EFFECTS OF GROUNDWATER DECLINE ON POPULUS EUPHRATICA AT HYPER-ARID REGIONS: THE LOWER REACHES OF THE TARIM RIVER IN XINJIANG, CHINA

We surveyed four plots at different groundwater depths in selected transects of the Tarim River of China. Water status, photosynthetic gas exchange, chlorophyll fluorescence, stem sap flow, chlorophyll content of Populus euphratica were analyzed for the effects imposed by the decline of groundwater. Water availability decreased with increasing groundwater depth, along with leaf water potential (LWP), net photosynthetic rate (P-N), water use efficiency (WUE), photochemical quenching (qP) of P. euphratica and photochemical quantum efficiency of PSII in the light (Phi(PSII)). Additionally, higher transpiration rate (E), non-photochemical quenching (NPQ) and a greater yield of dissipation by down-regulation (Y-(NPQ)) of P. euphratica were observed with increasing groundwater depth. These changes suggested that declined groundwater level could significant decrease the photosynthetic ability and integrative physiological activity of P. euphratica. Although P. euphratica could improve on its ability to uptake water, maintain normal water content by osmotic adjustment, and increase stomatal conductance and transpiration, such a survival strategy of high water-consumption but low water-use efficiency might accelerate the decline of the groundwater level due to positive feedback effects. This may result in a more serious survival crisis of P. euphratica at the lower reaches of the Tarim River.