中文摘要：

对植物水分利用效率的研究，可以揭示植物的内在耗水机制，为区域森林生态系统经营与维护提供依据．本研究以侧柏幼树为研究对象，通过室内控制试验设置不同的土壤水分梯度，分别用气体交换法和稳定同位素法对其不同土壤含水量条件下的瞬时水分利用效率（WUEga）和短期水分利用效率（WUEcp）进行研究．结果表明：受气孔导度（gs）的影响，叶片净光合速率（Pn）和蒸腾速率（t）随土壤含水量的增加呈现相同的变化趋势，均在土壤含水量为70％～80％田间持水量（FC）时达到最大值；叶片WUEgs则在土壤含水量最低（35％～45％FC）时达到最大值（7．26mmol·m^-2·s^-1）．叶片可溶性糖、枝条韧皮部渗出液的δ13C都在土壤含水量最低（35％～45％FC）条件下达到最大值，且叶片可溶性糖的δ13C明显高于枝条韧皮部渗出液的δ13C，未产生明显分馏；而叶片WUEcp也在土壤含水量最低（35％～45％FC）时达到最大值（7．26mmol·m^-2·s^-1）．相同条件下，叶片WUEga和WUEcp存在一定差异（平均相差0．52mmol·m^-2·s^-1），WUE。。时空变异性较大，而WUEcp更具有代表性．侧柏幼树通过降低生理生态活动和提高水分利用效率来适应干旱的土壤条件．

英文摘要：

The determination of plant foliar water use efficiency will be of great value to improve our understanding about mechanism of plant water consumption and provide important basis of regional forest ecosystem management and maintenance, thus, laboratory controlled experiments were carried out to obtain Platycladus orientalis sapling foliar water use efficiency under five different soil water contents, including instantaneous water use efficiency （WUEgs ） derived from gas exchange and short-term water use efficiency （WUEcp） eaculated using carbon isotope model. The results showed that, controlled by stomatal conductance （gs）, foliar net photosynthesis rate （P,） and transpiration rate （Tr） increased as soil water content increased, which both reached maximum va-lues at soil water content of 70%-80% field capacity （ FC ） , while WUEgs reached a maximum of 7.26 mmol · m^-2 · s^-1 at the lowest soil water content （35%-45% FC）. Both δ13C of water-soluble leaf and twig phloem material achieved maximum values at the lowest soil water content （35%-45% FC）. Be- sides, δ13C values of leaf water-soluble compounds were significantly greater than that of phloem exudates, indieating that there was depletion in 13C in twig phloem compared with leaf water-soluble compounds and no obvious fraetionation in the process of water-soluble material transportation from leaf to twig. Foliar WUEcp also reached a maximum of 7.26 mmol · m^-2 · s^-1 at the lowest soil water content （35%-45% FC）. There was some difference between foliar WUEg~ and WUE~p under the same condition, and the average difference was 0.52 mmol·m^-2 · s^-1. The WUEga had great space- time variability, by contrast, WUEcp was more renresentative. It was concluded that P. orientalis sapling adapted to drought condition by increasing water use efficiency and decreasing physiological activity.