Abstract: In this study, rice (Oryza sativa L.) cultivar “Liao Star 1” was used as the material to examine the effects of high CO₂ concentration and drought stress on photosynthetic physiology of rice seedlings under CO₂ concentrations of 380±10 and 760±20 μmol·mol^-1, aiming to provide a theoretical basis for explaining how plants cope with complex environmental changes. PEG-6000 (0, 5%, 10%, and 15%) was used to simulate rhizosphere drought stress. Single drought stress inhibited the growth of rice seedlings, decreased plant height, root length, fresh weight and dry weight, with stronger inhibitory effect on aboveground part than that on underground part, and on fresh weight than on dry weight. Under drought stress, net photosynthetic rate (P_n), stomatal conductance (G_s) and transpiration rate (T_r) were significantly decreased compared with the control. Under light (5%) and moderate (10%) drought stress, intercellular CO₂ concentration (C_i) decreased, and the decrease of P_n was largely due to stomatal factors. Under severe (15%) drought stress, C_i increased, and the decrease of P_n was largely due to non-stomatal factors. The chloroplast pigment contents, initial fluorescence yield (F_o), largest PSII photo-chemical quantum yield (F_v/F_m), and potential photochemical efficiency (F_v/F_o) did not change significantly under mild drought stress. F_o did not change significantly under moderate drought stress, but significantly increased under severe drought stress. Under moderate and severe drought stress, F_v/F_m, F_v/F_o and Chla, Chlb, Car contents were significantly decreased. Compared with the control, high CO₂ concentration alone promoted the growth of rice seedlings, plant height and root length, with stronger promotion effect on root than plant height. The dry weight of underground part increased significantly due to its promoting effect on dry weight than fresh weight. High CO₂ concentration significantly increased P_n, and decreased G_s, T_r and C_i. Photosynthesis was also increased by the increases of F_v/F_m, F_v/F_o and Chla, Chlb, Car contents. Under high CO₂ and water stress, high CO₂ promoted the growth of rice seedlings compared with water stress alone, P_n, G_s and T_r were also significantly increased. Photosynthesis was promoted by the increases of F_v/F_m, F_v/F_o and contents of chlorophyll and decrease of F_o, and thus reduced damage induced by water stress. The decrease of P_n under severe water stress transformed from non-stomatal factors to stomatal factors.

Key words: rice, elevated CO₂ concentration, water stress, photosynthetical response.