Abstract: For Cucumis sativus L. (cv. Boxin525), four different soil moisture levels were designed, which were normal irrigation (CK, soil relative humidity was 70%-80%), light water stress (T1, soil relative humidity was 60%-70%), moderate water stress (T2, soil relative humidity was 50%-60%) and severe water stress (T3, soil relative humidity was 35%-45%), respectively. Light response curves of cucumber leaves were measured under each treatment using LI-6400 portable photosynthesis system. Four different models (i.e. rectangular hyperbola, non-rectangular hyperbola, exponential and Ye Z P model) were applied to simulate light response curves of cucumber leaves, and the simulation effects were evaluated by statistics methods. Based on the experimental data and simulation results, the effect of water stress on light response curves of cucumber was discussed. The results indicated that water stress caused decline of net photosynthetic rate (Pn), at a PAR (photosynthetically active radiation) level of 800μmol·m-2·s-1, Pn decreased by 17.92%, 26.49% and 50.00% under T1, T2 and T3 treatments, respectively, compared with CK. The simulated light response curves showed a similar trend with the measured ones in response to water stress, and the variation amplitude of Pn-PAR curve decreased as water stress aggravated. Water stress affected light response curve parameters of cucumber leaves significantly. The initial quantum efficiency of cucumber leaves derived from the four mentioned models all increased at first and dropped afterwards with the aggravation of water stress, and the dark respiration rate was the highest under T2 treatment. Compared to CK, the average light saturation point calculated by four models decreased by 24.28%, 31.99% and 38.33%, respectively, under T1, T2 and T3, and the average maximum net photosynthetic rate declined by 23.88%, 33.19% and 55.78%, respectively, under T1, T2 and T3. The deviation degree of simulated light response curve and mean relative error of light response parameters achieved the lowest under CK, and the simulation effects of light response curve were weakened by water stress. The Ye Z P model performed best in simulation of the light response curve, next was the exponential model, the non-rectangular model, and the rectangular hyperbola model was the least effective one. The study indicated water stress significantly influenced light response of photosynthesis of cucumber leaves, and reduced photosynthetic capacity of leaves.

Key words: Cucumber, Photosynthesis, Water stress, Light response model, Light response parameters