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 (12/8A degrees C and 100 mu mol m(-2) s(-1) PPFD) for 3 days and then recovered under normal temperature and high irradiance (28/18A degrees C and 600 mu mol 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 (center dot-) 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 (I broken vertical bar(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 mu mol m(-2) s(-1)) further aggravated the decrease in P (N), g (s), I broken vertical bar(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), I broken vertical bar(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.