ECOPHYSIOLOGICAL RESPONSES OF TWO XEROPHYTES ATRAPHAXIS FRUTESCENS AND ELAEAGNUS ANGUSTIFOLIA TO THE CHANGE OF GROUNDWATER DEPTH IN ARID AREA Ⅱ. LEAF PHOTOSYNTHESIS AND PHOTOSYNTHETIC RESPONSE TO LIGHT AND TEMPERATURE
沙木蓼和沙枣对地下水位变化的生理生态响应Ⅱ.叶片光合作用及其对温度和光的反应

The eco_physiological responses of dominant trees (Atraphaxis frutescens and Elaeagnus angustifolia) to ground water availability were investigated in a controlled environment at the Plant Transpiration Consume Water Observation S tation in Minqin Desert Botanical Garden, Gansu province. In this experiment, th e depths of groundwater (DGW) were designed as 1.4 m, 2.4 m and 3.4 m. In each t rea tment, the two desert species were transplanted to the observation sites with di fferent DGW after growing naturally for 3 or 4 years. The results showed that: 1 ) the changes of DGW resulted in the significant changes of net photosynthetic r ate (Pn), the transpiration rate (E), stomatal conductance (Gs), lea f contents o f chlorophyll a (Chl a), chlorophyll b (Chl b) and the total chlorophyll (Chl a + Chl b), the ratio of Chl a and Chl b (Chl a/Chl b), the quantum yield efficien cy and the CO 2 carboxylation efficiency; 2) although these two desert species were well adapted to the water stress environment at 3.4 m DGW, their adaptive s trategies were different. In response to the water stress environment, A. fru tescens reduced its photosynthetic rate and maintained the level of water use efficiency, while E. angustifolia largely decreased the transpiration rat e and thus increased the water use efficiency; 3) in different DGW, the optimal temperature of photosynthesis in these two desert species did not change signif icantly, but the net photosynthetic rate under the optimal temperature range dec lined markedly. Since E. angustifolia is a nitrogen fixation species, fur ther studies are needed to verify whether or not the differences of drought adap tation between these two species are related to the nitrogen fixation.