Critical effects on the photosynthetic efficiency and stem sap flow of poplar in the Yellow River Delta in response to soil water

To explore the critical relationships of photosynthetic efficiency and stem sap flow to soil moisture, two-year-old poplar saplings were selected and a packaged stem sap flow gauge, based on the stem-heat balance method, and a CIRAS-2 portable photosynthesis system were used. The results show that photosynthetic rates (Pn), transpiration rates (Tr), instantaneous water use efficiency (WUE) and the stem sap flow increased initially and then decreased with decreasing soil water, but their critical values were different. The turning point of relative soil water content (Wr) from stomatal limitation to nonstomatal limitation of Pn was 42%, and the water compensation point of Pn was 13%. Water saturation points of Pn and Tr were 64% and 56%, respectively, and the WUE was 71%. With increasing soil water, the apparent quantum yield (AQY), light saturation point (LSP) and maximum net photosynthetic rate (Pnmax) increased first and then decreased, while the light compensation point (LCP) decreased first and then increased. When Wr was 64%, LCP reached a lower value of 30.7 µmol m−2 s−1, and AQY a higher value of 0.044, indicating that poplar had a strong ability to utilize weak light. When Wr was 74%, LSP reached its highest point at 1138.3 µmol∙m−2 s−1, indicating that poplar had the widest light ecological amplitude and the highest light utilization efficiency. Stem sap flow and daily sap flow reached the highest value (1679.7 g d−1) at Wr values of 56% and 64%, respectively, and then declined with increasing or decreasing Wr, indicating that soil moisture significantly affected the transpiration water-consumption of poplar. Soil water was divided into six threshold grades by critical values to maintain photosynthetic efficiency at different levels, and a Wr of 64–71% was classified to be at the level of high productivity and high efficiency. In this range, poplar had high photosynthetic capacity and efficient physiological characteristics for water consumption. The saplings had characteristics of water tolerance and were not drought resistant. Full attention should be given to the soil water environment in the Yellow River Delta when planting Populus.