The recovery of photosynthesis in tomato subsequent to chilling exposure

The overall success of a plant in coping with low temperature sensitivity of photosynthesis is dependent not only on the maximum extent of inhibition suffered for a given time of low temperature exposure but also on the persistence of the inhibition after normal growth temperatures are restored. Thus the capacity of recovery and the speed with which a plant can recover from the effects of chilling exposure are important parameters in determining how devastating the chilling event will be on season-long growth and yields. We have studied the recovery of CO₂-saturated photosynthesis from the injury caused by exposing intact tomato plants (Lycopersicon esculentum Mill. cv. Floramerica) or detached tomato leaves to a temperature of 1°C in the dark for varying periods of time. We found that net photosynthesis was fully recovered within 12 h after returning the plants to 25°C in the dark, even after chilling exposures as long as 45 h. This was true for intact plants as well as for detached leaves that were supplied with water. When chilling took place in the light (4°C, 1000 E · m^-2 · s^-1, PAR) inhibition of photosynthesis was more severe and appeared more quickly and the recovery was slower and incomplete. A 12 h chilling exposure in the light resulted in injury to net photosynthesis that was not fully recovered even after 50 h. Chilling damage to photosynthesis developing in the light was distinguished from chilling in the dark by the decreased photosynthetic quantum yield. Not only did high intensity illumination enhance chilling damage of photosynthesis but bright light subsequent to the chilling exposure also delayed the recovery of photosynthesis. At none of the three ambient CO₂ concentrations investigated (300, 1500 and 5000 1.1^-1) did the recovery of photosynthesis depend on stomatal conductance.     

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.     

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.     

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.     

Protection of photosynthesis and antioxidative system by 24-epibrassinolide in Solanum melongena under cold stress

This study was carried out to understand the mechanism of protection of plants under cold stress by exogenous 24-epibrassinolide (EBR). The eggplant (Solanum melongena L.) seedlings were pretreated with five concentrations of EBR (0, 0.05, 0.1, 0.2 and 0.4 °M) and then exposed to day/night temperatures of 10/5 °C for 8 d. The results show that EBR, especially 0.1 °M EBR, dramatically alleviated growth suppression and a decrease in chlorophyll content and photosynthetic rate caused by the cold stress. In addition, EBR also decreased malondialdehyde content and O₂ ^·? production rate induced by the cold stress, and increased the activities of superoxide dismutase, guaiacol peroxidase, catalase, and ascorbate peroxidase, and proline content. The results of the present study suggest that exogenous EBR could improve cold tolerance of eggplant by regulating photosynthesis and antioxidative systems.     

Chill-induced decrease in capacity of RuBP carboxylation and associated H2O2 accumulation in cucumber leaves are alleviated by grafting onto figleaf gourd

BACKGROUND AND AIMS: Chilling results in a significant decrease in Rubisco content and increased generation of reactive oxygen species (ROS) in cucumber (Cucumis sativus), a chilling-sensitive species. The role of roots in the regulation of the tolerance is unknown. Here, cucumber plants grafted onto figleaf gourd (Cucurbita ficifolia), a chilling-tolerant species were used to study the role of roots in the regulation of shoot functioning and the associated root-to-shoot communication. METHODS: Gas exchange and chlorophyll fluorescence were measured using an infrared gas analyser combined with a pulse amplitude fluorimeter during chilling at 14 degrees C or 7 degrees C and subsequent recovery. At the same time, Rubisco content and activity and ROS generation were spectrophotometrically assayed. Abscisic acid and cytokinin concentrations in xylem sap were also determined by enzyme-linked immunosorbent assay. KEY RESULTS AND CONCLUSIONS: Grafted plants showed a significantly higher light-saturated rate of CO(2) assimilation (A(sat)) than own-rooted plants when roots were gradually cooled, but no differences were detected when shoots were cooled. Chill at 7 degrees C irreversibly reduced A(sat), and significantly decreased maximum carboxylation activity, Rubisco content and initial Rubisco activity. However, grafted plants showed weaker inhibition, together with decreased electron flux in the water-water cycle. Higher activity of antioxidant enzymes with less ROS production was found in grafted plants. In addition, ABA concentration increased by 48.4-fold whilst cytokinin concentration decreased by 91.5% in the xylem sap of own-rooted plants after exposure to a 7 degrees C chill. In comparison, ABA and cytokinin concentrations increased by 10.5-fold and 36.9%, respectively, for the grafted plants. Improved plant growth was also observed in grafted plants after the chill. These results suggest that some signals coming from chilling-resistant roots (i.e. ABA and cytokinins) protect leaf photosynthesis in shoots of chilling-sensitive plants.     

Dark chilling inhibition of photosynthesis and symbiotic nitrogen fixation in soybean during pod filling

The growth stage of a soybean [Glycine max (L.) Merrill] plant may influence its physiological response to dark chilling. Opposed to vegetative development, the intense nutrient and energy requirements of the developing seeds during pod filling could cause additional chilling damage and decreased recovery capacity. Previously, we investigated dark chilling tolerance during vegetative development in two soybean genotypes, 'Maple Arrow' and 'Fiskeby V' and consistently found that photosynthesis and symbiotic nitrogen fixation (SNF) was less affected by dark chilling in 'Maple Arrow'. In this study we describe the dark chilling response of the same genotypes during pod filling. Our aim was to establish whether the potential selection criteria for dark chilling tolerance, identified during vegetative development, was equally sensitive during pod filling. The results indicate that photosynthesis is less affected by dark chilling in 'Maple Arrow' than in 'Fiskeby V', not only during vegetative development, but also during the critical reproductive stage of pod filling. 'Fiskeby V' also lacks the ability to restore normal photosynthetic capacity during an extended recovery treatment. The decrease of nodule ureide content indicates that SNF was inhibited to a similar extent in both genotypes. Nodule ureide content was reduced more than stem ureide content, suggesting that the former is a more sensitive indicator of chilling stress effects on SNF. The results indicate that certain photosynthetic and fluorescence parameters are sensitive indicators of dark chilling tolerance throughout plant development and should prove valuable in future breeding programmes aimed at increasing the chilling tolerance of soybean.     

Photoinhibition of photosystem I at chilling temperature and subsequent recovery in Arabidopsis thaliana

Chilling-induced photoinhibition and subsequent recovery was studied in Arabidopsis thaliana exposed to 4 degrees C and 150 micromol photons m(-2) s(-1). PSII showed progressive damage with a 14% decrease in quantum yield after 8 h exposure. In contrast, the damage to PSI leveled off after 8 h with a decrease in in vitro NADP+ photoreduction activity of around 32%. In vivo P700 measurements demonstrated that antenna efficiency was decreased by the photoinhibitory treatment. Measurements of P700 and immunoblotting demonstrated that the damaged PSI was not degraded during the 8 h light-chilling treatment, but after 12 h recovery at 20 degrees C, no damaged PSI remained in the thylakoids. Thus, degradation of damaged PSI is a step in the recovery and not a direct result of photodamage. Unlike photodamaged PSII, the PSI core complex is not repaired but completely degraded. In contrast, light harvesting complex I proteins have a slow turnover. PSII recovered completely within 8 h after transfer to 20 degrees C whereas PSI activity recovered very slowly, and the amount of PSI on a leaf area basis remained low even after 1 week at 20 degrees C. The results show that damage, protein turnover and recovery are well separated processes in Arabidopsis.     

Inhibition of photosynthesis by chilling in the light

The photosynthetic activity of leaf slices from Spinacia oleracea L., Cucumis sativus L. and Nerium oleander L. was measured in 25° C immediately after preincubation for 2.5 h at various photon flux densities (PFD) with chilling at 4°C, or at a moderate (450 μmol m⁻² s⁻¹) PFD with various temperatures below 25°C. Inhibition of photosynthesis was evident in C. sativus and 45°C-grown N. oleander after preincubation at 4°C at all PFD. The inhibition was most severe at fluxes in excess of the moderate PFD under which the plants were grown. Photosynthesis in S. oleracea and 20°C-grown N. oleander was not inhibited at 4°C unless the PFD was in excess of this moderate PFD. The inhibition of photosynthesis was initiated in C. sativus below 13°C, and in 45°C-grown N. oleander below 8°C. A phase transition in the polar lipids from the thylakoids of these plants was detected at about the same temperatures. For S. oleracea and 20°C-grown N. oleander preincubated under the same conditions, there was no inhibition of photosynthesis and no phase transition above 0°C. These results show that some component of photosynthesis was disrupted in the light at temperatures below that of the phase transition in the thylakoid polar lipids.     

Effect of 24-epibrassinolide on pollen viability during heat-stress in tomato

Tomato pollen when treated with 24-epibrassinolide (EBR), it was observed that in vitro pollen germination was more tolerant to high temperature. A significantly higher in vitro pollen germination, enhanced tube growth and low pollen bursting were observed in presence of EBR at 35 degrees C. Since a preconditioning treatment of tomato plant was not required for these effects, it is concluded that EBR treatment increased basic thermotolerance of germinating pollen. This study provided the first evidence for EBR induced thermotolerance in germinating pollen.     

Long-term chilling of young tomato plants under low light and subsequent recovery

The influence of unfavourable climatic conditions at the onset of the growth period on chilling-sensitive tomato (Lycopersicon esculentum Mill., cv. Abunda) was studied by exposing young plants to combinations of low temperature and low light (60–100 mol quanta · m^–2 · s^–1) for several weeks. When the temperature did not decrease below a critical point (8 ° C) no loss of developmental capacity of the plants was detected. However, while new leaves were readily formed upon return to normal growth conditions (22/18 °C, day/night, in a greenhouse), net accumulation of biomass showed a lag phase of approximately one week. This delay was accompanied by a strong, irreversible inhibition of photosynthesis in the fully expanded leaves which had been exposed to the chilling treatment. When plants were subjected to temperatures below 8 ° C, survival rates decreased after three weeks at 6 ° C and irreversible damage of apical meristematic tissue occurred. Drought-hardening prior to chilling ensured survival at 6 ° C and protected the plants against meristem loss.Abreviation Chl chlorophyll Thanks are due to G.P. Telkamp for technical assistance. This research is financially supported by the Netherlands Technology Foundation (STW, Utrecht, The Netherlands), and is coordinated by the Foundation for Biological Research (BION, 's-Gravenhage, The Netherlands).     

Amelioration of chilling stress by triadimefon in cucumber seedlings

Cucumber (Cucumis satvus L.) seeds were imbibed in distilled water (control) and 10 mg l^–1 triadimefon (TDM) for 10 h and then grown in a plant growth chamber with a light/dark temperature of 28/20 °C and a photoperiod of 14 h with a light intensity of 60 µmol m^–2 s^–1. 14-day-old seedlings were exposed to chilling stress with a light/dark temperature of 6/3 °C for 4 d. TDM improved the growth rate of cucumber seedling subjected to chilling stress and increased photosynthetic pigments contents and relative water content compared with the control at the end of chilling stress. Chilling stress decreased protein content and the activities of SOD, CAT and POD, but it increased proline, H₂O₂ and MDA accumulation, and relative electrical conductivity. TDM ameliorated the injury caused by chilling stress by preventing decreases in protein content and the activities of SOD, CAT and POD and by inhibiting increases in proline, H₂O₂ and MDA contents, and relative electrical conductivity, which suggested that TDM ameliorated the negative effect of chilling stress.     

Photooxidative inhibition of nitrate reductase during chilling

The effect of a temperature close to the freezing point (chilling) on the nitrate reductase system of leaf discs of Cucumis sativus L. cv. Kleine Groene Scherpe was determined in the absence and presence of light. The capacity of leaf discs in the light (250 μE m⁻²s⁻¹) at 20°C to increase in vivo and in vitro nitrate reductase activity, was unaffected by chilling pretreatment in the dark, but 4 h of chilling pretreatment in the light (250 μE m⁻²s⁻¹) decreased the capacity to less than 50% of the unchilled control. The chilling inhibition of the capacity to increase nitrate reductase activity was of a photooxidative nature since it only occurred in the presence of light and oxygen. Plants grown at a low light intensity (65 μE m⁻²s⁻¹) lost 95% of their capacity to increase nitrate reductase activity, while plants grown at 195 μE m⁻²s⁻¹ retained 80% of their nitrate reducing capacity after 6 h chilling pretreatment in the 250 μE m⁻²s⁻¹ light. Previously induced nitrate reductase activity was also affected by light during chilling. A lag phase of 7 h preceded a fast phase of decrease in activity. Both in vivo and in vitro activity decreased to 15% of the control value after 18 h of chilling in the light. It is concluded that the induction mechanism of nitrate reductase is primarily affected by photooxidation during chilling. The decrease in nitrate reductase activity is attributed to a decrease in the amount of activity enzyme.     

低氧胁迫下24-表油菜素内酯对黄瓜幼苗根系生长及其无氧呼吸同工酶表达的影响

采用营养液水培,研究了低氧胁迫下24-表油菜素内酯(EBR)对黄瓜幼苗根系生长及其无氧呼吸同工酶表达的影响.结果表明:低氧胁迫增强了黄瓜幼苗根系丙酮酸脱羧酶(PDC)、乙醇脱氢酶(ADH)、乳酸脱氢酶(LDH)同工酶的表达,低氧胁迫下施用外源EBR的第3天PDC、ADH同工酶的表达量分别提高了18.8％、28.8％,而第6、第9天PDC、ADH、LDH同工酶的表达减弱,比单纯低氧处理分别降低19.5％、25.6％、53.4％及26.4％、26.0％、28.4％;低氧胁迫至第9天,黄瓜幼苗根系的生长受到了显著抑制(P＜0.05),而低氧胁迫下施用EBR,黄瓜幼苗根系的生长受抑制程度减轻,其根系总长、干重、根尖数较单纯低氧处理显著增加(P＜0.05),低氧抑制了黄瓜幼苗根系的生长,低氧胁迫下营养液添加EBR可调节黄瓜根系无氧呼吸同工酶的表达,缓解低氧胁迫对黄瓜幼苗根系的伤害.     

Interaction between light and chilling temperature on the inhibition of photosynthesis in chilling-sensitive plants*

Abstract. Fully expanded leaves of 25°C grown Phaseolus vulgaris and six other species were exposed for 3 h to chilling temperatures at photon flux densities equivalent to full sunlight. In four of the species this treatment resulted in substantial inhibition of the subsequent quantum yield of CO₂ uptake, indicating reduction of the photochemical efficiency of photosynthesis. The extent of inhibition was dependent on the photon flux density during chilling and no inhibition occurred when chilling occurred at a low photon flux density. No inhibition occurred at temperatures above 11.5°C, even in the presence of the equivalent of full sunlight. This interaction between chilling and light to cause inhibition of photosynthesis was promoted by the presence of oxygen at normal air partial pressures and was unaffected by the CO₂ partial pressure present when chilling occurred in air. When chilling occurred at low O₂ partial pressures, CO₂ was effective in reducing the degree of inhibition. Apparently, when leaves of chilling-sensitive plants are exposed to chilling temperatures in air of normal composition then light is instrumental in inducing rapid damage to the photochemical efficiency of photosynthesis.     

低氧胁迫下24-表油菜素内酯对黄瓜幼苗叶片光合特性及多胺含量的影响

采用1/2 Hoagland营养液培养,研究了低氧胁迫下24-表油菜素内酯(EBR)对黄瓜幼苗叶片光合特性及多胺含量的影响.结果表明：低氧胁迫下黄瓜幼苗的净光合速率(P_n)、气孔导度(g_s)、蒸腾速率(T_r)、胞间CO₂浓度(C_i)显著下降,而叶绿素含量显著提高,幼苗生长受抑;低氧胁迫显著提高了黄瓜幼苗叶片的腐胺(Put)、亚精胺(Spd)、精胺(Spm)、多胺(PAs)含量和Put/PAs,但降低了(Spd+Spm) /Put.低氧胁迫下,外源EBR不仅显著提高了黄瓜幼苗的P_n、g_s、T_r及叶绿素含量,也显著提高了黄瓜幼苗叶片的游离态Spm、结合态Spd、Spm及束缚态Put、Spd、Spm含量,促进了PAs的进一步积累,且降低了Put/PAs,提高了(Spd+Spm)/Put．可见,外源EBR调节了黄瓜幼苗内源多胺含量及形态的变化,维持了较高的光合性能,促进了叶面积和干物质量的显著增加,缓解了低氧胁迫对黄瓜幼苗的伤害.     

Light-dependent damage to photosynthesis in olive leaves during chilling and high temperature stress

Abstract The leaves of olive are long lived and likely to experience both chilling and high temperature stress during their life. Changes in photosynthetic CO₂ assimilation resulting from chilling and high temperature stress, in both dim and high light, are investigated. The quantum yield (φ) of photosynthesis at limiting light levels was reduced following chilling (at 5°C for 12 h), in dim light by approximately 10%, and in high light by 75%; the difference being attributed to photoinhibition. Similar reductions were observed in the light-saturated rate of CO₂ uptake (A_max). Decrease in A_max correlated with a halving of the leaf internal CO₂ concentration (c_i), suggesting an increased limitation by stomata following photoinhibition. Leaves were apparently more susceptible to photoinhibitory damage if the whole plant, rather than the leaf alone, was chilled. On return to 26 °C, I he photosynthetic capacity recovered to pre-stress levels within a few hours if leaves had been chilled in high light for 8 h or less, but did not fully recover from longer periods of chilling when loss of chlorophyll occurred. Leaves which were recovering from chilling in high light showed far more damage on being chilled a second time in high light. Three hours in high light at 38 °C reduced φ by 80%, but φ recovered within 4h of return to 26 °C. Although leaves of Olive are apparently less susceptible to photoinhibitory damage during chilling stress than the short-lived leaves of chilling-sensitive annual? crops, the results nevertheless show that photoinhibition during temperature stress is potentially a major factor influencing the photosynthetic productivity of Olive in the field.     

Blockade of heavy metals accumulation in Chlorella vulgaris cells by 24-epibrassinolide

This study was conducted on the influence of 24-epibrassinolide (24-epiBL) mixed with varying concentrations of heavy metals (copper, lead, cadmium, zinc) upon the growth and accumulation of these heavy metals in the cell of the alga Chlorella vulgaris Beijerinck (Chlorophyceae). Heavy metals at the concentration of 10^–3 M, alone or mixed with 24-epiBL, showed a lethal effect on C. vulgaris. At metal concentrations of 10^–6–10^–4 M, a combination with 24-epiBL appeared to have a stronger stimulatory effect on a number of cells than a single metal (a stronger inhibitory effect). 24-EpiBL at the concentration of 10^–8 M in combination with heavy metals (in the range 10^–6–10^–4 M) blocked metal accumulation in algal cells. 24-EpiBL has an anti-stress effect on C. vulgaris contaminated by heavy metals. The inhibitory effect on metal accumulation of 24-epiBL mixed with different heavy metals was arranged in the following order: zinc > cadmium > lead > copper. This process is correlated with the stimulation of growth of C. vulgaris. The stimulatory effect of 24-epiBL mixed with heavy metals leading to an increased pH in the medium (5.28–6.20) was significantly higher than the impact due to the increased acidity in the medium due to metals alone (pH 3.10–5.85). Lower pH increased the toxicity of heavy metals in C. vulgaris cells.