中文摘要：

采用盆栽试验研究了正常供水（土壤含水量为最大持水量的85％）和干旱胁迫（土壤含水量为最大持水量的45％）条件下3年生西洋参叶片的光合日变化。结果表明，（1）干旱胁迫导致净光合速率（Pn）、气孔导度（Gs）、蒸腾速率（Tr）和水分利用效率（WUE）下降，分别比对照下降43．4％、17．4％、24．3％和25．7％。（2）开花期西洋参叶片Pn、Gs、Tr的日变化均表现为单峰型曲线。在干旱胁迫下，Pn、Gs和Tr均出现较大的日下降变化。（3）在干旱胁迫下胞间CO2浓度（Ci）升高，这说明在干旱胁迫下虽然Pn和Gs都呈下降趋势，但光合日下降以及胁迫导致的光合降低主要是由非气孔因素引起的。

英文摘要：

Three-year-old American ginseng （Panax quinquefolius L. ）plants were cultivated in a pot experiment to investigate diurnal changes of the gas exchange characteristics under well-watered （85% of soil field capacity） and water-deficiency or drough-stress （45% of soil field capacity） conditions. The results showed that drought-stress led to marked decrease of the photosynthetic rate （Pn） , stomatal conductance （Gs） , transpiration rate （Tr） and water use efficiency （WUE） , by 43.4% , 17.4% ,24.3% and 25.7% respectively, compared with those under well-watered conditions. Diurnal changes of Pn, Gs and Tr showed single peak. The diurnal change of the Pn, Gs, Tr showed larger decline under the drought stress conditions. Drought stress could improve intercellular CO2 concentration （Ci） , suggesting that although both Pn and Gs decreaced under the stress condition, Gs was not a limiting factor for the decline of photosynthesis.