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.     

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.     

Brassinosteroids protect photosynthesis and antioxidant system of eggplant seedlings from high-temperature stress

The effects of 24-epibrassinolide under high temperature in eggplant (Solanum melongena L.) seedlings were studied by investigating the plant growth, chlorophyll content, photosynthesis and antioxidant systems. High temperature significantly inhibited the plant growth and markedly decreased the chlorophyll content, net photosynthetic rate, stomatal conductance and transpiration rate, while it increased intercellular CO₂ concentration. In a similar manner, high temperature also decreased significantly maximum quantum efficiency of PSII, potential photochemical efficiency, the quantum efficiency of PSII, photochemical quenching, the excitation capture efficiency of open centers, and increased non-photochemical quenching. Application of 0.05–0.2 μM EBR remarkably promoted the plant growth and alleviated high-temperature-induced inhibition of photosynthesis. Under high temperature, reactive oxygen species levels and lipid peroxidation were markedly increased, which were remarkably inhibited by application of 0.05–0.2 μM EBR. The activities of antioxidative enzymes such as superoxide dismutase, peroxidase, catalase and ascorbate peroxidase, and contents of ascorbic acid and reduced glutathione were significantly increased during high-temperature treatments, and these increases were more pronounced than those of EBR at 0.05–0.2 μM treatment. The EBR treatment also greatly enhanced contents of proline, soluble sugar and protein under high-temperature stress. Taken together, it can be concluded that 0.05–0.2 μM EBR could alleviate the detrimental effects of high temperatures on plant growth by increasing photosynthetic efficiency and enhancing antioxidant enzyme systems. Addition of 0.1 μM EBR had the best ameliorative effect against high temperature, while the addition of 0.4 μM EBR had no significant effects.     

Effect of exogenous 24-epibrassinolide on chlorophyll fluorescence,leaf surface morphology and cellular ultrastructure of grape seedlings (<Emphasis Type="Italic">Vitis vinifera</Emphasis> L.) under water stress

The effects of 24-epibrassinolide (EBR) on chlorophyll fluorescence, leaf surface morphology and cellular ultrastructure of grape seedlings (Vitis vinifera L.) under water stress were investigated. The grape seedlings were subjected to 10 % (w/v) polyethylene glycol (PEG-6000) and treated with 0.05, 0.10 or 0.20 mg L^?1 EBR, respectively. EBR application increased chlorophyll contents, the effective photochemical quantum yield of PSII, maximum photochemical efficiency of PSII, maximal fluorescence and non-photochemical quenching coefficient under water stress in each concentration. Compared with water stress control, higher stomatal density and stomatal length were observed in young leaves under EBR treatments, but not in mature leaves. In-depth analysis of the ultrastructure of leaves indicated that water stress induced disappearance of nucleus, chloroplast swelling, fractured mitochondrial cristae and disorder of thylakoid arrangement both in young leaves and mature leaves. However, EBR application counteracted the detrimental effects of water stress on the structure of the photosynthetic apparatus better in young leaves than in mature leaves. Compared to the other treatments, treatment of 0.10 mg L^?1 EBR had best ameliorative effect against water stress. These results suggested that exogenous EBR could alleviate water stress-induced inhibition of photosynthesis on grape possibly through increasing chlorophyll content, lessening the stomatal and non-stomatal limitation of photosynthesis performance.     

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-表油菜素内酯对干旱胁迫下辣椒叶片快速叶绿素荧光诱导动力学曲线的影响

以辣椒品种“超辣九号”为试材,采用15%的PEG6000模拟干旱,研究了0.1μmol·L^-1外源24-表油菜素内酯(EBR)处理对干旱胁迫下辣椒叶片快速叶绿素荧光诱导动力学曲线(OJIP)的影响。结果表明:干旱胁迫降低了辣椒叶片的光化学效率和光合性能,导致干旱光抑制的发生。干旱胁迫既损伤了辣椒叶片PSⅡ供体侧放氧复合体(OEC),同时也对PSⅡ反应中心和受体侧造成伤害,阻碍了光合电子传递;干旱胁迫还导致单位叶面积有活性反应中心数目(RC/CS)的下降,并降低了单位叶面积吸收的光能(ABS/CS)、捕获的光能(TRo/CS)和进行电子传递的能量(ETo/CS),同时诱导了单位叶面积热耗散(DIo/CS)的增加。这说明辣椒遭受干旱胁迫后启动了相应的防御机制,一方面通过PSⅡ的可逆失活减少光能吸收与传递,另一方面通过促进热耗散减少过剩激发能的积累。EBR处理改善了干旱胁迫下辣椒叶片PSⅡ受体侧的电子传递,缓解了单位叶面积有活性反应中心数目的减少,优化了光合电子传递的进行,并维持相对较高的热耗散能力,从而减轻了干旱光抑制程度,对干旱胁迫下辣椒叶片光合机构和光合性能起到保护作用。     

Response of photosynthesis to high light and drought for Arabidopsis thaliana grown under a UV-B enhanced light regime

Arabidopsis thaliana grown in a light regime that included ultraviolet-B (UV-B) radiation (6 kJ m⁻² d⁻¹) had similar light-saturated photosynthetic rates but up to 50% lower stomatal conductance rates, as compared to plants grown without UV-B radiation. Growth responses of Arabidopsis to UV-B radiation included lower leaf area (25%) and biomass (10%) and higher UV-B absorbing compounds (30%) and chlorophyll content (52%). Lower stomatal conductance rates for plants grown with UV-B radiation were, in part, due to lower stomatal density on the adaxial surface. Plants grown with UV-B radiation had more capacity to down regulate photochemical efficiency of photosystem II (PSII) as shown by up to 25% lower φ_PSII and 30% higher non-photochemical quenching of chlorophyll fluorescence under saturating light. These contributed to a smaller reduction in the maximum photochemical efficiency of PSII (F _v/F _m), greater dark-recovery of F _v/F _m, and higher light-saturated carbon assimilation and stomatal conductance and transpiration rates after a four-hour high light treatment for plants grown with UV-B radiation. Plants grown with UV-B were more tolerant to a 12 day drought treatment than plants grown without UV-B as indicated by two times higher photosynthetic rates and 12% higher relative water content. UV-B-grown plants also had three times higher proline content. Higher tolerance to drought stress for Arabidopsis plants grown under UV-B radiation may be attributed to both increased proline content and decreased stomatal conductance. Growth of Arabidopsis in a UV-B-enhanced light regime increased tolerance to high light exposure and drought stress.     

Exogenous 24-epibrassinolide ameliorates high temperature-induced inhibition of growth and photosynthesis in Cucumis melo

This study was carried out to better understand the role of 24-epibrassinolide (EBR) in thermotolerance of melon (Cucumis melo L.). The melon seedlings were pretreated with various concentrations of EBR (0, 0.05, 0.1, 0.5, 1.0, and 1.5 mg dm^?3) as foliar spray and then exposed to a high temperature (HT) stress. Exogenous EBR (0.5–1.5 mg dm^?3) alleviated HT-caused growth suppression. In parallel, 1.0 mg dm^?3 EBR attenuated the decrease in chlorophyll content, net photosynthetic rate, stomatal conductance, maximum quantum efficiency of photosystem (PS) II, quantum yield of PS II, and photochemical quenching of chlorophyll a fluorescence in HT-stressed plants, and inhibited transpiration rate and non-photochemical quenching. Furthermore, exogenous EBR also significantly reduced the content of malondialdehyde (MDA) and increased the content of soluble proteins and free proline, and activities of antioxidant enzymes including superoxide dismutase, guaiacol peroxidase, catalase, and ascorbate peroxidase under the HT stress. The results show that protective effects of EBR against the HT stress in the melon seedlings were most likely mediated through the improvement of photosynthesis and the stimulation of antioxidant capacity.     

Differential photosynthetic performance of three Mediterranean shrubs under grazing by domestic goats

The impact of grazing by domestic goats, Capra hircus, on the photochemical apparatus of three co-ocurring Mediterranean shrubs, Erica scoparia, Halimium halimifolium, and Myrtus communis was evaluated. Seasonal course of gas exchange, chlorophyll fluorescence and photosynthetic pigment concentrations were measured in the field in grazed and ungrazed plants. Net photosynthetic rate was higher in grazed plants of E. scoparia and H. halimifolium in May, while there were not significant differences in M. communis. Photosynthetic enhancement in grazed plants of E. scoparia could be explained largely by higher stomatal conductance. On the other hand, the lack of differences in stomatal conductance between grazed and ungrazed plants of H. halimifolium could indicate that carboxylation efficiency, and ribulose-1,5-bisphosphate (RuBP) regeneration may have been enhanced by grazing. Overall grazing has little effect on the photochemical (PSII) apparatus, however grazed plants of M. communis showed chronic photoinhibition in the short term. Finally, seasonal variations recorded on photosynthesis, photochemical efficiency and pigment concentrations may be a physiological consequence of environmental factors, such as summer drought and competition for light, rather than an adaptation to grazing.     

Effects of salt stress on photosynthesis,PSII photochemistry and thermal energy dissipation in leaves of two corn (<Emphasis Type="Italic">Zea mays</Emphasis> L.) varieties

The effect of four different NaCl concentrations (from 0 to 102 mM NaCl) on seedlings leaves of two corn (Zea mays L.) varieties (Aristo and Arper) was investigated through chlorophyll (Chl) a fluorescence parameters, photosynthesis, stomatal conductance, photosynthetic pigments concentration, tissue hydration and ionic accumulation. Salinity treatments showed a decrease in maximal efficiency of PSII photochemistry (F_v/F_m) in dark-adapted leaves. Moreover, the actual PSII efficiency (ϕ_PSII), photochemical quenching coefficient (q_p), proportion of PSII centers effectively reoxidized, and the fraction of light used in PSII photochemistry (%P) were also dropped with increasing salinity in light-adapted leaves. Reductions in these parameters were greater in Aristo than in Arper. The tissue hydration decreased in salt-treated leaves as did the photosynthesis, stomatal conductance (g _s) and photosynthetic pigments concentration essentially at 68 and 102 mM NaCl. In both varieties the reduction of photosynthesis was mainly due to stomatal closure and partially to PSII photoinhibition. The differences between the two varieties indicate that Aristo was more susceptible to salt-stress damage than Arper which revealed a moderate regulation of the leaf ionic accumulation.     

The effect of CaCl<Subscript>2</Subscript> on calcium content,photosynthesis, and chlorophyll fluorescence of tung tree seedlings under drought conditions

The study investigated the effects of different CaCl₂ concentrations (2, 5, and 10 mM) on photosynthetic enzymatic activities, photosynthesis, and chlorophyll fluorescence of tung tree seedlings under drought conditions. Plants were sprayed with either CaCl₂ or distilled water until run-off. Irrigation was then withheld to induce drought stress. The strength of drought stress was evaluated by relative leaf water content and soil water content, which was 27.3 and 9.5% on day 0 and day 12, respectively. Drought stress decreased activities of ribulose-1,5-bisphosphate carboxylase/oxygenase and phosphoenolpyruvate carboxylase, chlorophyll (a+b) content, net photosynthetic rate, stomatal conductance, transpiration rate, electron transport rate, the maximal quantum yield of PSII photochemistry, and effective quantum yield of PSII in tung tree seedlings. The CaCl₂ pretreatments alleviated the negative effect of drought stress to some degree on all the parameters mentioned above.     

The interacting effect of urea and fenoxaprop-P-ethyl on photosynthesis and chlorophyll fluorescence in Perilla frutescens

We studied the effect of herbicide and nitrogen supply on photosynthesis in Perilla frutescens L. Britt. Plants were exposed to combined treatment of urea and herbicide, fenoxaprop-P-ethyl (FPE), in various concentrations. FPE reduced significantly chlorophyll (Chl) content, photosynthetic rate, and stomatal conductance, but increased significantly intercellular CO₂ concentration; thus, FPE inhibited significantly the photosynthetic capacity. In addition, FPE also decreased significantly the PSII photochemical efficiency, effective quantum yield of photochemical energy conversion in PSII, PSII potential activity, and photochemical quenching of variable Chl fluorescence. It also decreased nonphotochemical quenching. It indicated that FPE impaired PSII and blocked the electron transport in light reaction. The urea treatment at moderate concentration (1–4 g L^?1) could antagonize the negative effect of FPE, while the high urea concentration (8 g L^?1) aggravated this effect. The treatment with urea (4 g L^?1) and then with FPE (1.33 mL L^?1) enhanced Chl content index, photosynthetic rate, and stomatal conductance by 12.5, 36.1, and 28.5% compared to FPE treatment alone. Thus, we suggested to treat plants first with urea (4 g L^?1) and then by FPE (1.33 mL L^?1) as the best and the safest method to balance the fertilization and weeding.     

Physiological behaviors of <Emphasis Type="Italic">Acer mono</Emphasis> under drought and low light

The seedlings of Acer mono Maxim. were exposed to two watering regimes (well watered (100% of field capacity) and drought (30% of field capacity)) and two light levels (high light (100% of full sunlight) and low light (15% of full sunlight)) in a greenhouse to assess growth, photosynthesis, and foliar nutrient traits of woody plants under drought and low light. Drought significantly reduced growth and gas exchange characteristics of A. mono, including net photosynthetic rate (P _N), stomatal conductance (g _s), intercellular CO₂ concentration (C_i), and photosynthetic nitrogen use efficiency (PNUE). Also, drought reduced relative water content (RWC) and foliar C and N concentrations, but increased the C/N ratio. P _N and C concentration were lower under drought and low light than in any other treatment, indicating that deep shade might seriously decrease C assimilation. However, the negative effect induced by drought was alleviated by improving RWC and maintaining C balance, and therefore low-light seedlings accumulated more biomass than those under high light when they were exposed to drought. Our results showed that trade-off and facilitation effects of drought and low light might be complementary and alter in different species.     

Effects of exogenous glycinebetaine on growth, CO2 assimilation, and photosystem II photochemistry of maize plants

Effects of exogenous glycinebetaine (GB, 2–50 mM) on growth, photosynthetic gas exchange, PSII photochemistry, and the activities of key enzymes involved in CO₂ fixation in maize plants were investigated. Growth, CO₂ assimilation rate, and stomatal conductance increased at low GB concentrations (2–20 mM) but decreased significantly at high GB concentrations (30–50 mM). Leaf relative water content and water potential remained unchanged at low GB concentrations but decreased at high GB concentrations. The maximal efficiency of PSII photochemistry was unchanged either at low or high GB concentrations. The actual PSII efficiency ( Φ _PSII) and photochemical quenching (q_P) increased at low GB concentrations but decreased at high GB concentrations. At low GB concentrations, there were no significant changes in the efficiency of excitation energy capture by open PSII reaction centres (F_v′/F_m′) and non‐photochemical quenching (q_N). At high GB concentrations, F_v′/F_m′ decreased while q_N increased significantly. There were no changes in the activities of phosphoenolpyruvate carboxylase, pyruvate phosphate dikinase, and ribulose‐1,5‐bisphosphate carboxylase in control and GB‐fed plants. However, there was a linear correlation between CO₂ assimilation rate and stomatal conductance in control and GB‐fed plants. Moreover, there were no significant differences in O₂ evolution rate between control and GB fed‐plants under saturated CO₂ conditions. The results suggest that exogenous GB application at certain concentrations can enhance CO₂ assimilation rate, which can be explained by an increased stomatal conductance.     

In situ estimation of net CO2 assimilation, photosynthetic electron flow and photorespiration in Turkey oak (Q. cerris L.) leaves: diurnal cycles under different levels of water supply

Diurnal time courses of net CO₂ assimilation rates, stomatal conductance and light-driven electron fluxes were measured in situ on attached leaves of 30-year-old Turkey oak trees (Quercus cerris L.) under natural summer conditions in central Italy. Combined measurements of gas exchange and chlorophyll a fluorescence under low O₂ concentrations allowed the demonstration of a linear relationship between the photochemical efficiency of PSII (fluorescence measurements) and the apparent quantum yield of gross photosynthesis (gas exchange). This relationship was used under normal O₂ to compute total light-driven electron fluxes, and to partition them into fractions used for RuBP carboxylation or RuBP oxygenation. This procedure also yielded an indirect estimate of the rate of photorespiration in vivo. The time courses of light-driven electron flow, net CO₂ assimilation and photorespiration paralleled that of photosynthetic photon flux density, with important afternoon deviations as soon as a severe drought stress occurred, whereas photochemical efficiency and maximal fluorescence underwent large but reversible diurnal decreases. The latter observation indicated the occurrence of a large non-photochemical energy dissipation at PSII. We estimated that less than 60% of the total photosynthetic electron flow was used for carbon assimilation at midday, while about 40% was devoted to photorespiration. The rate of carbon loss by photorespiration (R₁) reached mean levels of 56% of net assimilation rates. The potential application of this technique to analysis of the relative contributions of thermal de-excitation at PSII and photorespiratory carbon recycling in the protection of photosynthesis against stress effects is discussed.     

Grafting onto Cucurbita moschata rootstock alleviates salt stress in cucumber plants by delaying photoinhibition

To determine how the use of a given rootstock can influence the functioning of the photosynthetic apparatus of the scion under salt stress, the growth, gas exchange, photosystem II (PSII) efficiency, xanthophyll cycle, and chloroplast ultrastructure of nongrafted, self-grafted, and pumpkin-grafted (hereafter referred to as rootstock-grafted) cucumber (Cucumis sativus L.) plants were investigated at day 15 after being treated with 90 mM NaCl. The reductions in plant growth of the rootstock-grafted plants were lower than those of the nongrafted and self-grafted plants under 90 mM NaCl. The net photosynthetic rate, stomatal conductance, maximal and effective quantum yield of PSII photochemistry, photochemical quenching coefficient, and effective quantum-use efficiency of PSII in the light-adapted state of the nongrafted and self-grafted plants were significantly decreased under 90 mM NaCl. However, these reductions were alleviated when the cucumber plants were grafted onto the pumpkin (Cucurbita moschata Duch.) rootstock. The intercellular CO₂ concentrations were significantly increased in the nongrafted and self-grafted plants under 90 mM NaCl, whereas it was decreased in the rootstock-grafted plants. Nonphotochemical quenching (NPQ) and the deepoxidation state of the xanthophyll cycle were significantly increased under 90 mM NaCl, particularly in the rootstockgrafted plants, suggesting the rootstock-grafted plants had higher potential to dissipate excess excitation energy and reduce the probability of photodamage to PSII. Under 90 mM NaCl, the number of grana was reduced, the thylakoids were swollen, and starch granules accumulated in all plants. However, the damage of chloroplast ultrastructure was alleviated in the rootstock-grafted plants. Taken together, the use of C. moschata rootstock alleviated salt stress in cucumber plants by delaying photoinhibition, probably due to a lower incidence of both stomatal and nonstomatal factors limiting photosynthesis.     

Nitrogen Metabolism and Photosynthesis in Leymus chinensis in Response to Long-term Soil Drought

Key enzyme activities related to nitrogen metabolism, gas-exchange, chlorophyll fluorescence, and lipid peroxidation were determined in Leymus chinensis (Trin.) Tzvel. plants under four soil moisture regimes (control: 75%–80% of field moisture capacity, mild drought: 60%–65%, and moderate drought: 50%–55% as well as severe drought: 35%–40%). Severe drought significantly decreased the key enzyme activities of nitrogen anabolism such as nitrate reductase (NR, EC 1.6.6.1), glutamine synthetase (GS, EC 6.3.1.2), and glutamate dehydrogenase (GDH, EC 1.4.1.2) but increased the key enzyme activities of nitrogen catabolism such as asparaginase (AS, EC 6.3.5.4) and endopeptidase (EP, EC 3.4.24.11), especially after long-term soil drought. Plant biomass, leaf-biomass ratio between the green leaf and total plant biomass, net photosynthetic rate, stomatal conductance, the maximal efficiency of PSII photochemistry, the actual quantum yield, and the photochemical quenching were significantly reduced by severe water stress. Plant malondialdehyde (MDA) concentration increased with the increase in water stress, particularly at the late-growth stage. Our results suggest that the key enzymes of nitrogen metabolism may play an important role in the photosynthetic acclimation of L. chinensis plants to long-term soil drought.     

Photoinhibition-induced reduction in photosynthesis is alleviated by abscisic acid,cytokinin and brassinosteroid in detached tomato leaves

Detached leaves of tomato (Lycopersicon esculentum Mill.) experienced photoinhibition associated with sharp reductions in net photosynthetic rate (Pn), quantum efficiency of PSII (Φ_PSII) and photochemical quenching (qP) even though they were exposed to mild light intensity (400 μmol m⁻² s⁻¹ PPFD) at 28°C. Photoinhibition and the reduction in Pn, Φ_PSII and qP, however, were significantly alleviated by 1 mg l⁻¹ ABA, 0.1 mg l⁻¹ N-(2-chloro-4-pyridyl)-N′-phenylurea (CPPU) and 0.01 mg l⁻¹ 24-epibrassinolide (EBR). Higher concentrations, however, reduced the effects or even exacerbated the occurrence of photoinhibition. Superoxide dismutase and ascorbate peroxidase activity in leaves increased with the increases in ABA concentration within 1–100 mg l⁻¹, CPPU concentration within 0.1–10 mg l⁻¹ and EBR concentration within 0.01–1.0 mg l⁻¹. Catalase and guaiacol peroxidase activity also increased with the increase in EBR concentration but CPPU and ABA treatments at higher concentrations caused a decrease. Malondialdehyde (MDA) content decreased with the increase in CPPU concentration. ABA and EBR, however, decreased MDA concentration only at 1 and 0.01 mg l⁻¹, respectively. In conclusion, detached leaves had increased sensitivity to PSII photoinhibition. Photoinhibition-induced decrease in photosynthesis, however, was significantly alleviated by EBR, CPPU and ABA at a proper concentration.     

Comparative effect of water, heat and light stresses on photosynthetic reactions in Sorghum bicolor(L.) Moench

Five varieties of Sorghum bicolor (L.) Moench., differing in their drought tolerance under field conditions have been used to study the effect of individual components of drought stress, namely high light intensity stress, heat stress and water stress, on their photosynthetic performance. Chlorophyll content, chlorophyll fluorescence, ribulose-1,5-bisphosphate carboxylase (Rubisco, EC 4.1.1.39) content, phosphoenolpyruvate carboxylase (PEPcase, EC 4.1.1.31) activity and photo-synthetic oxygen evolution were used as key parameters to assess photosynthetic performance. The results indicated that photochemical efficiency of photosystem II (PSII) was severely reduced by all three stress components, whereas PEPcase activity was more specifically reduced by water stress. Degradation of Rubisco and chlorophyll loss occurred under high light and water stress conditions. Of the four drought-tolerant varieties, E 36-1 showed higher PEPcase activity, Rubisco content and photochemical efficiency of PSII, and was able to sustain a higher maximal rate of photosynthetic oxygen evolution under each stress condition as compared to the other varieties. A high stability to stress-induced damage, or acclimation of photosynthesis to the individual components of drought stress may contribute to the high yields of E 36-1 under drought conditions. In the E 36-1 variety markedly higher levels of the chloroplastic chaperonin 60 (cpn 60) were observed under all stress conditions than in the susceptible variety CSV 5.Key words: Chlorophyll fluorescence, drought stress, oxygen evolution, phosphoenopyruvate carboxylase, Sorghum.