The effects of 24-epibrassinolide (EBR) spray application on gas-exchange, chlorophyll fluorescence characteristics, Rubisco activity, and carbohydrate metabolism were investigated in cucumber (Cucumis sativus L. cv. Jinchun No. 3) plants grown in a greenhouse. EBR significantly increased the light-saturated net CO(2) assimilation rate (A(sat)) from 3 h to 7d after spraying, with 0.1 mg l(-1) EBR proving most effective. Increased A(sat) in EBR-treated leaves was accompanied by increases in the maximum carboxylation rate of Rubisco (V(c,max)) and in the maximum rate of RuBP regeneration (J(max)). EBR-treated leaves also had a higher quantum yield of PSII electron transport (phi(PSII)) than the controls, which was mainly due to a significant increase in the photochemical quenching (q(P)), with no change in the efficiency of energy capture by open PSII reaction centres (F'(v)/F'(m)). EBR did not influence photorespiration. In addition, significant increases in the initial activity of Rubisco and in the sucrose, soluble sugars, and starch contents were observed followed by substantial increases in sucrose phosphate synthase (SPS), sucrose synthase (SS), and acid invertase (AI) activities after EBR treatment. It was concluded that EBR increases the capacity of CO(2) assimilation in the Calvin cycle, which was mainly attributed to an increase in the initial activity of Rubisco.

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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 o C and 100 μmol m -2 s -1 PPFD) for 3 days and then recovered under normal temperature and high irradiance (28/18 o 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 O 2˙formation and H 2 O 2 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. -light-adapted maximum fluorescence; F o -minimal fluorescence of dark-adapted state; F m -maximal fluorescence of dark-adapted state; F v /F m -optimal quantum yield of PSII; FM -fresh mass; g s -stomatal conductance; LT -low temperature; LTBR -low temperature; EBR-pretreatment; MDA -malondialdehyde; NT -normal temperature; NTBR -normal temperature/EBR-pretreatment; NPQ -nonphotochemical quenching; Φ PSII , -effective quantum yield of PSII photochemistry; P N -net photosynthetic rate; PPFD -photosynthetic photon flux density; q P -photochemical quenching coefficient; Rubisco -ribulose-1,5-bisphosphate carboxylase/oxygenase; ROS -reactive oxygen species; SOD -superoxide dismutase.