The Effects of Drought Stress on Photosynthetic and Chlorophyll Fluorescence Characteristics of Populus euphratica and P. pruinosa

In this study,two-year-old Populus euphratica and P.pruinosa seedlings were grown in pots with varying levels of drought stress to evaluate the effects of drought on photosynthesis and chlorophyll fluorescence.Specifically,the seedlings were subjected to one of the three following water treatments for 48 h: 70%~80%(normal water supply),50%~60%(moderate drought stress) and 30%~40%(severe stress) of the field water holding capacity.The results revealed that the Pn values of P.euphratica and P.pruinosa passed through 3 states under moderate and severe drought stress,a sharp decrease,slow increase and then another decrease.After being subjected to drought stress for 16 and 24 days,the drop in Pn was primarily caused by the stomatal limit,whereas this drop was primarily caused by non-stomatal limits after being subjected to drought stress for 48 days.In addition,there were no significant differences in the Pn,WUE,Fv/Fm,Fv/Fo and qP observed in response to slight drought stress and the normal water supply for either species evaluated in this study,which indicates that slight drought stress does not result in a serious impact on the photosynthetic physiological functions of P.euphratica and P.pruinosa.However,the Gs,Pn,Fv/Fm,Fv/Fo and qP values observed in both species obviously decreased in response to severe drought stress,while NPQ increased,which demonstrates that severe drought stress has a strong effect on the photosynthetic capacity and PSII photochemical activity of P.euphratica and P.pruinosa.Moreover,significant negative correlations between Pn and Fv/Fm,Fv/Fo and Φ PSII were observed,as indicated by correlation coefficients of-0.42,-0.26 and-0.41,respectively.Conversely,Fv/Fm and Fv/Fo and Φ PSII were significantly positively correlated,as indicated by correlation coefficients of 0.39 and 0.35,respectively.Finally,when subjected to the same levels of drought stress,the values of Pn,Gs,Ci,WUE,Fv/Fm,Fv/Fo,qP and NPQ were higher in P.euphratica than in P.pruinosa,which indicates that the adaptability of P.euphratica to drought stress is greater than that of P.pruinosa.