Chlorophyll Fluorescence Characteristic of Populus euphratica under Drought Stress in the Lower Reaches of Tarim River

In two typical monitoring sections in the lower reaches of Tarim River,soil water content,leaf water potential and chlorophyll fluorescence parameter of Populus euphratica near four ecological monitoring wells with different groundwater level were measured.The results showed that the deeper groundwater level,the fiercer drought stress imposed to Populus euphratica,and series of chlorophyll fluorescence parameters of Populus euphratica,such as photochemical quantum yield of PS Ⅱ in the light(Yield),electron transport rate(ETR),photochemistry quenching(qP)and relative chlorophyll content and so on,decreased evidently with the decrease of groundwater level,and there was greater decrease of chlorophyll fluorescence parameters near the monitoring well with deeper groundwater level.Non-photochemistry quenching(qN,NPQ)and the Yield for dissipation by downregulation(YNPQ),as an available chlorophyll fluorescence parameter to weigh the self-protective capability of PSⅡ,increased remarkably with photosynthetic active radiation(PAR).The potential maximum quantum yields(Fv/Fm)of all Populus euphratica samples maintained optimum values,indicating that Populus euphratica in the lower reaches of the Tarim River could well maintain their biological functions of PSⅡ at range 4—8 m of groundwater level.Irreversible damage from photo-inhibition did not yet occur in the PSⅡ of those Populus euphratica owing to good adaptability and drought tolerance although drought stress occurred actually.