Abstract: In this paper, the effects of drought stress simulated by PEG on the photosynthesis of Populus pseudo-simonii were monitored by gas exchange and chlorophyll fluorescence techniques. The results showed that in the early phase of drought stress, net photosynthetic rate (P_n), transpiration rate (T_r), stomata conductance (g_s),and intercellular CO₂ concentration (C_i) decreased with increasing stress, and the decrease of P_n was mainly related to the decreased g_s. In the late phase, C_i increased gradually, while F_v/F_m and F_v/F_o decreased significantly. POD activity increased in the early phase, but decreased in the late phase. All of these indicated that in the early phase of drought stress, stomata closure was the main factor limiting P. pseudo-simonii photosynthesis, while in the late phase, the main limiting factor was non-stomatal.A short period of drought stress to the stress resulted in the decrease of PS Ⅱ efficiency and POD activity.

Key words: Mixed cropping, Gramineous plants, Root exudates, Apoplast Fe, Root Fe(Ⅲ) reducing capacity