Brassinosteroids Alleviate Heat-Induced Inhibition of Photosynthesis by Increasing Carboxylation Efficiency and Enhancing Antioxidant Systems in <Emphasis Type="Italic">Lycopersicon esculentum</Emphasis>

To investigate the effects of exogenously applied brassinosteroids on the thermotolerance of plants, leaf CO₂ assimilation, chlorophyll fluorescence parameters, and antioxidant enzyme metabolism were examined in tomato (Lycopersicon esculentum Mill. cv. 9021) plants with or without 24-epibrassinolide (EBR) application. Tomato plants were exposed to 40/30°C for 8 days and then returned to optimal conditions for 4 days. High temperature significantly decreased the net photosynthetic rate (P _n), stomatal conductance (G _s), and maximum carboxylation rate of Rubisco (V _cmax), the maximum potential rate of electron transport contributed to ribulose-1,5-bisphosphate (RuBP), as well as the relative quantum efficiency of PSII photochemistry (Ф_PSII), photochemical quenching (q _P), and increased nonphotochemical quenching (NPQ). However, only slight reversible photoinhibition occurred during heat stress. Interestingly, EBR pretreatment significantly alleviated high-temperature-induced inhibition of photosynthesis. The activities of antioxidant enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPOD), and catalase (CAT) increased during heat treatments, and these increases proved to be more significant in EBR-treated plants. EBR application also reduced total hydrogen peroxide (H₂O₂) and malonaldehyde (MDA) contents, while significantly increasing shoot weight following heat stress. It was concluded that EBR could alleviate the detrimental effects of high temperatures on plant growth by increasing carboxylation efficiency and enhancing antioxidant enzyme systems in leaves.     

24-epibrassinolide improves cucumber photosynthesis under hypoxia by increasing CO2 assimilation and photosystem II efficiency

Seedlings of the hypoxia-sensitive cucumber cultivar were hydroponically grown under hypoxia for 7 d in the presence or absence of 24-epibrassinolide (EBR, 2.1 nM). Hypoxia significantly inhibited growth, while EBR partially counteracted this inhibition. Leaf net photosynthetic rate (P _N), stomatal conductance, transpiration rate, and water-use efficiency declined greatly, while the stomatal limitation value increased significantly. The maximum net photosynthetic rate was strongly reduced by hypoxia, indicating that stomatal limitation was not the only cause of the P _N decrease. EBR markedly diminished the harmful effects of hypoxia on P _N as well as on stomata openness. It also greatly stimulated CO₂ fixation by the way of increasing the carboxylation capacity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), ribulose-1,5-bisphosphate regeneration, Rubisco activity, and the protection of Rubisco large subunit from degradation. Our data indicated that photosystem (PS) II was damaged by hypoxia, while EBR had the protective effect. EBR further increased nonphotochemical quenching that could reduce photodamage of the PSII reaction center. The proportion of absorbed light energy allocated for photochemical reaction (P) was reduced, while both nonphotochemical reaction dissipation of light energy and imbalanced partitioning of excitation energy between PSI and PSII increased. EBR increased P and alleviated this imbalance. The results suggest that both stomatal and nonstomatal factors limited the photosynthesis of cucumber seedlings under hypoxia. EBR alleviated the growth inhibition by improving CO₂ asimilation and protecting leaves against PSII damage.     

Overexpression of the rubisco activase gene improves growth and low temperature and weak light tolerance in Cucumis sativus

Rubisco activase (RCA) is an important enzyme that can catalyze the carboxylation and oxygenation activities of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), which is involved in the photosynthetic carbon reduction cycle. Here, we studied the effects of changes in RCA activity on photosynthesis, growth and development, as well as the low temperature and weak light tolerance of RCA overexpressing transgenic cucumber (Cucumis sativus) plants. CsRCA overexpression increased the plant height, leaf area and dry matter, and decreased the root/top ratio in transgenic cucumber plants compared with the wild‐type (WT) plants. Low temperature and low light stress led to decreases in the CsRCA expression and protein levels, the photosynthetic rate (Pn) and the stomatal conductance (Gs), but an increase in the intercellular CO₂ (Ci) concentration in cucumber leaves. The actual photochemical efficiency and maximal photochemical efficiency of photosystem II in cucumber seedlings also declined, but the initial fluorescence increased during low temperature and weak light stress. Transgenic plants showed a lower decrease in the CsRCA expression level and actual and maximal photochemical efficiencies, as well as increases in the Ci and initial fluorescence relative to the WT plants. Low temperature and low light stress resulted in a significant increase in the malondialdehyde (MDA) content; however, this increase was reduced in transgenic plants compared with that in WT plants. Thus, the overexpression of CsRCA may promote the growth and low temperature and low light tolerance of cucumber plants in solar greenhouses.     

Chill-induced inhibition of photosynthesis was alleviated by 24-epibrassinolide pretreatment in cucumber during chilling and subsequent recovery

To investigate whether brassinosteroids (BRs) could be used to alleviate chill-induced inhibition of photosynthesis in cucumber (Cucumis sativus L) during chilling and subsequent recovery, the effects of exogenously applied 24-epibrassinolide (EBR) on gas exchange, chlorophyll fluorescence parameters, and antioxidant enzyme activity were studied. Cucumber plants were exposed to chilling under low light (12/8°C and 100 μmol m⁻² s⁻¹ PPFD) for 3 days and then recovered under normal temperature and high irradiance (28/18°C and 600 μmol m⁻² s⁻¹ PPFD) for 6 days. Chilling significantly decreased the net photosynthetic rate (P _N) and stomatal conductance (g _s), and increased rate of O₂ ^·− formation and H₂O₂ and malondialdehyde (MDA) content in cucumber leaves, but did not influence the optimal quantum yield of PSII (F_v/F_m). Chilling also decreased the effective quantum yield of PSII photochemistry (Φ_PSII) and photochemical quenching (q_P), but induced an increase in nonphotochemical quenching (NPQ), and the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX). High irradiance (600 μmol m⁻² s⁻¹) further aggravated the decrease in P _N, g _s, Φ_PSII and q_P, and enhanced the increase in reactive oxygen species (ROS) generation and accumulation in the first day of recovery after chilling. However, high irradiance induced a sharp decrease in F_v/F_m and NPQ, as well as the activities of SOD and APX on the first day of recovery. EBR pretreatment significantly alleviated chill-induced inhibition of photosynthesis during chilling stress and subsequent recovery period, which was mainly due to significant increases in g _s, Φ_PSII, q_P and NPQ. EBR pretreatment also reduced ROS generation and accumulation, and increased the activities of SOD and APX during chilling and subsequent recovery. Those results suggest that EBR pretreatment alleviates the chill reduction in photosynthesis and accelerated the recovery rate mainly by increasing of the stomatal conductance, the efficiency of utilization and dissipation of leaf absorbed light, and the activity of the ROS scavenging system during chilling and subsequent recovery period.     

Responses of leaf photosynthesis,pigments and chlorophyll fluorescence within canopy position in a boreal grass (<Emphasis Type="Italic">Phalaris arundinacea</Emphasis> L.) to elevated temperature and CO<Subscript>2</Subscript> under varying water regimes

The effects of elevated growth temperature (ambient + 3.5°C) and CO₂ (700 μmol mol⁻¹) on leaf photosynthesis, pigments and chlorophyll fluorescence of a boreal perennial grass (Phalaris arundinacea L.) under different water regimes (well watered to water shortage) were investigated. Layer-specific measurements were conducted on the top (younger leaf) and low (older leaf) canopy positions of the plants after anthesis. During the early development stages, elevated temperature enhanced the maximum rate of photosynthesis (P _max) of the top layer leaves and the aboveground biomass, which resulted in earlier senescence and lower photosynthesis and biomass at the later periods. At the stage of plant maturity, the content of chlorophyll (Chl), leaf nitrogen (N_L), and light response of effective photochemical efficiency (Φ_PSII) and electron transport rate (ETR) was significantly lower under elevated temperature than ambient temperature in leaves at both layers. CO₂ enrichment enhanced the photosynthesis but led to a decline of N_L and Chl content, as well as lower fluorescence parameters of Φ_PSII and ETR in leaves at both layers. In addition, the down-regulation by CO₂ elevation was significant at the low canopy position. Regardless of climate treatment, the water shortage had a strongly negative effect on the photosynthesis, biomass growth, and fluorescence parameters, particularly in the leaves from the low canopy position. Elevated temperature exacerbated the impact of water shortage, while CO₂ enrichment slightly alleviated the drought-induced adverse effects on P _max. We suggest that the light response of Φ_PSII and ETR, being more sensitive to leaf-age classes, reflect the photosynthetic responses to climatic treatments and drought stress better than the fluorescence parameters under dark adaptation.     

Foliar application of 24-epibrassinolide alleviates high-temperature-induced inhibition of photosynthesis in seedlings of two melon cultivars

Brassinosteroids (BRs), an important class of plant steroidal hormones, play a significant role in the amelioration of various biotic and abiotic stresses. 24-epibrassinolide (EBR), an active brassinosteroid, was applied exogenously in different concentrations to characterize a role of BRs in tolerance of melon (Cucumis melo L.) to high temperature (HT) stress and to investigate photosynthetic performance of HT-stressed, Honglvzaocui (HT-tolerant) and Baiyuxiang (HTsensitive), melon variety. Under HT, Honglvzaocui showed higher biomass accumulation and a lower index of heat injury compared with the Baiyuxiang. The exogenous application of 1.0 mg L^?1 EBR, the most effective concentration, alleviated dramatically the growth suppression caused by HT in both ecotypes. Similarly, EBR pretreatment of HTstressed plants attenuated the decrease in relative chlorophyll content, net photosynthetic rate, stomatal conductance, stomatal limitation, and water-use efficiency (WUE), as well as the maximal quantum yield of PSII photochemistry (F_v/F_m), the efficiency of excitation capture of open PSII center, the effective quantum yield of PSII photochemistry (Φ_PSII), photochemical quenching coefficient, and the photon activity distribution coefficients of PSI (α). EBR pretreatment further inhibited the increase in intracellular CO₂ concentration, leaf transpiration rate, minimal fluorescence of dark-adapted state, nonphotochemical quenching, thermal dissipation, and photon activity distribution coefficients of PSII. Results obtained here demonstrated that EBR could alleviate the detrimental effects of HT on the plant growth by improving photosynthesis in leaves, mainly reflected as up-regulation of photosynthetic pigment contents and photochemical activity associated with PSI.     

<Emphasis Type="Bold">Role of microRNAs and their target genes in salinity response in plant</Emphasis>s

We examined the effects of 2,4-epibrassinolide (EBR) application on photosynthesis, antioxidant enzyme activity, and Rubisco activase (RCA) gene expression in wheat (Triticum aestivum L.) seedlings under a combination of drought and heat stress. The net photosynthetic rates (P_n) of wheat seedlings decreased significantly, the photosynthetic capability was inhibited, and the activities of superoxide (SOD), peroxidase (POD), catalase (CAT), and RCA as well as the initial and total activity of Rubisco declined under the combined stress. These decreases and inhibitory effects were significantly ameliorated by exogenous EBR application. Three subunits (45–46, 41–42, and 38–39 kDa) of RCA were observed in wheat seedlings. The abundances of the 38–39 kDa and 41–42 kDa subunits were significantly lower in plants subjected to stressful conditions than in unstressed plants. Interestingly, a marked increase in 45–46 kDa RCA was observed under heat or heat combined with drought stress. The abundance of 38–39 kDa RCA in seedlings exposed to heat, drought, or their combination was significantly enhanced by EBR pretreatment, which paralleled the changes in initial Rubisco activity and P_n, but was not consistent with observed mRNA abundance. These results indicated that the larger subunit of RCA (45–46 kDa), which is more thermostable and increased in response to moderate heat stress, and the smaller isoform (38–39 kDa) of RCA may play important roles in maintaining the photosynthetic capability by EBR under stress conditions.     

Will rising CO2 protect plants from the midday sun? A study of photoinhibition of Quercus myrtifolia in a scrub‐oak community in two seasons

Over a large part of the photoperiod, light energy absorbed by upper canopy leaves saturates photosynthesis and exceeds the energetic requirements for light‐saturated linear electron flow through photosystem II (J_PSII), so that photoinhibition results. From a theoretical consideration of the response of light‐saturated photosynthesis to elevated atmospheric CO₂ partial pressure (pCO₂) it may be predicted that, where light‐saturated photosynthesis is Rubisco‐limited, an increase in pCO₂ will stimulate J_PSII. Therefore, the proportion of absorbed quanta dissipated photochemically will increase and the potential for photoinhibition of photosynthesis will decrease. This was tested by measuring modulated chlorophyll a fluorescence from Quercus myrtifolia Willd. growing in the field in open‐top chambers, at either current ambient or elevated (ambient + 35 Pa) pCO₂ on Merritt Island, Florida, USA. During spring and summer, light‐saturated photosynthesis at current ambient pCO₂ was Rubisco‐limited. Consistent with theoretical prediction, J_PSII was increased and photoinhibition decreased by elevated pCO₂ in spring. In the summer, when growth had largely ceased, an acclimatory decrease in the maximum Ribulose 1,5 bisphosphate saturated carboxylation capacity (V_{c max}) removed the stimulation of J_PSII seen in the spring, and photoinhibition was increased in elevated pCO₂. It is concluded that, for Q. myrtifolia growing in the field, the effects of elevated pCO₂ on J_PSII and photoinhibition will reflect seasonal differences in photosynthetic acclimation to elevated pCO₂ in a predictable manner.     

Mechanisms underlying leaf photosynthetic acclimation to warming and elevated CO2 as inferred from least‐cost optimality theory

The mechanisms responsible for photosynthetic acclimation are not well understood, effectively limiting predictability under future conditions. Least‐cost optimality theory can be used to predict the acclimation of photosynthetic capacity based on the assumption that plants maximize carbon uptake while minimizing the associated costs. Here, we use this theory as a null model in combination with multiple datasets of C₃ plant photosynthetic traits to elucidate the mechanisms underlying photosynthetic acclimation to elevated temperature and carbon dioxide (CO₂). The model‐data comparison showed that leaves decrease the ratio of the maximum rate of electron transport to the maximum rate of Rubisco carboxylation (J_max/V_cmax) under higher temperatures. The comparison also indicated that resources used for Rubisco and electron transport are reduced under both elevated temperature and CO₂. Finally, our analysis suggested that plants underinvest in electron transport relative to carboxylation under elevated CO₂, limiting potential leaf‐level photosynthesis under future CO₂ concentrations. Altogether, our results show that acclimation to temperature and CO₂ is primarily related to resource conservation at the leaf level. Under future, warmer, high CO₂ conditions, plants are therefore likely to use less nutrients for leaf‐level photosynthesis, which may impact whole‐plant to ecosystem functioning.     

Grafting of <Emphasis Type="Italic">Cucumis sativus</Emphasis> onto <Emphasis Type="Italic">Cucurbita ficifolia</Emphasis> leads to improved plant growth,increased light utilization and reduced accumulation of reactive oxygen species in chilled plants

The effects of chilling at 14 and 7°C on plant growth, CO₂ assimilation, light allocation, photosynthetic electron flux and antioxidant metabolism were examined in cucumber (Cucumis sativus L. cv. Jinyan No. 4, CS) plants with figleaf gourd (Cucurbita ficifolia Bouché, CF) and cucumber as rootstocks, respectively. Growth inhibition by chilling at 7°C was characterized by irreversible inhibition of CO₂ assimilation in grafted plants with cucumber as rootstock and scion (CS/CS) but this effect was significantly alleviated by grafting onto CF roots (CS/CF). Chilled CS/CF plants exhibited a higher photosynthetic activity and lower proportion of energy dissipation than chilled CS/CS plants. Chilling resulted in a greater decrease in the electron flux in photosystem (PS) II (J _PSII) than the rate of energy dissipation either via light-dependent (J _NPQ) or via constitutive thermal dissipation and fluorescence (J _f,D) in CS/CS plants. In parallel with the reduction in J _PSII, electron flux to oxygenation (J _o) and carboxylation by Rubisco (J _c) all decreased significantly whilst alternative electron flux in PS II (J _a) increased, especially in CS/CS plants. Moreover, CS/CF plants exhibited higher activity of antioxidant enzymes, lower antioxidant content and less membrane peroxidation relative to CS/CS plants after chilling.     

Effects of light quality on the chloroplastic ultrastructure and photosynthetic characteristics of cucumber seedlings

To understand how light quality influences plant photosynthesis, we investigated chloroplastic ultrastructure, chlorophyll fluorescence and photosynthetic parameters, Rubisco and chlorophyll content and photosynthesis-related genes expression in cucumber seedlings exposed to different light qualities: white, red, blue, yellow and green lights with the same photosynthetic photon flux density of 100 μmol m^?2 s^?1. The results revealed that plant growth, CO₂ assimilation rate and chlorophyll content were significantly reduced in the seedlings grown under red, blue, yellow and green lights as compared with those grown under white light, but each monochromatic light played its special role in regulating plant morphogenesis and photosynthesis. Seedling leaves were thickened and slightly curled; Rubisco biosynthesis, expression of the rca, rbcS and rbcL, the maximal photochemical efficiency of PSII (Fv/Fm) and quantum yield of PSII electron transport (Ф_PSII) were all increased in seedlings grown under blue light as compared with those grown under white light. Furthermore, the photosynthetic rate of seedlings grown under blue light was significantly increased, and leaf number and chlorophyll content of seedlings grown under red light were increased as compared with those exposed to other monochromatic lights. On the contrary, the seedlings grown under yellow and green lights were dwarf with the new leaves etiolated. Moreover, photosynthesis, Rubisco biosynthesis and relative gene expression were greatly decreased in seedlings grown under yellow and green light, but chloroplast structural features were less influenced. Interestingly, the Fv/Fm, Ф_PSII value and chlorophyll content of the seedlings grown under green light were much higher than those grown under yellow light.     

Antisense RNA inhibition of Rubisco activase expression

Ribulose bisphosphate carboxylase (Rubisco) activase catalyzes the activation of Rubisco in vivo. Activase antisense DNA mutants of tobacco have been generated to explore the control that activase exerts on the photosynthetic process. These mutants have up to 90% reductions in activase protein levels as a consequence of an inhibition of activase mRNA accumulation. It is shown that photosynthesis, measured as the rate of CO₂ exchange (CER), is modestly decreased in plants exposed to high irradiances. The decreases in CER in the transgenic plants are accompanied by corresponding decreases in Rubisco activation, indicating that activase has a direct effect on photosynthetic rates in the antisense plants by influencing the activation state of Rubisco. It is concluded that in high light conditions, control of photosynthesis is largely shared between Rubisco and activase. Plant growth is also impaired in mutant plants that have severe reductions in activase. The inhibition of activase in the antisense plants does not have an impact on the accumulation of Rubisco large subunit or small subunit mRNAs or proteins. This indicates that the concerted expression of the genes for activase (Rca) and Rubisco (rbcL and rbcS) in response to light, developmental factors and circadian controls is not due to feedback regulation of rbcL or rbcS by the amount of activase protein.     

Physiological Roles of Brassinosteroids in Early Growth of <Emphasis Type="Italic">Arabidopsis:</Emphasis> Brassinosteroids Have a Synergistic Relationship with Gibberellin as well as Auxin in Light-Grown Hypocotyl Elongation

We examined the physiological effects of brassinosteroids (BRs) on early growth of Arabidopsis. Brassinazole (Brz), a BR biosynthesis inhibitor, was used to elucidate the significance of endogenous BRs. It inhibited growth of roots, hypocotyls, and cotyledonous leaf blades dose-dependently and independent of light conditions. This fact suggests that endogenous BRs are necessary for normal growth of individual organs of Arabidopsis in both photomorphogenetic and skotomorphogenetic programs. Exogenous brassinolide (BL) promoted hypocotyl elongation remarkably in light-grown seedlings. Cytological observation disclosed that BL-induced hypocotyl elongation was achieved through cell enlargement rather than cell division. Furthermore, a serial experiment with hormone inhibitors showed that BL induced hypocotyl elongation not through gibberellin and auxin actions. However, a synergistic relationship of BL with gibberellin A₃ (GA₃) and indole-3-acetic acid (IAA) was observed on elongation growth in light-grown hypocotyls, even though gibberellins have been reported to be additive to BR action in other plants. Taken together, our results show that BRs play an important role in the juvenile growth of Arabidopsis; moreover, BRs act on light-grown hypocotyl elongation independent of, but cooperatively with, gibberellins and auxin.     

Brassinosteroids improve photosystem II efficiency,gas exchange,antioxidant enzymes and growth of cowpea plants exposed to water deficit

Water deficit is considered the main abiotic stress that limits agricultural production worldwide. Brassinosteroids (BRs) are natural substances that play roles in plant tolerance against abiotic stresses, including water deficit. This research aims to determine whether BRs can mitigate the negative effects caused by water deficiency, revealing how BRs act and their possible contribution to increased tolerance of cowpea plants to water deficit. The experiment was a factorial design with the factors completely randomised, with two water conditions (control and water deficit) and three levels of brassinosteroids (0, 50 and 100 nM 24-epibrassinolide; EBR is an active BRs). Plants sprayed with 100 nM EBR under the water deficit presented significant increases in Φ_PSII, q_P and ETR compared with plants subjected to the water deficit without EBR. With respect to gas exchange, P _N, E and g _s exhibited significant reductions after water deficit, but application of 100 nM EBR caused increases in these variables of 96, 24 and 33%, respectively, compared to the water deficit + 0 nM EBR treatment. To antioxidant enzymes, EBR resulted in increases in SOD, CAT, APX and POX, indicating that EBR acts on the antioxidant system, reducing cell damage. The water deficit caused significant reductions in Chl a, Chl b and total Chl, while plants sprayed with 100 nM EBR showed significant increases of 26, 58 and 33% in Chl a, Chl b and total Chl, respectively. This study revealed that EBR improves photosystem II efficiency, inducing increases in Φ_PSII, q_P and ETR. This substance also mitigated the negative effects on gas exchange and growth induced by the water deficit. Increases in SOD, CAT, APX and POX of plants treated with EBR indicate that this steroid clearly increased the tolerance to the water deficit, reducing reactive oxygen species, cell damage, and maintaining the photosynthetic pigments. Additionally, 100 nM EBR resulted in a better dose–response of cowpea plants exposed to the water deficit.     

The rapid A–Ci response: photosynthesis in the phenomic era

Phenotyping for photosynthetic gas exchange parameters is limiting our ability to select plants for enhanced photosynthetic carbon gain and to assess plant function in current and future natural environments. This is due, in part, to the time required to generate estimates of the maximum rate of ribulose‐1,5‐bisphosphate carboxylase oxygenase (Rubisco) carboxylation (V_c,max) and the maximal rate of electron transport (J_max) from the response of photosynthesis (A) to the CO₂ concentration inside leaf air spaces (C_i). To relieve this bottleneck, we developed a method for rapid photosynthetic carbon assimilation CO₂ responses [rapid A–C_i response (RACiR)] utilizing non‐steady‐state measurements of gas exchange. Using high temporal resolution measurements under rapidly changing CO₂ concentrations, we show that RACiR techniques can obtain measures of V_c,max and J_max in ~5 min, and possibly even faster. This is a small fraction of the time required for even the most advanced gas exchange instrumentation. The RACiR technique, owing to its increased throughput, will allow for more rapid screening of crops, mutants and populations of plants in natural environments, bringing gas exchange into the phenomic era.     

Role of H2O2 dynamics in brassinosteroid‐induced stomatal closure and opening in Solanum lycopersicum

Brassinosteroids (BRs) are essential for plant growth and development; however, their roles in the regulation of stomatal opening or closure remain obscure. Here, the mechanism underlying BR‐induced stomatal movements is studied. The effects of 24‐epibrassinolide (EBR) on the stomatal apertures of tomato (Solanum lycopersicum) were measured by light microscopy using epidermal strips of wild type (WT), the abscisic acid (ABA)‐deficient notabilis (not) mutant, and plants silenced for SlBRI1, SlRBOH1 and SlGSH1. EBR induced stomatal opening within an appropriate range of concentrations, whereas high concentrations of EBR induced stomatal closure. EBR‐induced stomatal movements were closely related to dynamic changes in H₂O₂ and redox status in guard cells. The stomata of SlRBOH1‐silenced plants showed a significant loss of sensitivity to EBR. However, ABA deficiency abolished EBR‐induced stomatal closure but did not affect EBR‐induced stomatal opening. Silencing of SlGSH1, the critical gene involved in glutathione biosynthesis, disrupted glutathione redox homeostasis and abolished EBR‐induced stomatal opening. The results suggest that transient H₂O₂ production is essential for poising the cellular redox status of glutathione, which plays an important role in BR‐induced stomatal opening. However, a prolonged increase in H₂O₂ facilitated ABA signalling and stomatal closure.