Abstract: By using portable Li-6400 photosynthesis system, this paper studied the gas exchange in Radermachera sinica and Alchornea trewioides leaves in karst regions of Northwest Guangxi in rainy season (June, July, and August, 2009) and dry season (October, November, and December, 2009), and analyzed the relationships between leaf photosynthetic rate (P_n) and leaf nitrogen and chlorophyll contents. In both dry and rainy seasons, the diurnal changes of the two species leaf P_n, transpiration rate (T_r), and stomatal conductance (G_s) were all in single peak, but that of water use efficieny (WUE) was not significant. The leaf P_n, T_r, G_s, and nitrogen and chlorophyll contents were higher in rainy season than in dry season, but the WUE was in adverse, suggesting that the plants could decrease the G_s to limit evaporation and increase WUE to adapt stress environment. In the same seasons, A. trewioides had stronger photosynthetic and water use capabilities than R. sinica, because the leaf P_n, WUE, and nitrogen and chlorophyll contents of A. trewioides were all higher, suggesting that A. trewioides could be more competitive than R. sinica in the karst regions of Northwest Guangxi. The two plant species in same seasons and the same plant species in different seasons had different correlations between P_n and environmental factors, and photosynthetic active radiation (PAR) was the dominant factor affecting P_n.

Key words: SO₂, Spatial temporal distribution, Passive sampling, Acid rain, Hangzhou