Abstract: This study aims to explore the characteristics of stomatal and non-stomatal limitations on photosynthesis of flag leaves of medium mature indica rice (Oryza sativa) under natural conditions in Jiangxi. Leaf gas exchange of four rice cultivars (i.e.restorer line R7-35, sterile line Jingda 3S, F₁ hybrid of Jingda 3S/R7-35, and CK 9311) at the early earring stage were examined using a LI-6400 portable photosynthesis analyzer. The light-response curves of net assimilation rate (A_n), stomatal conductance (g_s), intercellular CO₂ concentration (C_i) and stomatal limitation value (L_s) were measured. The results showed that the saturated light intensity (I_sat) values, corresponding to the maximum A_n, for the four cultivars were 1500, 1800, 1200, and 1200 μmol·m^-2·s^-1, respectively. The g_s of R7-35, Jingda 3S and 9311 increased with the increases of light intensity (I). At higher I levels, non-stomatal limitation played a major role in limiting A_n of the four cultivars. The I values corresponding to the minimum C_i were 1600, 1600, 1400, and 1600 μmol·m^-2·s^-1, respectively. The saturation I values corresponding to the maximum L_s were 1400, 1200, 1400, and 600 μmol·m^-2·s^-1, respectively. If the decrease ofC_i and the increase of L_s were the main reasons for the decreases of photosynthetic rate, the criterion of stomatal factor would be biased. Cultivar 9311 showed a decrease ofC_i with the increases ofI until reaching 1600 μmol·m^-2·s^-1, an increase of L_s with the increases of I until reaching 600 μmol·m^-2·s^-1, and a nonlinear increase ofA_n with the increases of I until reaching 1200 μmol·m^-2·s^-1. These results indicated that a combination of C_i decrease and ofL_s increase would not necessarily lead to a decrease in A_n. Accordingly, we propose a new criterion for determining non-stomatal limitation whether it had I_sat and for determining stomatal limitation whether it had the maximum g_s.

Key words: Haitan Island, land use, ecological risk, exploratory spatial data analysis (ESDA).