The responses of gas exchange and chlorophyll fluorescence of field-growing Ulmus pumila seedlings to changes in simulated precipitation were studied in Hunshandak Sandland, China. Leaf water potential (Ψ wp ), net photosynthetic rate (P N ), stomatal conductance (g s ), and transpiration rate (E) were significantly increased with enhancement of precipitation from 0 to 20 mm (p<0.01), indicating stomatal limitation of U. pumila seedlings that could be avoided when soil water was abundant. However, P N changed slightly when precipitation exceeded 20 mm (p>0.05), indicating more precipitation than 20 mm had no significant effects on photosynthesis. Maximum photochemical efficiency of photosystem 2, PS 2 (F v /F m ) increased from 0.53 to 0.78 when rainfall increased from 0 to 10 mm, and F v /F m maintained a steady state level when rainfall was more than 10 mm. Water use efficiency (WUE) decreased significantly (from 78-95 to 23-27 μmol mol -1 ) with enhancement of rainfalls. P N showed significant linear correlations with both g s and Ψ wp (p<0.0001), which implied that leaf water status influenced gas exchange of U. pumila seedlings. The 20-mm precipitation (soil water content at about 15 %, v/v) might be enough for the growth of elm seedlings. When soil water content (SWC) reached 10 %, down regulation of PS2 photochemical efficiency could be avoided, but stomatal limitation to photosynthesis remained. When SWC exceeded 15 %, stomatal limitation to photosynthesis could be avoided, indicating elm seedlings might tolerate moderate drought.Additional key words: drought stress; leaf water potential; maximum photochemical efficiency; water use efficiency.
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