Abstract:

To explore the regulation mechanism of exogenous melatonin（MT）on the cold tolerance of cucumber plants，‘Jingyou 35’cucumber plants were used as materials，75 μmol · L^-1 MT was sprayed on the foliage to study its effects of exogenous MT on the photosynthesis，reactive oxygen species（ROS）accumulation，antioxidant enzyme activities of cucumber in winter solar greenhouse. The results showed that the photosynthetic rate（P_n），stomatal conductance（G_s），transpiration rate（T_r），maximum photochemical efficiency（F_v/F_m）and actual photochemical efficiency（Φ_PSII）decreased，while the intercellular CO₂ concentration（C_i）increased gradually，with the temperature decrease. Low temperature led to an increase in the accumulation of MDA，hydrogen peroxide（H₂O₂）and superoxide anion（$\mathrm{O}_{2}^{\bar{·}}$），as well as the electrolyte leakage rate（EL）. The activities of superoxide dismutase（SOD），peroxidase（POD），ascorbate peroxidase（APX）and glutathione reductase（GR）increased in the early days of low temperature stress，but subsequently decreased. Compared with the control，the MT treatment displayed a markedly increase in P_n，G_s，T_r，F_v/F_m and Φ_PSII，as well as the activity of Ribulose-1,5-diphosphate carboxylase（RuBPCase）and Rubisco activase（RCA）and their relative mRNA expressions，whereas an obviously decrease in C_i，after cold snap. The MT treated plants showed significantly lower MDA，H₂O₂ and $\mathrm{O}_{2}^{\bar{·}}$ accumulations and EL，while markedly higher activities of SOD，POD，APX and GR than those of the control. The yield of MT treatment increased by 16.3% compared with the control. These data suggest that exogenous MT can improve the cold tolerance of cucumber plants in solar-greenhouse by increasing the activities of Calvin cycle and photosystem，and enhancing the ROS scavenging activity during low temperature stress.

Key words: cucumber, melatonin, cold stress, photosynthesis, reactive oxygen species, cold tolerance