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Main conclusion In this review, emphasis is given to the most recent updates about morpho-anatomical, physiological, biochemical and molecular responses adopted by plants to cope with B excess. Abstract Boron (B) is a unique micronutrient for plants given that the range of B concentration from its essentiality to toxicity is extremely narrow, and also because it occurs as an uncharged molecule (boric acid) which can pass lipid bilayers without any degree of controls, as occurs for other ionic nutrients. Boron frequently exceeds the plant’s requirement in arid and semiarid environments due to poor drainage, and in agricultural soils close to coastal areas due to the intrusion of B-rich seawater in fresh aquifer or because of dispersion of seawater aerosol. Global releases of elemental B through weathering, volcanic and geothermal processes are also relevant in enriching B concentration in some areas. Considerable progress has been made in understanding how plants react to B toxicity and relevant efforts have been made to investigate: (I) B uptake and in planta partitioning, (II) physiological, biochemical, and molecular changes induced by B excess, with particular emphasis to the effects on the photosynthetic process, the B-triggered oxidative stress and responses of the antioxidant apparatus to B toxicity, and finally (III) mechanisms of B tolerance. Recent findings addressing the effects of B toxicity are reviewed here, intending to clarify the effect of B excess and to propose new perspectives aimed at driving future researches on the topic.
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Planta (2019) 250:1011–1032
Boron toxicity inhigher plants: anupdate
MarcoLandi1· TheoniMargaritopoulou2· IoannisE.Papadakis3 · FabrizioAraniti4
Received: 27 March 2019 / Accepted: 18 June 2019 / Published online: 24 June 2019
© Springer-Verlag GmbH Germany, part of Springer Nature 2019
Main conclusion In this review, emphasis is given to the most recent updates about morpho-anatomical, physiologi-
cal, biochemical and molecular responses adopted by plants to cope with B excess.
Abstract Boron (B) is a unique micronutrient for plants given that the range of B concentration from its essentiality to
toxicity is extremely narrow, and also because it occurs as an uncharged molecule (boric acid) which can pass lipid bilayers
without any degree of controls, as occurs for other ionic nutrients. Boron frequently exceeds the plant’s requirement in arid
and semiarid environments due to poor drainage, and in agricultural soils close to coastal areas due to the intrusion of B-rich
seawater in fresh aquifer or because of dispersion of seawater aerosol. Global releases of elemental B through weathering,
volcanic and geothermal processes are also relevant in enriching B concentration in some areas. Considerable progress has
been made in understanding how plants react to B toxicity and relevant efforts have been made to investigate: (I) B uptake and
in planta partitioning, (II) physiological, biochemical, and molecular changes induced by B excess, with particular emphasis
to the effects on the photosynthetic process, the B-triggered oxidative stress and responses of the antioxidant apparatus to B
toxicity, and finally (III) mechanisms of B tolerance. Recent findings addressing the effects of B toxicity are reviewed here,
intending to clarify the effect of B excess and to propose new perspectives aimed at driving future researches on the topic.
Keywords Boric acid· Boron partitioning· Boron-polyol complexes· Boron tolerance· Boron transporter· Oxidative
Boron (B) toxicity limits crop yield and quality in several
agricultural areas worldwide, and frequently occurs naturally
in alkaline and saline soils together with a low rainfall and
very scarce leaching (Camacho-Cristóbal etal. 2018; Landi
etal. 2012), in agricultural lands close to coastal area (Kabay
etal. 2010) or in areas with persistent geothermal activities
(Princi etal. 2016a). Indeed, ocean evaporation is the pre-
dominant source of B release in the biosphere (65–85%),
whereas natural chemical and mechanical weathering of
sedimentary rocks provide B compounds in soil and water
(Princi etal. 2016a). The most impactful source of highly
concentrated B, with an average of 5–6mg B l−1 (Kabay
etal. 2010), is certainly the seawater, whose intrusion occurs
naturally in most coastal aquifers, owing to the hydraulic
connection between groundwater and seawater, thereby
increasing B concentration in irrigation water (Reid 2010).
Differently to other pollutants, environmental B release
that is directly or indirectly attributable to human activi-
ties plays a minor role compared to the amplitude of the
environmental B-enrichment deriving from natural sources.
List of anthropogenic sources based on their increased con-
tribution to B release worldwide is agriculture, wood burn-
ing, power generation from coal and oil, glass manufacture,
use of borates/perborates, borate mining and processing,
* Ioannis E. Papadakis
1 Department ofAgriculture, Food andEnvironment,
University ofPisa, Via del Borghetto 80, 56124Pisa, Italy
2 Laboratory ofMycology, Department ofPhytopathology,
Benaki Phytopathological Institute, St. Delta 8,
14561Kifisia, Greece
3 Laboratory ofPomology, Department ofCrop Science,
Agricultural University ofAthens, Iera Odos 75,
11855Athens, Greece
4 Dipartimento AGRARIA, Università Mediterranea di Reggio
Calabria, Località Feo di Vito, SNC, 89124ReggioCalabria,
RC, Italy
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... The essentiality of this element was first reported in 1923 by Katherine Warrington who showed that the growth of Vicia faba (field bean) and other plants was reduced in the B deficiency, but it was rescued following the resupply of B (Warrington, 1923). Afterwards, it has been widely accepted that B is a necessary and beneficial element for different plants; the nutritional and physiological functions of B have been studied and reviewed elsewhere (Camacho-Cristóbal et al., 2018;Landi et al., 2019). Accordingly, B is an important element for cell wall formation and stability as well as the maintenance of plasma membrane functions (Brown et al., 2002). ...
... Moreover, B has been also associated with seed formation and development by directly impacting seed germination and seedling establishment (Zohaib et al., 2018). Notably, the influences of B on specialized metabolites biosynthesis such as antioxidants polyphenols has been also suggested since B nutrition is likely an important agent regulating reactive oxygen species (ROS) levels (Brown et al., 2002;Landi et al., 2019). B is also involved in RNA metabolism (pyrimidine biosynthesis) and indole-3-acetic acid oxidase root elongation (González-Fontes et al., 2015), phenolic metabolism, carbohydrate metabolism and translocation of photoassimilates (Graham and Webb, 1991); however, it is important to mention that the molecular mechanisms behind most of these functions remain largely unknown. ...
... In soil solution, B occurs mainly in the undissociated form of boric acid (H 3 BO 3 ), which greatly influences its availability (Brown et al., 2002). Generally, two extreme conditions, B deficiency and toxicity, are observed in arable lands drastically affecting crop production, yield, and quality, leading to significant economic losses (Camacho-Cristóbal et al., 2018;Landi et al., 2019). Noteworthy, studies on B fertilization have shown that the limits between deficiency and toxicity are very narrow and that the ideal dose is relatively variable across plant species (Camacho-Cristóbal et al., 2018;Landi et al., 2019). ...
Boron (B) is an essential nutrient for the plant, and its stress (both deficiency and toxicity) are major problems that affect crop production. Ethylene metabolism (both signaling and production) is important to plants' differently responding to nutrient availability. To better understand the connections between B and ethylene, here we investigate the function of ethylene in the responses of tomato (Solanum lycopersicum) plants to B stress (deficiency, 0 μM and toxicity, 640 μM), using ethylene related mutants, namely nonripening (nor), ripening-inhibitor (rin), never ripe (Nr), and epinastic (Epi). Our results show that B stress does not necessarily inhibit plant growth, but both B stress and ethylene signaling severely affected physiological parameters, such as photosynthesis, stomatal conductance, and chlorophyll a fluorescence. Under B toxicity, visible symptoms of toxicity appeared in the roots and margins of the older leaves through necrosis, caused by the accumulation of B which stimulated ethylene biosynthesis in the shoots. Both nor and rin (ethylene signaling) mutants presented similar responses, being these genotypes more sensitive and displaying several morphophysiological alterations, including fruit productivity reductions, in response to the B toxicity conditions. Therefore, our results suggest that physiological and metabolic changes in response to B fluctuations are likely mediated by ethylene signaling.
... Borax deficiency induces improper reproductive organ development and lowers plant output (Dell and Huang 1997;Chen et al. 2005;Nabi et al. 2006). Recently, from another perspective, B was assumed to be a rather toxic element, causing substantial damage to plant cells even at low levels (Reid et al. 2004;Landi et al. 2019). It is also necessary for blooming and fruit production (Nonnecke 1989). ...
Micronutrients play a crucial role in enabling crops and vegetables to achieve optimum productivity, improve storage quality, and overcome physiological disorders. The present study focuses on evaluating the effects of the foliar application of Zinc, Borax, and their combined application in enhancing onion yield and shelf life of Allium cepa L. (cultivars Red coral and Liberty). An experimental study was conducted for two years at Agriculture Research Unit, DIHAR Leh (34.1383˚N, 77.5725˚E) to assess the impact of different doses of ZnSO4, Borax, and their combination on onion yield and shelf-life improvement. Four treatments of ZnSO4 and Borax (T1: 0.5% ZnSO4; T2: 0.25% Borax; T3: 0.5% ZnSO4 þ 0.25% Borax and T4: control) were applied twice, once at 30 DAT and again after 45 DAT. Among four treatments, T3 showed a significant impact on growth parameters, yield, and onion shelflife followed by T1 and T2 compared to the control T4 treatment. Foliar sprays of ZnSO4 and Borax, rather than only ZnSO4 or Borax application, significantly improved plant height, leaves number, leaf area index, chlorophyll content, and onion yield. The storage examination revealed an important correlation between the concurrent application of ZnSO4 and Borax on onion, demonstrating a significant reduction in weight loss, an increase in Total Soluble Solids (T.S.S), and an extension of the onions’ shelf life. These findings hold the potential for significant implications in elucidating the optimal utilization of micronutrients for improving onion cultivation and preservation practices in cold desert regions of Ladakh.
... High levels of B in the soil as well as in irrigation water, especially in dry and semi-arid region, impede plant development and reduce agricultural output and quality (Choudhary et al. 2020). B-toxic conditions occurs naturally in alkaline and saline soils as well as a low rainfall and very scarce leaching (Camacho-Cristóbal et al. 2008), in agricultural lands close to coastal area or in areas with persistent geothermal activities (Landi et al. 2019). Furthermore, B-toxic soils also occur as a consequence of over-fertilization and/or irrigation with water containing high levels of B (Camacho-Cristóbal et al. 2008). ...
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The purpose of this research was to investigate whether sodium salicylate (NaSA) treatment could ameliorate the stress caused by B toxicity in Nigella damascena. In the study, the effects of untreated control, 1 mM NaSA (NaSA), 4 ppm B (B4), 8 ppm B (B8), B4 + NaSA, and B8 + NaSA on grain yield, oil content, and fatty acid composition of N. damascena were examined. The B4 and B8 applications reduced nearly all of the studied characteristics in N. damascena. Plant seed yield decreased by 45% and 55%, In B4- and B8-applications, respectively. Compared to the B4 and B8 applications, the B4 + NaSA and B8 + NaSA applications increased grain yield by 16% and 17%, respectively. NaSA application yielded the highest both β‑elemene and α‑selinene ratios. Application-Trait-biplot (AT-biplot) explained 76.1% of total variation. Based on the positive relationship between B and NaSA applications, the properties in this investigation, which mostly included fatty acids, were divided into five groups: (I) palmitic acid, total-saturated-fatty-acid (TSFA), eicosadienoic acid, linolenic acid, and total-unsaturated-fatty-acid (TUFA), (II) monounsaturated-fatty-acid (MUFA), myristic, stearic, and oleic acids, (III) cis-oleic acid, oil content and thousand-kernel-weight (TKW), (IV) plant grain yield, β‑elemene, α‑selinene, and TUFA/TSFA ratio and linoleic acid, (V) polyunsaturated-fatty-acid (PUFA). As seen in the biplot analysis, compared to the B4 application, the B4 + NaSA application brought the majority of examined features closer to values of control application. As a consequence, it was discovered that the B‑toxicity in N. damascena species may be alleviated by NaSA application, and the data collected under various stress circumstances can be examined visually using a biplot.
... The chelating function of anthocyanins reduces the occurrence of free metal ions in cells, thus decreasing the toxicity of metal ions to plants. Briefly, after the formation of the anthocyanin-metal complex, the toxic metal ions are sequestered in the vacuole of the peripheral cell layer, thus isolating the harmful metals or metalloids in physiologically less sensitive tissues (Landi et al., 2019;Estévez et al., 2021;. ...
... One of the significant limitations to crop production is the insufficient use of micronutrients, particularly Boron (B), which poses a substantial risk to the growth, yield, and quality attributes of cotton (Atique-ur-Rehman et al., 2022;Yeates et al., 2010). Physiological and biochemical processes of plants exhibit sensitivity to B deficiencies, while concentrations above acceptable levels result in reduced plant growth and development owing to toxic effects (Behera et al., 2023;Brdar-Jokanović , 2020;Landi et al., 2019). Boron plays an active role in flowering initiation, as well as in the production of seeds and fruits. ...
... The improvement of fertilization in nurseries can significantly reduce the high field mortality of seedlings while also offsetting the need for early field fertilization (Villar-Salvador et al. 2004;Schott et al. 2016). Previous studies examined the effects of B on the morphological, physiological, and biochemical responses of plants; however, many of these studies investigated food crops (Landi et al. 2019). Meanwhile, Lehto et al. (2010) reviewed the role of B in forest trees and forest ecosystems and identified some gaps in understanding B uptake by different tree species, as well as the management of B fertilization for many tree plantations. ...
Boron (B) is a well-known essential element for plant growth, but B supplementation is not widely considered for broad-leaved seedlings because of the lack of B deficiency symptoms. The effects of B on various broad-leaved seedlings in tree nurseries need to be discovered and studied. We selected five native hardwood species commonly used in plantation forestry in Taiwan. Moreover, five concentrations of B supplementation were designed to determine the growth response and nutrient uptake of these seedlings. Supplementation with 10 mg L−1 B nutrient solution, corresponding to 2 kg ha−1 B, in the growing season significantly increased the growth of most species. Excluding Zelkova serrata, which was confirmed as a B-sensitive species in our study, the seedling height, dry weight, and total leaf area of the other species were increased by 21.2–30.3%, 23.2–41.3%, and 14.2–37.9%, respectively, by B supplementation versus the control, suggesting the existence of a hidden hunger effect. Additionally, moderate B supplementation increased the levels of other nutrients, such as nitrogen, phosphorus, magnesium, sodium, and potassium, whereas decreases of calcium levels were observed under the highest B supplementation. Moreover, visible toxic symptoms such as chlorosis and necrosis of leaves and growth inhibition were observed and enhanced by increasing the concentration of B. Our results demonstrated that the B management in the tree nurseries of broad-leaved species is a key factor for obtaining high-quality seedlings, as optimum B supplementation helps improve their growth and nutrient uptake.
The aim was to evaluate the chemical and mineral composition, carbohydrate fractionation, in vitro dry matter digestibility (IVDMD), and in vitro gas production of Furcraea foetida (L.) Haw at different phenological stages. The experiment was carried out in a completely randomised design, in which 4 plants of the same size were selected at random, and from these plants, leaves were collected at 3 phenological stages: young (leaves at the apex of the plant), intermediate (leaves in the middle parts of the plant) and mature (leaves at the base of the plant). Higher levels of ether extract (P = 0.001) and cellulose (P = 0.010) were obtained at maturity. The concentration of Boron in intermediate (35.58 g/kg DM) and mature (27.79 g/kg DM) leaves, and Zn in young (58.19 g/ kg DM) and mature (54.98 g/kg DM) leaves decreased with the phenological stage. High levels of non-fibre carbohydrates influenced the in vitro gas production kinetics, with higher concentrations for young and intermediate stages. Under experimental conditions, Furcraea foetida is inferred to be a plant with a high energy content that preserves its nutrients in the different phenophases evaluated, a positive factor for its use as a forage plant for ruminant production in the semiarid region.
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Flowering time in plants is a complex process regulated by environmental conditions such as photoperiod and temperature, as well as nutrient conditions. While the impact of major nutrients like nitrogen, phosphorus, and potassium on flowering time has been well recognized, the significance of micronutrient imbalances and their deficiencies should not be neglected because they affect the floral transition from the vegetative stage to the reproductive stage. The secondary major nutrients such as calcium, magnesium, and sulfur participate in various aspects of flowering. Micronutrients such as boron, zinc, iron, and copper play crucial roles in enzymatic reactions and hormone biosynthesis, affecting flower development and reproduction as well. The current review comprehensively explores the interplay between microelements and flowering time, and summarizes the underlying mechanism in plants. Consequently, a better understanding of the interplay between microelements and flowering time will provide clues to reveal the roles of microelements in regulating flowering time and to improve crop reproduction in plant industries.
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Generally, moderate to high salinity conditions and excess boron (B) occur together as limiting factors for plant growth in the soils of arid and semiarid regions. To determine the combined effect of excessive boron, salinity stress, or both, five different levels of B (0, 0.3, 0.6, 1.2, and 1.8 mM) and 80 mM sodium chloride (NaCl) were applied to lavandin plants grown in a greenhouse. The results showed that under nonsaline conditions, biomass production in shoots and roots and photosynthetic pigment contents (chlorophyll (Chl) a, b, and Chl a + b) decreased with exceptionally high B applications compared to the control. Moreover, the bioconcentration (BCF) of B (in shoots and roots), potassium (K) concentrations (in roots), K/sodium (Na) and calcium (Ca)/Na ratios (in shoots), and Ca/B ratios (in shoots and roots) decreased for all B applications compared to the control. In contrast, all B applications caused a remarkable increase in the carotenoid (Car)/ Chl ratio, B concentrations (in shoots and roots), translocation (TF) of B, and net B accumulation compared to the control. In addition, under nonsaline conditions, concentrations of K (in shoots), Ca (in shoots and roots), and K/Na and Ca/Na ratios (in roots) were significantly increased by B applications compared with the control. Under saline conditions, significant decreases in Chl b, Chl a + b, BCF of B (in shoots and roots), and Ca/B ratio (in shoots) were observed in all B applications compared to the control. However, under saline conditions, B application caused significant increases in the Car/Chl ratio, TF of B, net B accumulation, and concentrations of B (in shoots and roots), K (in shoots), Ca, and Na (in shoots and roots) compared to the control. It was concluded that although it is not seen in the growth parameters, NaCl application could effectively alleviate the harmful effects of B toxicity in lavandin plants. Under saline conditions, notable decreases in the mean B concentration in shoots could be strong evidence for this hypothesis.
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Earlier our colleagues detected that the genes related to jasmonate (JA), ethylene, and cell wall modification were significantly regulated under boron (B) toxicity in wheat. Determination of regulation mechanisms of these novel genes under B toxicity is very important in Arabidopsis thaliana as a model plant. As key regulators, the microRNAs (miRNAs) regulate gene expression at the posttranscriptional level and respond to numerous abiotic stresses in plants. In this study, expression levels of miRNAs such as miR159, miR172, miR319, and miR394 targeting JA and ethylene-related transcription factors and also miR397 targeting laccase were determined in Arabidopsis thaliana under toxic B conditions. Stem-loop quantitative reverse transcription polymerase chain reaction was used to amplify mature miRNAs for expression analyses. Expression levels of miRNAs targeting transcription factors related to JA and ethylene metabolisms were induced remarkably in moderate B toxicity (condition 1B) but not in severe B toxicity (condition 3B). Most remarkable regulations were obtained in miR172 and miR319 in Arabidopsis thaliana. Expression level of miR397 did not remarkably change under B toxicity, indicating a lack of posttranscriptional regulation of laccase related to cell wall modification. Moreover, miRNAs targeting transcription factors related to JA and ethylene metabolisms might be oxidative stress-adaptive responses of Arabidopsis to B toxicity.
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Two independent experiments were performed to assess the role of thiourea (TU)-mediated nitric oxide (NO) in mitigating boron toxicity (BT) in bread wheat (Triticum aestivum L. cv. Pandas) and durum wheat (Triticum durum cv. Altıntoprak 98) plants. In the first experiment, plants of the two wheat species were grown under control (0.05 mM B) and BT (0.2 mM B) supplied to nutrient solution for 4 weeks after germination. These two treatments were also combined with TU spray at 200 or 400 mg L⁻¹ once a week during the period of stress. Boron toxicity reduced dry weights of shoot and root, leaf total chlorophyll, efficiency of photosystem II (Fv/Fm) and leaf relative water content, whereas it increased endogenous nitric oxide (NO), nitric oxide synthase (NOS), electrolyte leakage (EL), hydrogen peroxide (H2O2), malondialdehyde (MDA) and leaf B content. Reductions in total dry matter were 33% and 61% of control in cvs. Pandas and Altintoprak, respectively. Exogenous application of TU improved the plant growth attributes and led to further increases in NO in the leaves. An additional experiment was set up to further understand whether or not TU mediated NO production played a significant role in mitigation of BT using 0.1 mM scavenger of NO, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt (cPTIO) combined with the TU treatments by spraying once a week for 4 weeks. TU-induced BT tolerance was totally eliminated by cPTIO by reversing endogenous NO levels. BT enhanced the activities of superoxide dismutase (SOD; EC, catalase (CAT; EC., peroxidase (POD; EC. and lipoxygenase (LOX; EC. as well as the contents of soluble sugars (SS), soluble proteins and phenols, but decreased NR. TU treatments enhanced enzyme activities, but reduced contents of soluble sugars (SS), soluble protein and phenols. The present results clearly indicate that TU mediated endogenous NO significantly improved BT tolerance of wheat plants. This evidence was also supported by the increase in hydrogen peroxide (H2O2) and malondialdehyde (MDA) as well as plant growth inhibition with the application of TU combined with cPTIO.
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In this study, individual and combined effects of boron and sodium chloride salinity on growth, photosynthetic pigments (chlorophyll and carotenoids content), enzymatic activities (catalase and ascorbate peroxidase), hydrogen peroxide content, malondialdehyde content, proline accumulation, and some ion contents, such as B–, Na⁺, Cl–, K⁺, Ca²⁺, Mg²⁺ of purslane (Portulaca oleracea L.) were investigated. Five B levels (0, 4, 8, 16, 24, 40 mg/kg) and 100 mM NaCl were applied to the soil and mixed before sowing. Results showed that purslane growth was reduced significantly by higher B levels and salinity due to ion toxicity and osmotic stress. Also, content of photosynthetic pigments increased with both higher B levels and salinity, but they were decreased with combined effects of them. Tissue B–, Na⁺, Cl–, K⁺ and Ca²⁺ levels in shoot increased with applied NaCl, but B levels applied together with NaCl caused a decrease in B– content due to antagonistic effects between B– and Cl– ions. The MDA content, proline accumulation, and H2O2 content increased with higher B levels, but salinity caused a decrease in MDA content. The catalase and ascorbate peroxidase activities increased with B and salinity combination, but did not change with salinity. Increasing B reduced the catalase activity. It is suggested that purslane has the potential to manage the amount of soluble boron and also it has a promising potential that can be grown in B-rich and saline soils.
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Although a requirement for boron is a well‐established feature of vascular plants, its designation, for almost a century, as essential is challenged and, instead, the proposal is made that it has never been so as conventionally defined. This is because an alternative interpretation of published evidence negates its compliance with one of the criteria for essentiality, that its effects are direct. The alternative, here postulated, is that boron is, and always has been, potentially toxic, a feature which, for normal growth, development and reproduction, needed to be nullified. This was enabled by exploitation of boron's ability to be chemically bound to compounds with cis‐hydroxyl groups. Although particular cell wall carbohydrate polymers, glycoproteins and membrane glycolipids are among candidates for this role, it is here proposed that soluble phenolic metabolites of, or related to, the components of the pathway of lignin biosynthesis, themselves potentially toxic, are primarily used by vascular plants. When metabolic circumstances allow these phenolics to accumulate endogenously in the cytoplasm, their own inherent toxicity is also alleviated, partially at least, by formation of complexes with boron. This chemical reciprocity, enhanced by physical sequestration of the complexes in vacuoles and/or apoplast, thus achieves, in a flexible but indirect manner, a minimisation of the inherent toxicities of both boron and relevant phenolics. In these ways, the multifarious outcomes of impairments, natural or experimental, to this interplay are responsible for the lack of consensus to explain the diverse effects observed in the many searches for boron's primary metabolic role, here considered to be non‐existent. In particular, since a toxic element cannot have ‘deficiency symptoms’, those previously so‐called are postulated to be largely due to the expressed toxicity of phenylpropanoids. A principal requirement for the otherwise toxic boron is to nullify, by means of its indirect chemical and physical sequestration, such expression. In these ways, it is therefore neither an essential nor a beneficial element as currently strictly defined. This article is protected by copyright. All rights reserved.
Poplars (Populus species) are tolerant to boron (B) toxicity and have phytoremediation potential in B-contaminated soils. However, the detoxification strategy is largely unknown. To screen the key B transporter-like (BOR-like) genes for B compartmentation, Populus russkii plants were exposed to different levels of excess B and the plant growth, physiological responses, B distribution, and the expression patterns of BOR-like genes were characterized. P. russkii showed moderate tolerance to excess B although the plant growth was inhibited. The enhanced proline level and well-regulated antioxidant defense system were associated with B tolerance in leaves. The B absorbed by plants was predominantly allocated to leaves. Ten BOR-like genes were identified and seven of them showed tissue-specific expression patterns. PrBOR7 was identified as an important BOR-like gene possibly involved in the export of B from leaf cytoplasm because it was expressed specifically in leaves and induced by excess B. Yeast experiment assays verified that PrBOR7 functions as an efflux-type transporter and strongly improved cell tolerance to excess B. The expression patterns of BOR-like genes highlight the diversity of the family members in P. russkii, and PrBOR7 has potential as a candidate gene for B detoxification.
Boron (B) toxicity is a serious problem for the growth and development of plants in arid and semiarid areas. Jasmonates enhance the capacity of plants to grow under stressful environmental conditions. The objective of this experiment was to study the effect of foliar application of methyl jasmonate MeJA (at 0 and 100 μM) on biochemical and gene expression responses of two Iranian grape cultivars (‘Bidaneh Ghermez’ and ‘Rashe’) under B toxicity conditions (0, 10 and 20 mg B /kg soil). With increase in B levels of the soil, the leaf B content, malondialdehyde (MDA), hydrogen peroxide, total phenolics, total flavonoids, total protein and proline content, as well as total antioxidant activity and the activity of antioxidant enzymes (except for the ascorbate peroxidase) were significantly (P ≤ 0.01) increased. Under B toxicity conditions, foliar application of MeJA at 100 μM enhanced the activity of catalase, superoxide dismutase and phenylalanine ammonia lyase enzymes, and decreased MDA and proline content in the leaves of both cultivars. In addition, MeJA significantly (P ≤ 0.01) influenced the accumulation of B in grape leaves. B accumulation in the leaves of Rashe cultivar was lower than that of ‘Bidaneh Ghermez’. The expression of VvPAL and VvBOR1 genes in all treatments were lower than those of controls. MeJA enhanced the expression of VvPAL under B toxicity conditions while the expression of VvBOR1 gene was declined. The expression of VvBOR1 in ‘Rashe’ was significantly (P ≤ 0.01) lower than that of ‘Bidaneh Ghermez’ cultivar. According to the results, Rashe cultivar performed better as compared with Bidane Ghermez cultivar in response to increased soil B levels and MeJA treatment.
This study explores the possibility of using mycorrhization as a novel technique for diminishing the negative effects of boron (B) in the nutrient solution on seedlings of Carrizo citrange rootstock plants. For this, an experiment was planned for studying the physiological (gas exchange and chlorophyll fluorescence parameters), morphological (vegetative growth parameters), nutritional (organic solutes, carbohydrates) and oxidative stress responses of seedlings that were either mycorrhized (+AM, Rhizophagus irregularis; previously known as Glomus intraradices) or not mycorrhized (-AM), and irrigated with water containing different concentrations of B (0.5, 5 and 10 mg L −1). It was observed that an excess of B in the nutrient solution decreased the vegetative growth in both +AM and-AM plants, but this decrease was greater in-AM plants. Mycorrhized plants (+AM) under high B concentration accumulated less B in the leaves, and had a smaller reduction of net assimilation rate of CO 2 and lower MDA concentration than non-mycorrhized plants. Thus, it can be concluded that mycorrhization increased the tolerance to high boron concentration in the irrigation water of citrange Carrizo seedlings by reducing both the B concentration in the plant tissue and the B toxicity in the physiological processes. The study of organic solutes and carbohydrates also pointed to a different response model between +AM and-AM plants that could be related to the different tolerance observed between these plants.
In many parts of the world, soils deficient and/or toxic in micronutrients reduce potential soybean (Glycine max) yields. The objective of our study was to grow plants in low to high concentrations of boron (B) and zinc (Zn) to determine how B and Zn deficiencies and toxicities affect soybean growth and interact with other essential nutrients in roots, leaves, and seeds. We found that B significantly affected levels of all essential nutrients except manganese and iron, while Zn significantly affected all essential nutrients in at least one plant tissue tested. Some of the physiological responses and nutrient interactions were cultivar-dependent. This study showed how deficiencies and toxicities of B and Zn affect plant growth and how B and Zn fertility interacts with many of the other essential nutrients.
Citrus rootstocks, (i.e. Carrizo citrange, Cleopatra mandarin, and Forner-Alcaide nº5) influence the agronomical and physiological behavior of citrus trees under unfavorable environmental conditions. Citrus are very sensitive to boron (B) toxicity. However, when good-quality water is scarce, growers must use water treatment and desalination plant water for irrigation, which may have a B concentration that is above the threshold recommended for citrus trees (0.5 mg L⁻¹). There is little information on the relative tolerance of citrus rootstocks to an excess of B, and the physiological and biochemical mechanisms involved. In this work, the morphological, physiological, biochemical and nutritional responses of three citrus rootstock plants, Carrizo citrange (Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.), Cleopatra mandarin (Citrus reshni Hort. ex Tan.) and Forner-Alcaide nº 5 (Cleopatra mandarin × Poncirus trifoliata (L.) Raf.), under different concentrations of B in the irrigation water (0.25, 5 and 10 mg L⁻¹), grown in containers in a greenhouse, were studied. The results showed that Carrizo citrange was very sensitive to B toxicity, as the plants had the most reduced vegetative growth, the greatest concentration of B in their leaves, stem and roots, a greater concentration of malondialdehyde in leaves, and a lesser net assimilation of CO2. The other two genotypes did not show differences in the accumulation of B in their tissues. Nevertheless, Cleopatra mandarin showed a greater tolerance, as its photosynthetic system was less affected by B toxicity, probably due to its potent antioxidant system, which is based on a high activity of catalase, which restricts the accumulation of malondialdehyde in its leaves. Forner-Alcaide nº 5 had an intermediate tolerance, similar to Cleopatra mandarin than to Carrizo citrange, as per the accumulation of B in its leaves. Forner-Alcaide nº 5 was the only genotype whose root was not affected by B toxicity.
Boron (B) toxicity frequently affects plant performances and productivity, especially in arid and semi-arid environments. In this experiment, loquat seedlings were subjected to 25 μM (control) or 400 μM B (B excess) to test the hypothesis that (i) B alters sugar/polyol metabolism in polyol-producing tree species as loquat and (ii) changes of leaf and stem anatomy assist young tissues against toxic effect of B. Gas exchange was monitored from the beginning of the experiment (FBE) till one week after the first visible symptoms of B toxicity appeared in the upper part of the stems (147 d FBE). At 147 FBE, plant biometric parameters and pattern of B accumulation, leaf and stem anatomy, chlorophyll a fluorescence kinetics as well as biochemical measurements were assessed in top (asymptomatic) leaves and upper stem bark. Boron accumulated principally (in the row) in top leaves > top bark > top wood in B-stressed plants, but no changes in allocation pattern were found between controls and B-stressed plants. Excess B promoted the increase in the spongy layer of top leaves and caused the development of cork and numerous collenchyma cells with increased cell wall thickness. This mechanism, which has never been described before, can be considered an attempt to store excessive B in tissues where B ions are less harmful. The accumulation of sorbitol (B-complexing polyol) in top leaves and stem bark can be considered as a further attempt to detoxify B excess. However, B toxicity drastically affects the photosynthetic rate of top leaves, mainly due to non-stomatal limitations, i.e., reduction of ambient CO2 use efficiency and of photosystem II (PSII) efficiency, modification of the partitioning excess energy dissipation in PSII, thus leading to an increased level of lipid peroxidation. Our results suggest that changes in sugar metabolism associated with leaf and stem bark thickening partially assist (but not totally preserve) young tissues of loquat plants under B stress.