中文摘要：

在自然界，水通常是限制植物生长的重要因子。水分亏缺严重影响着植物的形态结构、光合生长和代谢水平（Seki et al．，2007），但植物也进化出了复杂的感知、响应和适应水分亏缺的机制。

英文摘要：

A pot experiment in the outdoor was conducted to investigate the effect of soil progressive drought on the physiological parameters of 2-year-old Populus euphratica seedlings in the lower reach of Tarim River. The seedlings were respectively grown under well-watered and progressive water deficit conditions by controlling the water supply. Chlorophyll fluorescence parameters of actual photochemical quantum yield of PSII in the light （tpps~1） , photochemical quenching index （qp） and non-photochemical quenching index （qN） , leaf stomatal conductance （ G ） and predawn water potential were measured during the experiment period. Results showed that the soil moisture content decreased to mild drought stress level in 6 days after a saturated irrigation, and reached to severe drought stress level in 15 days after a saturated irrigation in July. During the drying process of soil water content, the increasing soil drought resulted in decreases in the predawn leaf water potential at 6：00 am, φPSH,, qp and G of the seedlings, but increase in qN. These results would provide a scientific foundation for understanding drought resistant mechanism of P. euphratica seedlings.