) the technical aspects of insect
nets, (ii) the airflow characterization through screened openings, and (iii) the description of the
morpho-physiological and biochemical eects of heat stress on plant growth and yield with a view,
in particular, to the antioxidant responses to heat-induced oxidative stress. A literature review was
conducted, integrating peer-reviewed papers, books, technical journals, and conference proceedings
published by 2020, including technical and physical aspects of insect nets and plants’ responses to
high-temperature oxidative stress.
2. Technical Aspects of Anti-Insect Nets
The increasing consumer interest in fresh, sustainable, and high-quality year-round horticultural
products prompts the implementation of integrated pest management (IPM) strategies. From this
perspective, agro-textiles are a valuable tool for pest management, pollinator confinement, and pesticide
reduction. Farmers can rely on dierent types of insect nets that dier in manufacturing (material,
texture, porosity, weight, and number of meshes); radiometric (color, shading, and transmissivity);
and physical and mechanical properties [
28
]. To these purposes, farmers’ concerns are mainly about
the best nets, raising several questions. What materials and technical features are ideal for successful
exclusion? How do insect nets work? What are the drawbacks of nets?
A plastic net is a fabric obtained by processing plastic fibers by weaving or nonweaving
methods [
28
]. Woven nets are characterized by regular holes in which air flows due to the connection
of vertical warp and horizontal weft threads. In contrast, in a nonwoven net, the fabric is produced by
Biology 2020,9, 432 4 of 21
a dierent process such as extrusion or micro-perforation. The weaving process produces most insect
nets available on the market; round or flat plastic monofilaments made of high-density polyethylene
(HDPE) or polypropylene (PP) are woven on looms. In agreement with the National Greenhouse
Manufacturers Association (NGMA), polyamide (nylon) or multifilament nets in steel and brass or
polyethylene and acrylic are marketed, but they have several drawbacks compared to HDPE nets [
29
].
Steel and brass nets are very resistant and durable, but they are expensive and relegated to the industrial
and hobby sectors, while polyamide nets are lightweight but mechanically weak.
Depending on the texture, as discussed by Castellano et al. [
28
], three types of insect nets are
marketed: Italian, English, and Raschel textures. Italian texture (flat woven net) is produced by
overlapping weft and warp threads in orthogonal arrangement. The warp threads are spaced to
allow the passage of a weft thread between them, which results in a rigid and stable net. However,
when the number of threads per cm
2
is reduced, the net stability decreases, and the fabric frays when
cut. The English texture is a revised and improved version of the Italian one. Two pairs of warp threads
are twisted and trapped with weft threads avoiding net fraying. English nets are more stable, resistant,
and nondeformable. A complex structure characterizes Raschel-textured nets. The warp threads are
knotted to create longitudinal chains that twist and incorporate weft threads. Raschel and English
textures are valuable solutions for insect-proof screens. Moreover, they are recommended for anti-hail
and windproof nets, where higher tension and resistance are required.
The weft and warp threads form a regular hole pattern, called mesh, which is the square hole
formed at the intersection of a warp and weft thread, varying from 0.2 to 3.1 mm, depending on the
insect size to be excluded [
28
]. Insect nets available on the market are described by mesh number,
representing the number of holes per inch in each direction [
30
]. The insect’s exclusion is based on
avoiding insect thorax passage (“prison eect”) [
31
], and, theoretically, a net is ecient when the holes
are smaller than the thorax width of the insect to be excluded. This parameter also depends on the
insect sex [
32
]. Table 1shows the average thorax width of “key insects” and the hole size and mesh
number required for their eective exclusion from greenhouses. The hypothetical exclusion eciency
does not necessarily coincide with real eectiveness, achieving up to 90% control of a designated
pest [
33
]; for example, due to the shape of thrips (F. occidentalis) bodies, they can penetrate through
small holes of widespread commercial nets [
34
]. The reason that small holes do not ensure total
exclusion is correlated to the 3D arrangement of the threads. Usually, nets are considered flat structures,
but they are three-dimensional, and their eectiveness depends on several factors like the threads’
thickness, width and length of the hole, and its geometry [
34
]. Warp threads are usually closer together
than weft threads, forming a hole with a rectangular geometric structure; the overlapping of warp and
weft threads alters the geometric structure of the hole, allowing easy access of the insect [34].
Manufacturers do not have specific tools to evaluate insect nets’ efficiency. Therefore, several laboratory
experiments were carried out to assess the exclusion eciency of dierent types of nets in calm
conditions and at dierent air velocities and temperatures [
35
37
]. In recent years, the agro-textile
industry has tested and marketed innovative nets with improved airflow, due to thinner threads,
without aecting exclusion performance. A recent experiment carried out by Formisano et al. [
27
]
investigated the eects of a suboptimal growth environment induced by two 50-mesh nets with
dierent porosities (Biorete
®
50 mesh and Biorete
®
50 mesh AirPlus, Arrigoni S.p.A, Uggiate Trevano,
CO, Italy) on the production and quality attributes of Cucurbita pepo L. in controlled growing conditions.
The improved porosity of the 50-mesh AirPlus net, due to a thinner HDPE filament (Harlene HT
®
,
Arrigoni S.p.A, Uggiate Trevano, CO, Italy), resulted in increased quality traits of zucchini squash
without compromising yield. The 50-mesh AirPlus net led to an improvement in the inner microclimate,
with lower soil and air temperatures and relative humidity. A comparable study on cucumber showed
the positive eects of insect-proof screens with dierent porosities in containing cucumber beetles in
high tunnels while providing adequate ventilation [38].
The durability and mechanical stability of the nets are essential parameters, and fabrics with
complex textures confer enhanced mechanical characteristics, increasing the stability. However,
Biology 2020,9, 432 5 of 21
durability does not depend exclusively on the number and structure of the threads. Several elements,
such as environmental factors (temperature), chemical treatments, dirt, and UV radiation, aect the
mechanical and physical characteristics of plastic threads, leading to premature net deterioration.
UV radiation plays a crucial role in the lifetime and performance of nets [
39
]; hence, manufacturers
use additives to increase the UV stability of HDPE plastic polymers. The longevity of nets is directly
related to their resistance to UV radiation, which is expressed in the amount of kiloLangley (kLy) and
represents the number of years required to reduce the net tensile strength by 50%. For example, a net
with 600 kLy in a Mediterranean climate region (100–130 kLy) potentially has a lifetime of five to six
years [28].
Insect nets are usually made with transparent or white fibers; however, the industry has
recently tested multifunctional nets supplying protection and photoselection by adding colored
and UV-absorbing additives to HDPE polymers. Many authors reported that light modulation using
photoselective nets induces a “barrier eect” against pests while reducing the incidence of viral diseases
aecting horticultural crops. Antignus et al. [
40
] reported that UV-absorbing plastic screens were
eective in decreasing the dispersion rate of pests in greenhouses. Whiteflies detect solar radiation
in a specific light spectrum, and their findings showed that the lack of UV radiation in greenhouse
interferes with the flight and orientation of insects. Further studies conducted by Legarrea et al. [
41
]
investigated the impact that UV-absorbing nets had on the visual cues of two beneficial predators
(Orius levigatus and Amblyseius swirksii). The results obtained showed that the lack of UV radiation
created a favorable environment for Orius levigatus, in contrast to what occurred with Amblyseius swirksii.
In a comparative study, Ben-Yakir [
42
] evaluated the impact of colored photoselective nets (yellow,
red, and pearl ChromatiNets
, Polysack Plastic Industries, Nir-Yitzhak, Israel) on the containment of
aphids and aleyrodids involved in the transmission of the potato virus Y (PVY), cucumber mosaic
virus (CMV) in peppers, and the tomato yellow leaf curl virus (TYLC). Specifically, yellow and pearl
nets reduced aphid and whitefly infestation up to three-fold compared to red and conventional black
nets. Similarly, yellow and pearl nets reduced the incidence of CMV, PVY, and TYLC up to ten-, three-,
and four-fold, respectively.
Over the last two decades, various pest management methods were implemented, such as
insecticide-treated insect nets. Studies on cucumbers (Cucumis sativus L.) and African eggplants
(Solanum macrocarpon L.) demonstrated the ecacy of pyrethroid-treated nets in the management of
aphids and Lepidoptera, although providing lower ecacy in containing tiny insects such as whiteflies
(Trialeurodes vaporarium) and thrips (Frankliniella occidentalis) [
43
,
44
]. In a recent trial, Arthurs et al. [
45
]
tested the exclusion performance of a two-colored modern long-lasting insecticide net (LLIN) with a
larger mesh size (32 holes/cm
2
) compared to a conventional thrips exclusion screen. The results showed
lower thrips penetration in yellow-treated nets than in black ones. However, while insecticide-treated
nets resulted in considerable airflow increase, a larger hole size did not guarantee total thrips exclusion.
Insect nets are commonly used in agriculture, and their eectiveness is proved by many studies.
Nets represent a valuable eco-sustainable solution to limit the use of pesticides, thus exposing producers
and workers to lower risks. The requests of the globalized market have driven technicians, producers,
and researchers to consider insect nets as multifunctional tools that provide high exclusion eciency,
environmental and economic eco-sustainability, and that ensure high yields and high-quality products.
In previous decades, research has focused on improving airflow to limit the detrimental impact of
excessive temperatures in the warm Mediterranean regions. High temperatures, if critical thresholds are
not exceeded, can ensure an early production and an improvement in the quality of vegetables, such as
a higher antioxidant content. Despite extensive research on the plant response to high temperatures,
few studies have examined the possible improvement in quality caused by the insect nets, as well as
the most appropriate porosity level, to ensure a balance between the production, quality, and eciency
of exclusion.
Biology 2020,9, 432 6 of 21
Table 1.
Hypothetical exclusion eciency
1
of insect nets for the control of a designated pest, hole size, and mesh number of widespread insect nets and average
thorax width of “key insects”.
Insect Species Screen Hole Size Average Thorax Width 4(µm)
Microns Mesh Male Female Male Female Male Female
Frankliniella occidentalis 2192 132 190.6 258.0 184.4 245.5 215
Bemisia argentifolii 239 — — — — — 239
Trialeurodes vaporarium 288 — — — — — 288
Aphis gossypii 340 78 486.3 355 355
Bemisia tabaci 462 352 241.7 277.5 215.8 261.3
Myzus persicae — — — — 433.8
Liriomyza trifolii 640 40 — — 562.5 653.8 608
Reference [46] [35] [32] [46]
1
An insect net is theoretically eective when the width of its pores is equal or less than the thorax width of the insect to be excluded.
2
Thrips (Frankliniella occidentalis) are very thin and can
pass through common nets.
3
Thoracic width and hole size are not the only parameters to predict the ecacy of insect exclusion; hole geometry and the way in which holes were formed
are crucial elements as well. 4In this table, the thorax width was measured in the dorsal view.
Biology 2020,9, 432 7 of 21
3. Airflow Characterization of Screened Openings
To ensure optimal growth conditions in protected environments, it is necessary to provide adequate
ventilation, especially in warm Mediterranean regions. High solar radiation and insucient ventilation
cause a rapid rise in air temperature, exposing crops to severe stress aecting all growth stages and
crop production [
16
]. For sucient air exchange, vents should be 15% to 25% of the total area and
should cover the entire length of the greenhouse for balanced air distribution [
30
]. The air flowing
through the greenhouse moves according to a pressure gradient. The air exchange process occurs either
by natural (passive) or forced ventilation [
47
], each aimed at replacing warm indoor air with cooler air
from the outside. With natural ventilation, the airflow through the vents is triggered by temperature
dierences and wind pressure, but mainly wind contributes to air renewal [
48
]. The airflow drives
insects through the openings, and, therefore, insect nets are usually mounted on greenhouse openings
like doors and vents [
30
]. The exclusion performance depends on the mesh and hole geometry [
13
,
32
].
Fine-meshed nets, despite their theoretical better exclusion eciency, have the disadvantage of low
porosity (percentage of the ratio between open net area and total net area). Consequently, a high-static
pressure drop occurs [
14
], leading to inadequate air exchange and rising temperature and humidity [
49
].
Despite the availability of advanced solutions to increase net porosity without reducing mesh
size, thereby improving air exchange in protected environments, it is still necessary to estimate the
pressure drop that occurs through screened openings [
30
]. From a physical perspective, the air is a
viscous and compressible fluid with a variable velocity, which moves according to either the laminar
or turbulent regime. Viscous forces govern the movements in a laminar flow, while, in a turbulent
flow, inertial forces are also involved. Considering air as an incompressible fluid (constant density),
the only variable that discriminates from the turbulent and laminar flow is the Reynolds number (Re).
For insect net, the Reynolds number is defined as follows:
Re=ud
ν
where:
u=flux velocity (m/s),
d=thread diameter (m), and
ν=kinematic viscosity m2/s.
It is a dimensionless parameter that physically expresses how the inertial and viscous forces
acting on a fluid particle move at uvelocity. When air flows through a screened opening, the flow rate
decreases significantly with the pressure drop that occurs from the inside out. Therefore, a prediction
of the total pressure drop through insect-proof screens is necessary to ensure their correct sizing and,
consequently, sucient air exchange without compromising the exclusion eciency. The total pressure
drop
P
T
is the sum of the pressure drop caused by unscreened opening and insect screen [
49
] and is
given by:
PT=Po+Ps
where:
Po=pressure drop across the unscreened opening, and
Ps=pressure drop across the screen [Pa].
The pressure drop generated by insect nets can be assessed both through a “coecient of discharge”
included in Bernoulli’s equation [
50
52
] and by the motion equation of a fluid through a porous
medium (Forchheimer equation) [
53
,
54
]. Supposing that air moves by turbulent flow (Re >150), it is
possible to quantify the pressure drop and the airflow through an unscreened opening using Bernoulli’s
equation. A fluid movement through an opening is subjected to a contraction, causing in the flow an
eect known as vena contracta (V
c
), which represents the fluid flow point where the section is minimal,
Biology 2020,9, 432 8 of 21
the velocity is uniform, and the static pressure is equal to the surrounding air [
55
]. The ratio between
the vena contracta and the total area of a hole (A) defines the contraction coecient (Cc):
Cc=Ac
A
As a result of hole contraction, the velocity in the vena contracta is lower than ideal velocity (V
i
);
the equation that correlates the two velocities is defined as velocity coecient (Cv):
Cv=VC
Vi
Outside and inside the net, we have, respectively:
ρ
2V2
0+P0=ρ
2V2
i+Pi
where:
V=fluid velocity (m/s),
P=static pressure (Pa), and
ρ=fluid density Kg/m3.
For the ideal fluid, without friction, the velocity is dierent from the real one; assuming the
external velocity as zero, we obtain the equation that relates the ideal (or theoretical) velocity to the
static pressure variation:
Vi=s2P0Pi
ρ
The continuity equation, describing the airflow through an opening, can be defined as follows:
Q=AcVC=CcACvVi=CcACvs2P0Pi
ρ
The multiplication between the contraction coecient and the velocity coecient is defined as
the discharge coecient (C
d
), expressing the resistance that a specific opening oers to the airflow [
48
].
Therefore:
Q=CdAs2P0Pi
ρ
Experiments were carried out to determine the discharge coecients of the openings, as well as
the nets. The discharge coecients of vents ranged from 0.60–0.90 [
56
,
57
] as a function of the sharp
edge, whereas they ranged from 0.05 to 0.5 as a function of net porosity [
58
,
59
]. The flow resistance
is often expressed by the pressure loss coecient (K), correlated to the discharge coecient by the
following relationship:
K=1
C2
d
Based on previous observations, the pressure drop through an unscreened opening is given by
the equation below:
Po=1
2KρV2
Moreover, several researchers developed correction functions to adjust the pressure loss value by
correlating the pressure loss coecient to the aspect ratio (L/H) of the openings [
60
] and considering
the influence of flaps [
48
]. Usually, insect nets have an ideal Reynolds number below 150, which results
Biology 2020,9, 432 9 of 21
in a laminar flow [61]; therefore, it is known that the pressure loss coecient is a function of both the
porosity and Reynolds number [62].
In the literature, numerous research have linked the Kcoecient to dierent porosity values
with dierent Re values [
48
,
63
,
64
]. Net resistance to airflow can be evaluated by the physical laws
governing the movement of a fluid through porous media. From this viewpoint, nets are assumed
as solid porous structures consisting of interconnected holes. On a small scale, the pressure drop is
usually expressed by Forchheimer’s equation:
P
x=µ
Kv+ρY
K1/2|v|v
The infinitesimal pressure drop is the sum of a linear term, reflecting the flow resistance generated
by the viscosity
µ
and the specific permeability Kof the porous medium and a quadratic term depending
on the permeability of the medium Kand the inertial factor (Y) (relative to the pore characteristics) [
53
].
Dierent Kand Yvalues were reported by Miguel [
53
] and Valera [
54
] and were classified based on
screen porosity.
As cited by Succi and Vulpiani [
65
], the fluid flow in porous media is dominated by a high
prevalence of dissipative over convective processes. Therefore, at a low Reynolds number (Re <1),
the flow can be described by Darcy’s law (linear term of Forchheimer’s equation); in particular, the
nonlinear term can be ignored, and the flow velocity shows a linear trend with pressure loss:
P
x=µ
Kv
with a Reynolds’ number over the unit (1 <Re <100), nonlinear eects cannot be ignored [61,65].
The applicability of Bernoulli and Forchheimer’s equations is dependent on the Reynolds’ number.
At Re >150, the pressure drop can be determined by the discharge coecient of Bernoulli’s equation,
whereas the laminar flow rate (Re <150) by Forchheimer’s equation. Teitel [
66
] and Kittas et al. [
50
]
demonstrated that the variations in pressure drop obtained with the two mentioned methods were
relatively small. On the other hand, at Re >8, the pressure drop can be determined by the discharge
coecient [
66
], although it is not constant at all values of the Reynolds number, according to Teitel and
Shklyar [14].
Insect nets are eective ecological solutions in regulating pests. However, as shown in the
published literature, low-porous nets drastically decrease the ventilation rate, resulting in higher
relative humidity and temperature gradients in protected environments (Table 2). As mentioned
by Ajwang et al. [
41
], the airflow improvement can be achieved by adequately sizing the screened
openings according to the pressure drop produced by the net. A correction factor, relative to net
porosity, was proposed by Perez-Parra et al. [
67
] to improve the ventilation area. However, as suggested
by Fatnassi et al. [
68
], it is not always possible to compensate the pressure drop by increasing the
screened area; therefore, a forced ventilation system is required in this case.
Table 2.
Evaluation of anti-insect screens with dierent discharge coecients (C
d
), porosity (
ε
),
and mesh sizes on the temperature dierences (
T) and humidity between the inside and outside of the
greenhouses under real conditions and with computational fluid dynamics (CFD) simulation models.
Experimental
Conditions Treatments Eect on Microclimate Reference
Simulation
model
Evaluation of a model to predict the
eect of screen area/opening area ratio
on T (inside/outside). Net radiation
and wind velocity were set to
500 Wm2and 1 ms1, respectively.
For a screen area/opening area ratio of
one, the nets with a discharge
coecient of 0.1 and 0.5 resulted in a
T of 0.75 C and 4.5 C, respectively.
[58]
Biology 2020,9, 432 10 of 21
Table 2. Cont.
Experimental
Conditions Treatments Eect on Microclimate Reference
Multi-span
greenhouse
Eect on inner temperature and
humidity of two insect screens with
dierent porosities (
ε
=0.5 and
ε
=0.6)
Anti-insect nets with porosity of 0.5
and 0.6 resulted in 2.5 and 2-fold
increase in T, respectively, compared
to the unscreened greenhouse.
[50]
Four-span
greenhouse
Eect on inner temperature and
humidity of two insect screens with
dierent porosities (
ε
=0.2 and
ε
=0.4)
mounted on the roof and side openings
of a four-span greenhouse.
Anti-insect nets with porosity of 0.2
and 0.4 resulted in 3 and 2-fold
increases in air temperature and
humidity, respectively, compared to
the unscreened greenhouse.
[69]
Greenhouse
Eect of anti-thrips net (Cd=0.22) on
air temperature in a greenhouse in the
tropical region with small plants and
low transpiration rate.
Unripe plants (low transpiration rate)
grown under the anti-thrips net led to
a temperature increase of 5 C.
Dierently, mature plants (high
transpiration) under anti-thrips net
showed a temperature of 3 C.
[70]
Greenhouse
Eects of insect nets with dierent
porosities (53%, 34%, 33%, and 19%) on
vertical temperature distribution in
greenhouses with tomato crops at two
dierent growth stages
and two densities.
Fine net porosity resulted in a higher
air temperature. The highest
temperature peak was recorded at the
eaves height of the greenhouse.
Taller plants and higher plant density
resulted in lower air temperatures at
all vertical points.
[71]
CFD simulation
model
Evaluation of anti-Bemisia (ε=0.41)
and anti-thrips (
ε
=0.2) nets positioned
on the roof alone and roof and side
openings of a multi-span greenhouse
on the inner microclimate.
Both nets led to a significant increase in
temperature, as compared to the
unscreened control. Specifically,
unscreened control, anti-Bemisia,
and anti-thrips nets resulted in T of
2.4 7.1, and 5.1 C, respectively.
[72]
Greenhouse
Eects of dierent mesh sizes of nets
(40, 52, and 78 mesh) on microclimate
and air exchange rates
in the humid tropics.
The 78 and 52-mesh nets increased air
temperatures of 1–3 C. In addition,
the 78-mesh net determined an increase
in humidity of about twice as much as
observed with the 40-mesh net,
while 52-mesh net led to a rise of 50%.
[73]
Mono-span
greenhouse
Influence of dierent vent opening
positions (side-only, roof-only, and
combined roof and side openings) and
anti-aphid insect screens
on the microclimate.
The combined application of roof and
side openings resulted in a reduction
of the air temperature in the
greenhouse compared to the roof or
side vents alone.
[74]
4. Morphological, Physiological, and Biochemical Responses of Plants under Heat Stress
4.1. Eect of Heat Stress on Growth and Yield
It is well-documented that very intense solar radiation and thermal stress negatively aect
crop physiology with, for instance, significant yield and quality losses in cereals, legumes,
and vegetables [7,18]
. High temperatures aect all growth stages, especially germination and
reproduction. Common and early eects caused by high temperatures are necrosis; leaf elongation
(hyponastia); drying and burning of leaves, branches, twigs, and stems; fruit discoloration and damage;
leaf abscission; poor germination and rooting; loss of turgidity; and cell size reduction, leading to
a decrease in total biomass [
22
,
75
]. The plant can also manifest programmed cell death (PCD),
causing leaves, flowers, and fruits to fall and, in extreme cases, the whole plant to die [
76
]. Germination,
mostly the development of the embryo axis and its emergence, is particularly sensitive to temperature
fluctuations. Short exposure to high temperatures can lead to a reduction in the percentage of seed
germination or a total inhibition, as well as poor vigor and reduced plant, rootlets, and plumules
growth [77].
Biology 2020,9, 432 11 of 21
Considerable high temperature eects were recorded in several crops, aecting their quantitative
and qualitative characteristics. In Leguminosae such as the common bean (Phaseolus vulgaris L.) and
peanuts (Arachis hypogea L.), high temperatures reduced the yields [
78
,
79
]; similarly, in tomatoes
(Lycopersicum esculentum Mill.), Camejo et al. [
80
] reported a significant yield reduction due to defects
in embryo fertilization and meiosis. In many cultivated species, the eects of heat stress are more
evident in reproductive development than in vegetative growth. All plant tissues are susceptible to
high temperatures, and a few degrees increase during anthesis can lead to significant yield losses [
18
].
According to Zinn et al. [
81
], high temperatures shorten the number of days to anthesis, hampering the
optimal nutrients accumulation for embryo development. Further studies on tomatoes, snap beans,
and zucchinis showed tapetum degeneration and pollen sterility caused by PCD and endoplasmic
reticulum malformations [
82
,
83
]. Under heat stress, it is likely that the under-regulation of sucrose
synthetase and pollen vacuolar invertases occurs, as verified in tomatoes and cowpeas [
84
]. A further
relevant eect induced by high temperature is the abscission of reproductive organs due to increased
levels of abscisic acid (ABA) and ethylene (ET), combined with altered or reduced auxin (AUX)
biosynthesis [85].
4.2. Plant Physiological Response to Heat Stress
Heat stress aects a range of physiological processes that are essential for the proper functioning
of cell structures. High temperatures hamper water and nutrient uptake and impair most
physiological and photosynthetic functions, leading to reduced productivity and economic return [
86
].
The proper functioning of metabolic processes in plant tissues requires adequate tissue hydration.
High temperatures, however, lead to a rapid reduction in the water contents in leaf tissue and soil;
a decrease in root conductance, as in tomatoes [
87
], mass, and growth [
7
]; and a decline of the activity
of critical enzymes, such as nitrate reductase [
88
], essential for nutrient uptake, as well as for source
and sink activity [89].
Photosynthesis is the most sensitive to heat stress among plant physiological processes.
Complex reactions leading to CO
2
reduction involve thylakoid reactions (specialized internal
chloroplastic membranes) and carbon-fixing reactions. Foliar mesophyll cells are rich in chloroplasts,
with pigments for light absorption (chlorophylls). In chloroplasts, light energy is captured by two
distinct photosystem units (PSI and PSII) and used to trigger electron transfer to reduce NADP
+
and oxidize H
2
O. Therefore, under heat stress, an optimal performance of cell membranes might
support a better photosynthetic and respiratory eciency. However, high temperatures have shown
to aect cell structures negatively and, thus, photosynthesis as well. Specifically, they alter the
structure of chloroplasts [
25
], reduce the enzymatic activity of ribulose 1,5-biphosphate carboxylase
(RuBisCo) and its regeneration, as shown in cotton plants [
90
] and RuBisCo activase [
87
,
91
], induce the
closure of stomata by decreasing the CO
2
availability and, consequently, the activity of RuBisCo [
92
],
which is recognized to have a low anity toward CO
2
compared to O
2
[
93
], reduce carbon fixation
with oxygen evolution, and generate reactive oxygen species (ROS) [
80
,
94
]. Notably, damage to
photosynthetic pigments was observed, probably due to lipid peroxidation of chloroplasts and
thylakoids, the reduction or stop of PSII activity, and reduction of electron flux and maximum PSII
quantum eciency (Fv/Fm ratio) [
20
,
21
]. Chlorophyll’s lower accumulation is due to its reduced
biosynthesis, degradation, or eects of either due to the deactivation of crucial enzymes such as
5-aminolevulinate dehydratase, as studied in cucumbers [
95
,
96
]. Camejo et al. [
80
] also observed an
increase in the chlorophyll a/b ratio and a decrease in the chlorophyll/carotenoid ratio of heat-tolerant
tomato cultivars.
4.3. Biochemical Response to Heat Stress: The Role of Antioxidant Compounds
In response to heat stress, plants maintain their physiological function through self-regulating
mechanisms (i.e., homeostasis) by producing and accumulating a wide variety of osmoprotectants
(i.e., “compatible solutes”) to restore osmotic pressure [
97
]. Plant cells have numerous compounds,
Biology 2020,9, 432 12 of 21
like proline, glycin-betaine, betaine, soluble sugars, sugar alcohols or tertiary and quaternary
ammonium compounds, ubiquitin, dehydrins, and late-embryogenesis-abundant (LEA) proteins [
7
,
98
].
These compounds also prevent the deactivation of critical enzymes such as RuBisCo under high
temperatures, scavenging free radicals and stabilizing subcellular structures [
20
,
99
101
]. In addition to
compatible solutes, several authors also agree that soluble sugars, such as glucose and sucrose, play a
direct role in heat stress tolerance by regulating carbon allocation, acting as signal molecules [
102
,
103
],
protecting pollen cells by enhancing their quality, as in tomatoes [
104
], and acting as antioxidants and
ROS scavengers at high concentrations [105,106].
Thermal stress produces harmful reactive oxygen species (ROS, e.g., compounds with high
oxidizing activity and a strong tendency to donate oxygen atoms to other substances) [
7
], triggering a
“chain” reaction that can be stopped by antioxidant compounds. ROS can be divided into two main
categories: free radicals, such as hydroxyl radical (OH
), nitroxide radical (NO
), superoxide anion
(O
2•−
), and singlet oxygen (O
) and nonradical species, such as hydrogen peroxide (H
2
O
2
) and ozone
(O
3
) [
107
]. ROS production occurs mainly in chloroplast reaction centers, peroxisomes, and especially,
in the mitochondria by enzymatic and nonenzymatic pathways [
107
], by photo-oxidation reactions,
Haber-Weiss and Fenton reactions, mitochondrial electron transport chain reactions, and during
photo-inhibition [
108
,
109
]. The superoxide radical anion (O
2•−
) does not possess high reactivity. It is
not able to pass through the mitochondrial membrane, and its formation occurs spontaneously during
cellular respiration by cytochrome oxidase that releases partially reduced intermediate compounds,
including O2•− and H2O2.
Even though H
2
O
2
is not a radical species and does not cause any immediate risk to cell structures,
it is involved in the synthesis of reactive ROS. Its formation can also occur due to the enzyme superoxide
dismutase (SOD) from two molecules of superoxide anion. The hydroxyl radical (OH
) production,
which has a high reactivity towards biomolecules, causing considerable cellular damage, is based on
H2O2and O2•− use in Haber-Weiss and Fenton reactions:
O•−
2+H2O2OH+OH+O2(Haber Weiss reaction)
Fe2++H2O2OH+OH+Fe3+(Fenton reaction)
Overexposure to ROS causes oxidative stress that leads to the activation of many cellular antioxidant
systems. These are activated to avoid any damage to proteins, enzymes, lipids, photosynthetic pigments,
and other cellular components. Oxidative damage results in protein denaturation and membrane
instability; lipid peroxidation; photosynthetic reaction center damage; thylakoid membrane electron
leakage; impairment; reduced biosynthesis; and reduced accumulation of metabolites, carbohydrates,
enzymatic activity, and osmotic imbalance [
26
]. Oxidative stress is, therefore, the natural expression of
a damage that occurs when pro-oxidant factors (abiotic and biotic pressures) exceed the endogenous
antioxidant defenses.
One of the most frequent oxidative alterations occurs in lipids, causing a “chain mechanism”
(lipoperoxidation) in the polyunsaturated fatty acids of membrane phospholipids. The reaction chain
produces reactive compounds such as malondialdehyde (MDA), able to react with free amino groups of
proteins, phospholipids, and nucleic acids, inducing molecular structural alterations [
110
]. The reaction
ends when no more oxygen is available or by the action of antioxidants that donate an atom of
hydrogen or an electron, forming nonradical inactive species. However, ROS also acts as a molecular
signal, enabling complex metabolic reactions by which the plant activates thermal stress defenses.
Mittler et al. [
111
] highlighted the vital role of ROS in promoting transcription and translation processes
in chloroplasts, necessary to develop defenses against high temperature-induced oxidative stress.
Environmental stresses prompt ROS production in plants that react by modulating their antioxidant
metabolism [
76
]. Plants undergo high oxidative stress due to harmful ROS under thermal stress and
synthesize a wide range of antioxidants, which lead to an increased stress tolerance. The ROS removal
Biology 2020,9, 432 13 of 21
is necessary for cell survival, and several studies have shown that antioxidant compounds of enzyme
and non-enzyme origin are widely produced in all cell structures under stress conditions [107,111].
Eective plant defense chemicals are nonenzymic, low-weight antioxidant compounds
(i.e., “scavengers”), such as glutathione (GHS), ascorbic acid (AsA),
α
-tocopherol, phenolics, carotenoids,
anthocyanins, plant steroids, and flavonoids [
112
]. Their mode of action is based on altering cellular
metabolic functions, stabilizing membranes, and defending photosynthetic and respiratory functions
from ROS, synergistic acting with other enzymatic antioxidants and phytohormones. The AsA
exerts a protective action against peroxide, superoxide, and hydroxide radicals and singlet oxygen.
At the same time,
α
-tocopherol protects the cell membrane against lipid peroxidation. The GSH and
its oxidized form glutathione disulfide (GSSG) are abundantly present in the cytosol, the nucleus,
and mitochondria. GHS is a cofactor of several antioxidant enzymes (e.g., glutathione peroxidase
and glutathione transferase), eliminates hydroxyl radicals and singlet oxygen, and contributes to the
regeneration of vitamins C and E [113].
The role of antioxidant compounds in the plants’ adaptation to heat stress was studied in several
plant species. Tomato and watermelon plants grown under high temperatures showed a higher
accumulation of soluble phenols than observed in plants grown under optimal conditions [
114
].
The increased accumulation and reduced oxidation of phenols were probably due to the increased
enzyme activity of phenylalanine ammonia-lyase (PAL) and a lower activity in high temperatures
induced by polyphenol oxidase (PPO) and peroxidases (POX). Wahid et al. [
112
] reported that the
accumulation of anthocyanins caused a decrease in the osmotic leaf potential to maximize the absorption
and prevent water loss through transpiration, as well as acting as a UV screen. In a recent trial on
zucchinis grown under anti-insect nets, thermal stress increased the contents of hydrophilic and
lipophilic antioxidant activity, total phenols, and total ascorbic acid [
27
]. Camejo et al. [
94
] underlined
the photoprotective activity of carotenoids such as xanthophyll and terpenoids such as tocopherol in
the stabilization of thylakoid membranes. At the same time, zeaxanthin produced by the hydroxylation
of
β
-carotene performed similar functions in Arabidopsis [
115
]. Enzymatic antioxidants are usually
considered the most eective anti-ROS tools [116].
The first defense system of the plant is the SOD, which catalyzes the dismutation of the toxic
superoxide anion O2•− to molecular oxygen and H2O2:
2 O•−
2+2H+SOD
H2O2+O2
The hydrogen peroxide produced will act as a substrate for CAT and APX. The CAT is an
oxidoreductase of hydrogen peroxide and catalyzes the dismutation of H2O2to water and oxygen:
2H2O2CAT
2H2O+O2
However, the antioxidant compounds play a crucial role in activating the ascorbate-glutathione
(AsA-GHS) cycle involved in ROS detoxification [76].
The ascorbate-glutathione cycle (AsA-GHS) or Foyer-Halliwell-Asada pathway (Figure 1) includes
a series of chemical cascade reactions, described below:
First, the APX catalyzes the reduction of H
2
O
2
to H
2
O utilizing ascorbate as a specific
electron donor:
2H2O2+AsA APX
2H2O+2MDHA
The monodehydroascorbate (MDHA) is regenerated by monodehydroascorbate reductase (MDHAR):
NADH +H++2MDHA MDHAR
NAD++2AsA
Biology 2020,9, 432 14 of 21
However, monodehydroascorbate, if not rapidly reduced, breaks down into ascorbate and
dehydroascorbate (DHA). Dehydroascorbate (DHA) is reduced to ascorbate and oxidized glutathione
(GSSG) by dehydroascorbate reductase (DHAR):
2GSH +DHA DHAR
GSSG +AsA
After eliminating the harmful hydroperoxide, the GSSG must return to its reduced form (GSH)
to reacquire its antioxidant activity; this is achieved by an NADPH-dependent enzyme known as
glutathione reductase (GR) through the following reaction:
GSSG +NADPH +H+GR
2GSSG +NADP+
Figure 1.
Enzymatic and nonenzymatic active antioxidants in plant defense and the Foyer-
Halliwell-Asada cycle (also known as the AsA-GHS cycle) with its intermediates are reported.
The Foyer-Halliwell-Asada cycle starts with the reduction of hydrogen peroxide in water by ascorbate
peroxidase (APX). Abbreviations: SOD, superoxide dismutase; CAT, catalase; APX, ascorbate peroxidase;
MDHAR, monodehydroascorbate reductase; DHAR, dehydroascorbate reductase; GR, glutathione
reductase; MDHA, monodehydroascorbate reductase; DHA, dehydroascorbate reductase; GHS,
reduced glutathione; and GSSG, glutathione disulphide.
4.4. Heat Stress Impact on Product Quality
Thermal stress influences the morpho-physiological aspects of vegetables, thus undermining the
quality and causing significant economic loss. However, recent studies have shown that plants under
moderate heat stress can exhibit better-quality features [
117
]. In protected environments, thermal stress
induces physiological alterations and aects vegetables’ appearance, flavor, carbohydrate content,
and aromatic and antioxidant compounds.
For example, if white asparagus is exposed to thermal stress, the rapid opening of the
heads induces purple coloration, thus reducing their quality and economic value; moreover,
an increase in fibrousness, wilting of shoot tips, and imbalances in calcium assimilation were also
observed [118,119]
. Studies on onions revealed an increase in sulfur compounds (important for flavor)
as the temperature increased, as well as bulb splitting [
119
,
120
]. Similarly, carrot cultivars exposed to
high temperatures showed a better and more intense taste and an increased terpenes content but a
carotene reduction [
121
]. In broccoli, temperatures around 25
C caused head deformation, premature
ripening, and discoloration [
122
]. However, as reported by Mølmann et al. [
123
], high temperatures
induced a higher accumulation of anthocyanins, glucosinolates, phenols, and flavonoids that led to a
less sweeter taste than in broccoli that was exposed to lower temperatures (12
C). Similar findings
were obtained in Chinese cabbage [
124
]. In lettuce, temperatures above 15–18
C determined a higher
incidence of physiological disorders, such as loose head, tipburn, and leaf chlorosis. In contrast, a higher
accumulation of bitter compounds and vitamins C and E but a lower accumulation of carotene were
recorded [
117
,
119
,
125
,
126
]. Similarly, in tomatoes, heat stress led to an increase in vitamin C content
and antioxidant compounds, contrasted by a decrease of the lycopene content and macronutrients such
Biology 2020,9, 432 15 of 21
as magnesium, calcium, and potassium. Additionally, for peas, tomatoes, melons, and watermelons,
a lower sugar content was observed [119,124,126].
Several studies showed a relationship between the expression of antioxidant enzymes, temperature,
and genetic tolerance to heat stress. The scientific literature suggests explicitly that antioxidant activity
increases over a range of certain temperature levels. Chakrabortty and Pradhan [
127
] reported that
catalase, ascorbate peroxidase, and superoxide dismutase enzymes increased up to 50
C. On the other
hand, the activity of peroxidase and glutathione reductase demonstrated a decrease in the temperature
range of 20–50 C.
Temperature is not the only variable to play an important role in enzymatic antioxidant activation
and expression. Studies on field crops indicate that the expression of antioxidant enzymes increases in
heat-resistant species at all stages of growth. For example, there was a higher accumulation of GHS
and GHS/GSSG ratio [128], GST (glutathione S-transferase), POX, APX, CAT, SOD, and GR [129,130].
5. Conclusions
Scientists and producers are being motivated by climate change and consumers’ appreciation
of healthy foods to broaden their vision on conventional production processes. In particular, this is
encouraging them to adopt multidisciplinary approaches to improve productivity, including novel
breeding targets, pest control strategies, and stress reduction tools. The introduction of insect-protection
physical measures has provided a safe tool for the environment, oering the suitable defense against
harmful insects, as well as new alien species, as part of the attempts to increase greening and
environmental sustainability. Nowadays, growers have a wide range of insect nets available that
dier in manufacturing and performance, helping them to choose the most suitable ones for their
purposes. However, the use of anti-insect nets demands careful assessment of the eects they have on
the microclimate, particularly in warm climatic regions, where the radiation surplus can cause a rapid
and detrimental increase in temperature that will ultimately has to be overcome to avoid a significant
drop in production or, in exceptional circumstances, the total loss of production. In a planet exposed to
global warming, there is an urgent need to draw the attention of engineers, producers, and researchers
to find the right compromise between insect protection and favorable climatic conditions for plant
growth. Researchers have focused most of their attention on improving the airflow of anti-insect nets to
avoid detrimental increases in temperature and suboptimal growth environments while continuing to
exclude insects and not aecting the quality of the final product. Most of this research was conducted
in a simulated environment using computational fluid dynamics (CFD) models. It is now necessary to
increase knowledge on more realistic growth conditions and to study the insect net interaction with
crops. The reviewed literature indicated that high temperatures induce high adaptive responses in
edible vegetables. Plants’ defense mechanism of producing antioxidant compounds against harmful
ROS is an excellent quality boost for vegetables until a certain threshold. Given these considerations,
we believe that it is necessary to investigate these aspects to develop mathematical models that can
predict the performance of insect nets in more realistic conditions to be able also to correlate it with
vegetable qualities. These models would make it possible to develop versatile insect nets that can
provide physical protection, improve airflow, and increase the quality of vegetables while preserving
the yields.
Author Contributions:
Writing—original draft preparation, L.F.; writing—review and editing, L.F., C.E.-N., G.C.,
S.D.P., and Y.R.; and supervision, S.D.P. and Y.R. All authors have read and agreed to the published version of
the manuscript.
Funding: This research received no external funding.
Acknowledgments:
The authors are grateful to Arrigoni S.p.A (Uggiate Trevano, Italy) for providing technical
information about the insect nets manufacturing.
Conflicts of Interest: The authors declare no conflict of interest.
Biology 2020,9, 432 16 of 21
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... This latter is also partially confirmed by our results, particularly when the improvement of the net CO 2 assimilation (A CO2 ) by the PH3 fraction is considered.The increase in H 2 O 2 and MDA (a general indicator of lipid peroxidation in the cell membrane) observed under N-limiting conditions is consistent with the results obtained in tomato and lettuce, respectively(Francesca et al. 2022;El-Nakhel et al. 2023b). As recently highlighted byFormisano et al. (2020), plants possess an efficient antioxidant system composed of enzymes such as CAT (catalase) and APX (ascorbate peroxidase), among others. As expected, in N-limiting conditions, the levels of CAT and APX were also more pronounced. ...
Article
The application of protein hydrolysates (PH) biostimulants is considered a promising approach to promote crop growth and resilience against abiotic stresses. Nevertheless, PHs bioactivity depends on both the raw material used for their preparation and the molecular fraction applied. The present research aimed at investigating the molecular mechanisms triggered by applying a PH and its fractions on plants subjected to nitrogen limitations. To this objective, an integrated transcriptomic‐metabolomic approach was used to assess lettuce plants grown under different nitrogen levels and treated with either the commercial PH Vegamin® or its molecular fractions PH1(>10 kDa), PH2 (1–10 kDa) and PH3 (<1 kDa). Regardless of nitrogen provision, biostimulant application enhanced lettuce biomass, likely through a hormone‐like activity. This was confirmed by the modulation of genes involved in auxin and cytokinin synthesis, mirrored by an increase in the metabolic levels of these hormones. Consistently, PH and PH3 upregulated genes involved in cell wall growth and plasticity. Furthermore, the accumulation of specific metabolites suggested the activation of a multifaceted antioxidant machinery. Notwithstanding, the modulation of stress‐response transcription factors and genes involved in detoxification processes was observed. The coordinated action of these molecular entities might underpin the increased resilience of lettuce plants against nitrogen‐limiting conditions. In conclusion, integrating omics techniques allowed the elucidation of mechanistic aspects underlying PH bioactivity in crops. Most importantly, the comparison of PH with its fraction PH3 showed that, except for a few peculiarities, the effects induced were equivalent, suggesting that the highest bioactivity was ascribable to the lightest molecular fraction.
... In addition, with the gradual development of experimental instruments and science and technology, research on insects has gradually transitioned from a focus on basic structure, external morphology, receptor classification, etc., to a focus on physiological and biochemical processes, genetic and receptor internal composition and related proteins [8][9][10][11]. With the rapid development of next-generation sequencing technology for transcriptome analysis, a considerable number of scholars have identified and classified genes and related proteins in insects and connected them with their daily life and reproductive behaviours [12][13][14]. Insects have chemoreceptors; for instance, numerous studies have found that they are located within specialized sensilla, most of which are scattered on the antennae, mouthparts, legs, and oviposition organs [15][16][17][18][19]. ...
Article
Full-text available
The legs of insects play an important role in their daily behaviour, especially reproduction. Entomologists have performed much research on the role of the leg in different behaviours of beetles, an important group in the insect family, but relatively little has been done to study the ultrastructure and transcriptome of their legs. Hence, we systematically studied the ultrastructure and gene expression of the leg of G. cantor, a polygynous beetle, and compared its male and female diversity. In this study, we found the fore-leg, mid-leg and hind-leg of the female were significantly longer than those of the male. From the perspective of intuitive structural differences, we also compared the ultrastructures of the adhesion structure (tarsal) of males and females. The tarsal functional structure of the adult leg mainly includes sensilla and an adhesion structure. The sensilla on the tarsal joint mainly include sensilla chaetica (SCh II, SCh III) and sensilla trichodea (ST II). The adhesion structure includes disc-shaped bristles (di), lanceolate bristles (la), serrated bristles (se), spatula-shaped bristles (spl) and mushroom-shaped bristles (mus). Although there was no significant difference in sensillum distribution or type between males and females, there were significant differences in the distribution and species of adhesion structures between the fore-leg, mid-leg, and hind-leg of the same sex and between males and females. Therefore, different adhesion structures play different roles in various behaviours of beetles. On the other hand, the transcriptome results of male and female legs were screened for a subset of olfaction- and mechanics-related genes. We discovered that the male leg showed upregulation of 1 odorant binding protein (OBP), 2 Olfactory receptors (ORs) and 2 Chemosensory proteins (CSPs). Meanwhile, the female leg showed upregulation of 3 OBPs, 1 OR, 1 Gustatory receptor (GR) and 3 Mechanosensitive proteins (MSPs). An in-depth examination of the ultrastructure and molecular composition of the legs can elucidate its function in the reproductive behavior of G. cantor. Moremore, this investigation will serve as a cornerstone for subsequent research into the underlying behavioral mechanisms.
... The optimum temperature for zucchini cultivation is ranging from 24°C-27°C. Zucchini cultivation in the greenhouse is also increasing due to the high demand for fresh fruits throughout the year in the local and international markets [9]. To get a successful crop the most crucial aspect is the planting time as it exerts influence on vegetative growth, flowering habit, and quality of fruits and allows the plant to grow and respond over a certain quantum of period and produce more yield. ...
Article
In the agricultural sector, the vitally important factors to achieve higher production in a crop are fertilizer management and planting time. To maintain healthy plant growth and to boost crop production and productivity, fertilizer application, and planting dates have a pivotal role in the present scenario of climate change. However, the effect of fertilizer and planting dates on zucchini in agro-climatic conditions of Assam has not been properly studied. So, during the year 2019-20, it was assessed in the Experimental Farm, Department of Horticulture, Assam Agricultural University, Jorhat. Four fertilizer treatments 45: 48: 48 kg NPK/ha, 60: 64: 64 kg NPK/ha, 75: 80: 80 kg NPK/ha and keeping one treatment as control with three planting dates 1 st December, 15 th December, 1 st January was imposed in twelve treatment combinations. The zucchini seedlings were planted at a 60 cm x 60 cm distance in a plot size of 3.24 m 2 in a split-plot design. During the growing period, the diameter of fruit, length of fruit, weight of fruit, number of days to opening of flower to harvest, number of days to first harvest, number of days to last harvest, number of harvests, number of fruits per plant and fruit yield per plant/ plot/ hectare were recorded. From this trial, it is concluded that early planting and maximum fertilizer dose were better in terms of the above physical and yield attributes. Therefore, based on the findings of the experiment, it is hereby inferred that planting of zucchini on the 15 th of December with a fertilizer dose of 75: 80: 80 kg NPK/ha showed the best result with maximum yield (82.59 t) per hectare area with (3.46) B:C ratio in Assam growth condition.
... Цей вид входить до першої десятки овочів із найвищою економічною та харчовою цінністю, високим національним виробництвом, переважно в південно-центральній частині Бразилії, також зростає протягом літнього сезону в Єгипті та в усьому світі [17][18][19]. Проте зростання попиту споживачів на місцевих і міжнародному ринках на свіжі плоди цукіні цілий рік призвели до збільшення площі насаджень їх у теплицях [20,21]. ...
Article
Full-text available
Nowadays, thereareaconsiderablenumberoftechnologicalinnovations in the field of agriculture, aimed at increasing stress-resistance, yield capacity, and the quality of grown products by decreasing the application of chemical means. The use of bio-stimulators, such as plant extracts or micro-organisms is a promising direction, which improves plant growth and effective use of the available soil resources, reclaims its fertility, decreases industry-related load, etc. The purpose of the article is to study the application of bio-stimulators for pre-sowingseed treatment and their impact on zucchini plants. Pre-sowing zucchini seeds treatment with bio-stimulator is an important reserve for raising the yield and improving the product quality, as well as the plants’ growth and healthby stimulating natural processes. Taking into account that zucchini has a high yield potential per unit of area during a short vegetation period, it is expedient to improve the farming method of its cultivation. For example, it is advisable to use silica combinations, Trichoderma or rhizo-bacteria, and plant extracts, which stimulate the plants’ growth. The application of P. putida S1Pf1 andPseudomonas spp. 5Vm1К bacteria strains results in increasing the durationof blooming, the number of flowers and fruits. It has been determined that zucchini seeds treatment with Stimulate® and chitosan assists in seed germination and, at the combination of Eucalyptus camaldulensis leaf extract + K2SiO3 + Trichoderma viride, it is possible to get the highest yield of fruits. It has been found that as a result of zucchini seedtreatment with Emistim C and Vermisol bio-stimulators the germinating energy and field germination increases, the yield grows and product quality improves, abiotic stress and phyto-toxic effect of pesticides decreases, the amount of residual pesticides diminishes, etc. Thus, bio-stimulators play a vital role in the nutrient cycle, the control of abiotic stress, and other important processes in zucchini plants, which enables to consider them promising agricultural practices.
... Цей вид входить до першої десятки овочів із найвищою економічною та харчовою цінністю, високим національним виробництвом, переважно в південно-центральній частині Бразилії, також зростає протягом літнього сезону в Єгипті та в усьому світі [17][18][19]. Проте зростання попиту споживачів на місцевих і міжнародному ринках на свіжі плоди цукіні цілий рік призвели до збільшення площі насаджень їх у теплицях [20,21]. ...
Article
Full-text available
Сьогодні існує значна кількість технологічних інновацій у галузі сільського господарства, спрямованих на підвищення стресостійкості, врожайності і якості вирощеної продукції шляхом зменшення використання хімічних засобів. Використання біостимуляторів, таких як рослинні екстракти або мікроорганізми, є перспективним напрямом, який покращує зростання рослин, а також ефективне використання наявних ресурсів ґрунту, що відновлює його родючість, зменшує антропогенне навантаження тощо. Метою статті є дослідження застосування біостимуляторів для передпосівної обробки насіння та їх вплив на рослини цукіні. Передпосівна обробка насіння цукіні біостимулятором є важливим резервом підвищення врожайності та поліпшення якості продукції, покращення росту та здоров’я рослин, стимулюючи природні процеси. Зважаючи, що цукіні має високий потенціал урожайності на одиницю площі за короткий період вегетації, доцільно поліпшити агротехнологію його вирощування. Наприклад, доцільне застосування кремнеземних сполук, Trichoderma або ризобактерій, що стимулюють ріст рослин, і рослинних екстрактів. Використання штамів бактерій P. putida S1Pf1 і Pseudomonas spp. 5Vm1К призводить до збільшення тривалості цвітіння, кількості квіток і плодів. Визначено, що обробка насіння цукіні біостимулятором Stimulate® і хітозаном сприяє проростанню насіння, а за умови поєднання Eucalyptus camaldulensis leaf extract + K2SiO3 + Trichoderma viride – можна отримати найвищий урожай плодів. Визначено, що у разі обробки насіння цукіні біостимуляторами Емістим С і Вермісол підвищується енергія проростання та польова схожість, з більшується врожайність і поліпшується якість продукції, зменшується абіотичний стрес і фітотоксичний вплив пестицидів, знижується кількість залишкових пестицидів тощо. Отже, біостимулятори відіграють життєво важливу роль у кругообігу поживних речовин, контролі абіотичного стресу та інших важливих процесах рослин цукіні, що дозволяє віднести їх до перспективних агротехнічних прийомів.
... According to the report by Greenpeace [1], the number of extreme hot days has doubled over the past ten years. This heat warning has increasingly attracted the attention of many scientists and farmers, and several reports regarding heat stress in greenhouse vegetables were published [2,3]. Cherry tomato (Solanum lycopersicum) is an important vegetable, which is mostly grown in a greenhouse, with 30-40 million tons of average annual total production in South Korea [4]. ...
Article
Full-text available
As global warming increases day/night temperatures as well as frequencies of heat waves, studying physiological responses in long-term heat stress is required for sustainable yield production in the future. In this study, effects of long-term heat stress on photosynthetic, morphological, and yield parameters of three cherry tomato accessions, HR17, HR22, and HR24, were evaluated. The experiment was conducted under two temperature greenhouse conditions, where temperature set-point for ventilation was 30 °C and 35 °C during the day for 57 days, respectively. Plants were harvested on the 35th days and 57th days after heat treatments, and their physiological and morphological characteristics and yield traits were measured. Under control conditions, HR17 and HR22 had 0.5–0.6 harvest index, while HR24 had 0.3 harvest index. On 35th days after heat treatment, although yield loss percentage of HR17 was high (43%), it produced the highest fruit yield among all three accessions. However, after longer heat treatment, HR24 produced the highest fruit yields among all accessions with the smallest yield loss (34%). Furthermore, yield loss was highly associated with reductions in nitrogen use efficiency and water content in plant body under heat stress. The results of this study will provide breeders with a new insight into selecting heat-tolerant genotypes in cherry tomatoes.
... With the incorporation of IPSs in greenhouses, average ventilation has been decreased, being very difficult to achieve high-performance microclimate conditions while preventing the entrance of insects into crops [11]. The effect on ventilation by the installation of IPSs can lead to misaligning the heat and mass exchange with the exterior, leading to unpunctual growth of crops and development and spread of fungal diseases [1,[12][13][14]. The effect of installing IPS on natural ventilation and the microclimate of greenhouses has been studied by many authors before. ...
Article
Full-text available
Insect-proof screens are a frequent passive method to restrict the entrance of insects into greenhouses. However, the installation of these screens also has a negative effect on natural ventilation, which is reflected in the turbulence and velocity of the airflow inside the greenhouse. The turbulent characteristics of airflow through an insect-proof screen installed in the greenhouse windows have not been studied thoroughly in the literature. The present work focuses on the use of two simultaneous 3D sonic anemometers to study the impact of the use of a 13 × 30 threads·cm−2 insect-proof screen on the turbulence properties of the micro and microscale airflow turbulence. Four tests have been carried out in windward-oriented side windows of a Mediterranean greenhouse. Results demonstrate that the approach of using two simultaneous 3D sonic anemometers for the first time allows one to observe that the effect is different for the three components of the velocity vector field, and there is a strong connection between the simultaneous conditions inside and outside of the greenhouse. Useful information and data on the effect of using a 13 × 30 threads·cm−2 insect-proof screen are also provided. To give details on the impact of screens in the turbulent properties of ventilation is essential for any commercial distribution, as well as providing important data in the design and development of more efficient insect-proof screens.
... The size of the targeted insects determines the type of screen used: the smaller the insect, the denser the screen. Dense (fine-mesh) screens with low porosity significantly reduce ventilation and result in higher air temperature and humidity within the canopy than in the case of the same crop grown in the open field, negatively affecting plant growth and enhancing disease development (Agrafioti et al., 2020;Formisano, El-Nakhel, Corrado, De Pascale, & Rouphael, 2020). Thus, fine-tuning between the screen's aerodynamic characteristics and the structure on which it is placed is essential. ...
Article
Insect-proof screenhouses are commonly used to grow plants in warm climates. However, there is relatively little literature on their microclimate compared to greenhouses. This study presents computational fluid dynamics (CFD) results of airflow, temperature, and humidity ratio patterns in a screenhouse with a roof consisting of a large flat insect-proof screen and impermeable walls. First, vertical profiles of velocity, temperature, and humidity at the center of the screenhouse were obtained by 2D steady-state CFD simulations and validated by experimental results. Root mean square error (RMSE) values were used to measure the differences between the two. The lowest RMSE values among simulations with different turbulence models were 0.49 K, 1.26 g kg⁻¹, and 0.05 m s⁻¹ (with the RNG turbulence model) for temperature, humidity ratio, and air velocity, respectively. The main deviation of the CFD results from the experimental results was observed with the air velocity in the upper region of the screenhouse. Inflow and outflow in the leeward and windward parts of the flat roof were observed, respectively. This resulted in large-scale airflow within the screenhouse opposite the outside wind direction at the canopy level. The results suggested that the leeward section of the screenhouse is warmer than the windward one and has a lower humidity ratio. Large-scale rotating airflow formed in the center of the screenhouse, close to the roof, a large area with a humidity ratio similar to ambient conditions.
Article
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There are many technological innovations in the field of agriculture to improve the sustainability of farmed products by reducing the chemicals used. Uses of biostimulants such as plant extracts or microorganisms are a promising process that increases plant growth and the efficient use of available soil resources. To determine the effects of some biostimulants' treatments on the photosynthetic pigments and biochemicals composition of zucchini plants, two experiments were conducted in 2019 and 2020 under greenhouse conditions. In this work, the effects of beneficial microbes (Trichoderma viride and Pseudomonas fluorescens), as well as three extracts from Eucalyptus camaldulensis leaf extract (LE), Citrus sinensis LE, and Ficus benghalensis fruit extract (FE) with potassium silicate (K2SiO3) on productivity and biochemical composition of zucchini fruits, were assessed as biostimulants. The results showed that E. camaldulensis LE (4,000 mg/L) + K2SiO3 (500 mg/L) and T. viride (10⁶ spore/ml) + K2SiO3 (500 mg/L) gave the highest significance yield of zucchini fruits. Furthermore, the total reading response of chlorophylls and carotenoids was significantly affected by biostimulants' treatments. The combination of K2SiO3 with E. camaldulensis LE increased the DPPH scavenging activity and the total phenolic content of zucchini fruits, in both experiments. However, the spraying with K2SiO3 did not observe any effects on the total flavonoid content of zucchini fruits. Several phenolic compounds were identified via high-performance liquid chromatography (HPLC) from the methanol extracts of zucchini fruits such as syringic acid, eugenol, caffeic acid, pyrogallol, gallic acid, ascorbic acid, ferulic acid, α-tocopherol, and ellagic acid. The main elemental content (C and O) analyzed via energy-dispersive X-ray spectroscopy (EDX) of leaves was affected by the application of biostimulants. The success of this work could lead to the development of cheap and easily available safe biostimulants for enhancing the productivity and biochemical of zucchini plants.
Article
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In a global climate change environment, assuring optimal growing conditions is a difficult challenge, compromising the food supply for a rapidly rising population. The climatic conditions in the protected environment lead to high temperatures and fast insect development, impacting productivity and vegetables qualitative attributes. Consumers' interest in healthy food requires sustainable tools to manage biotic and abiotic factors and, from this perspective, anti-insect nets represent an excellent "green" solution. For this purpose, our goal was to compare two different anti-insect nets on microclimate, production, and qualitative traits of Cucurbita pepo L. fresh fruits. The experiment was conducted in three separate polyethylene high tunnels, with 50 mesh anti-insect nets of different porosities being installed on the openings of two tunnels, while the third tunnel was a control without nets. Microclimate measurements, as well as yield, physiological, and phytochemicals variables, were assessed. The 50 mesh net led to a decrease in marketable yield (22.5%), fruit number (18.0%), CO2 net assimilation rate (6.0%), and transpiration rate (29.5%). Total soluble solids, antioxidant activities and total ascorbic acid concentration had an opposite trend. The 50 mesh AirPlus net improved quality aspects of zucchini fruits by increasing total ascorbic acid, total phenols, and antioxidant compounds, with no negative impact on yield.
Article
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High temperatures represent a limitation for growth and development of many crop species. Several studies have demonstrated that the yield reduction of tomato under high temperatures and drought is mainly due to a photosynthetic decline. In this paper, a set of 15 tomato genotypes were screened for tolerance to elevated temperatures by cultivating plants under plastic walk-in tunnels. To assess the potential tolerance of tomato genotypes to high temperatures, measurements of chlorophyll fluorescence, pigments content and leaf functional traits have been carried out together with the evaluation of the final yields. Based on the greenhouse trials, a group of eight putative heat-sensitive and heat-tolerant tomato genotypes was selected for laboratory experiments aimed at investigating the effects of short-term high temperatures treatments in controlled conditions. The chlorophyll fluorescence induction kinetics were recorded on detached leaves treated for 60 min at 35 °C or at 45 °C. The last treatment significantly affected the photosystem II (PSII) photochemical efficiency (namely maximum PSII quantum efficiency, Fv/Fm, and quantum yield of PSII electron transport, ΦPSII) and the non-photochemical quenching (NPQ) in the majority of genotypes. The short-term heat shock treatments also led to significant differences in the shape of the slow Kautsky kinetics and its significant time points (chlorophyll fluorescence levels minimum O, peak P, semi-steady state S, maximum M, terminal steady state T) compared to the control, demonstrating heat shock-induced changes in PSII functionality. Genotypes potentially tolerant to high temperatures have been identified. Our findings support the idea that chlorophyll fluorescence parameters (i.e., ΦPSII or NPQ) and some leaf functional traits may be used as a tool to detect high temperatures-tolerant tomato cultivars.
Conference Paper
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The photoselective, light-dispersive shade nets can be used as an alternative to protect crops from adverse environmental conditions such as; excessive solar radiation, heat and drought stress, wind and hail, birds, flying pests, thus improving crop's production, yield and quality. The physiological parameters discussed in the review include: vegetable growth parameters (leaf area, leaf chlorophyll), tissue structure, fruit ripening, physiological disorders, pest and disease incidence, fruit quality parameters (soluble solids content and titratable acidity), bioactive compounds (antioxidant activity, ascorbic acid, carotenoid and flavonoid contents) and aroma volatile compounds at harvest. Also, it is evident in the reviewed literature that light quality influences the biosynthesis, accumulation and retention of vegetable phytochemicals, as well as the decay development during storage. These new strategies to modulate light quality should be conveyed to vegetable producing farmers, thus allowing them to preserve the freshness and post-harvest quality of vegetables for an extended period of time, and to meet the consumers demand for vegetables with high nutritional value all year round. Research on light manipulation in horticultural systems is necessary for a sustainable and market-oriented open field and greenhouse vegetable production in the future.
Chapter
Following an overview of climate change and global crop productivity, the book is divided into 4 sections: the problem-changing biosphere (climatic change and variability, and agricultural contributions to greenhouse gas emissions); crop ecosystem responses to climatic change (rice, maize and sorghum, soyabean, cotton, root and tuberous crops, vegetable crops, tree crops, productive grasslands, rangelands, crassulacean acid metabolism crops, crop-weed interactions, pests and population dynamics, soil organic matter dynamics, and interactive effects of ozone, ultraviolet-B radiation, sulfur dioxide and carbon dioxide); mitigation strategies (crop breeding strategies for the 21st century, and role of biotechnology in crop productivity in a changing environment); and economic and social impacts (global, regional and local food production and trade in a changing environment).
Article
The present study was carried out to investigate the effect of individual drought, heat, and combined drought and heat stress on tomato plants. Combined stress resulted in the higher accumulation of Proline (101.9%), MDA (38.55%), H2O2 (101.19%), and lower levels of RWC (53.84%). Individual drought and heat stress decreased photosynthetic pigments like total chlorophyll content by (45.45%) and (25.35%), respectively, higher rates of pigment reduction (79.42%) were observed under combined drought and heat stress. Combined stress decreased PSII efficiency (Fv/Fm), quantum yield (ΦPSII), and photochemical efficiency (qp) and increased non-photochemical quenching (NPQ) levels. Moreover, the gas exchange parameters E, A, and Pn decreased by 5.36%, 36.45%, and 51.00%, respectively, in comparison to control plants. Antioxidant enzymes, SOD, APX, CAT, and GR showed a two- to threefold increase under combined drought and heat stress; however, the non-enzymatic antioxidants AsA and GSH displayed one–twofold increase under combined stress. Moreover, 2- to 2.5-fold decrease was observed in MDHAR and DHAR enzyme transcripts under combined stress conditions. The transcripts corresponding to AsA–GSH pathway enzymes SOD, APX, GR, DHAR, and MDHAR were up-regulated by 8- to 12-fold under combined drought and heat. Furthermore, DREB and LEA transcripts were up-regulated under drought and combined stress and down-regulated under drought stress. In the same manner, HSP70 and HSP90 transcripts were up-regulated under heat and combined stress; however, the transcription levels got down-regulated under drought stress. Additionally, rbcL and RCA transcripts were down-regulated especially under combined stress in comparison to individual drought and heat conditions. PSIP680 relative expression levels were up-regulated under drought stress; however, the transcripts were down-regulated under heat and combined stress. Taken together, the results suggest that the combined stress has a predominant effect over individual stress.
Article
As high tunnel vegetable production acreage increases in the United States, so does the need for management strategies tailored to their unique growing environment. Cucumbers are an ideal crop in these systems; they can be vertically trellised to maximize the production area and provide high yields to balance the increased costs associated with high tunnel construction. One of the most limiting factors in cucurbit production in general is the cucumber beetle complex and the bacterial pathogen they transmit. In this study, we investigated the optimal size of netting installed on high tunnels to prevent cucumber beetle colonization while maintaining ventilation to reduce heat stress. Of the three mesh sizes investigated across 4 yr, the intermediate mesh with a pore size of 0.72 × 0.97 mm was optimal to exclude cucumber beetles, maintain ventilation, and produce the highest yields for both cucumber and melon plants. The smallest (0.16 mm2) and intermediate mesh sizes resulted in secondary pest outbreaks (e.g., aphids), which did not occur in open tunnels and to a lesser extent in tunnels covered with the largest (1.00 × 4.00 mm) mesh. Despite these secondary pests, yield was higher in small- and intermediate-sized mesh treatments due to relief from cucumber beetle infestations, including striped (Acalymma vittatum Fabr. (Coleoptera: Chrysomelidae)) and spotted (Diabrotica undecimpunctata howardi Barber (Coleoptera: Chrysomelidae)) beetles. Overall, we conclude that insect exclusion netting is an effective method to exclude cucumber beetles from high tunnels, but mesh size should be carefully considered when weighing the collective effects on yield and primary/secondary pest abundance.
Article
Thrips are major pests of vegetables and ornamental plants grown under protective structures, and can penetrate all but the finest insect screens; i.e. thrips exclusion screening has recommended hole-diameter < 0.2 mm. We investigated a modern long-lasting insecticidal net (LLIN), with relatively larger hole size compared with thrips exclusion screening, as a barrier treatment for Western flower thrips, Frankliniella occidentalis Pergande under laboratory and greenhouse conditions. Theoretically, treated nets permit larger mesh sizes compared with untreated insect screening, improving light penetration and ventilation inside protected environments. In residual exposure bioassays, the LT50 and LT90 values for adult female F. occidentalis exposed to a commercial 0.4% w/w deltamethrin net (D-Terrence) with hole size 1.8 mm diameter, were 3.96 and 8.99 min, compared with 1.86 and 5.30 min, respectively, for males. In greenhouse cage tests, 43% fewer thrips penetrated black D-Terrence netting when compared with equivalent untreated nets, confirming the benefit of the insecticidal treatments. In another test, fewer thrips penetrated yellow D-Terrence netting when compared with black net. However, treated netting failed to prevent all thrips from penetrating through the net and, in subsequent tests, establishing on bean plants covered by the D-Terrence nets. We conclude that LLIN show promise in greenhouse applications, but require screening with smaller hole sizes (i.e. < 1.8 mm) for protection protect against thrips or similarly sized pests.
Book
About the book: This book provides an integrated approach to crop growth and development and the technical aspects of greenhouse cultivation and climate management. It combines an analysis of the relationship between crop production and ambient climate with an explanation of the processes that determine the climate in a protected environment. With the ability to modify the environment comes the need for growers to strike a balance between the costs and benefits of technology. This book outlines the methods and gives several examples of how to make ‘optimal’ choices about technology. Sustainable management of shoot and root environment is discussed, as well as the pros and cons of vertical farming. The processes addressed in this book, like crop growth, energy balance and mass exchange, apply to any kind of greenhouse. Therefore, in spite of the word ‘technology’, this is not a book about high-tech greenhouses only. Greenhouse horticulture: technology for optimal crop production is an easy-to-read textbook for all those interested in protected cultivation, from university students and teachers to professional advisers in the field and managers of horticultural companies. You can find it at URL: https://www.wageningenacademic.com/doi/book/10.3920/978-90-8686-879-7
Article
Shading and insect-proof screens are widely used in agriculture for passive microclimate control and for insect exclusion. It is an efficient tool for crop production in adverse climatic and environmental condition. As screens are made of a porous material, the protected environment usually interacts with the outside and hence screens provide only moderate microclimatic modifications. Nevertheless, such modifications might be crucial for certain horticultural processes, thus may strongly influence production and quality. Depending on the type of screen, structure configuration, crop and climatic region, recent studies have shown that compared to open field conditions, screens reduce solar radiation and air velocity by about 15–39% and 50–87%, respectively; increase air relative humidity by 2–21%; decrease air temperature and evapotranspiration by 2.3–2.5 °C and 17.4–50% respectively. This paper seeks to review recent advances regarding effects of such screens on microclimate, crop water use and production. Therefore, the ultimate objective of this review is to assist both researchers and growers. For researchers the review provides up-to-date information of the recent studies as well as knowledge gaps that call for future research. For growers and extension service experts this review would assist in choosing the appropriate screen for a specific application, based on the current knowledge.