Abstract: In a phytotron, the effects of three temperature regimes (day/night 25 ℃/18 ℃, optimal temperature; 15 ℃/9 ℃, suboptimal temperature; and 9 ℃/7 ℃, low temperature) on the low-light (75-85 μmol·m^-2·s^-1) tolerance of two Cucumis sativus cultivars (shade-susceptible Jinyan 2 and shade-tolerant Deltastar) seedling leaves in their photosynthesis were studied. The results showed that under low light, the SPAD, net photosynthesis rate (P_n), stomatal conductance (G_s), transpiration rate (T_r), water use efficiency (WUE), actual photochemical efficiency of PSⅡ in the light (Φ_PSⅡ), and photochemical quenching of chlorophyll fluorescence (q_P) of cucumber leaves decreased, with the decrement getting more with decreasing temperature, while the superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities were in adverse. During the recovery process after low-light stress relieved, the parameters of gas exchange and chlorophyll fluorescence of the leaves recovered gradually, and the recovery of some gas exchange parameters lagged to that of chlorophyll fluorescence parameters. Under low light, the lower the temperature, the more damage the photosynthesis apparatus suffered, and the lesser tolerance to low light the cucumber leaves had in their photosynthesis. During the low temperature and low light treatment period, the decrease of P_n, Φ_PSⅡ, and q_P was more obvious for Jinyan 2 than for Deltastar; and during the relief period, the recovery of these parameters was slower for Jinyan 2 than for Deltastar. It was indicated that Jinyan 2 had weaker tolerance to low temperature and/or low light in its photosynthesis than Deltastar.

Key words: reproductive allocation, community succession, dominant species, homoeostasis