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 (P_n), transpiration rates (T_r), 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 (W_r) from stomatal limitation to nonstomatal limitation of P_n was 42%, and the water compensation point of P_n was 13%. Water saturation points of P_n and T_r 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 (P_nmax) increased first and then decreased, while the light compensation point (LCP) decreased first and then increased. When W_r 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 W_r 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 W_r values of 56% and 64%, respectively, and then declined with increasing or decreasing W_r, 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 W_r 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.

为了探索光合作用效率和茎汁流量与土壤水分的关键关系，选择了两岁的杨树树苗，并根据茎热平衡法和CIRAS-2便携式光合作用系统包装了打包的茎汁流量计。被使用。结果表明，随着土壤水分的减少，光合速率（P _n），蒸腾速率（T _r），瞬时水分利用效率（WUE）和茎液流量先增大后减小，但其临界值不同。土壤相对含水量的转折点（w ^ _{- [R} ）从气孔限制到非气孔限制P _Ñ为42％，P _n的水补偿点为13％。P _n和T _r的水饱和点分别为64％和56％，WUE为71％。随着土壤水分的增加，表观量子效率（AQY），光饱和点（LSP）和最大净光合速率（P _n最大）增加，然后再下降，而光补偿点（LCP）先减小后增加。当W _r为64％时，LCP达到较低的值30.7 µmol m ^-2  s ^-1，并且AQY的较高值0.044，表明白杨具有较强的利用弱光的能力。当W _r为74％时，LSP达到最高点1138.3 µmol∙m ^-2  s ^-1，表明杨树具有最大的光生态幅度和最高的光利用效率。树干液流与每日液流达到最高值（1679.7克d ^-1处）w ^ _{- [R}值的56％和64％，分别的值，然后用增加或减少下降W¯¯ _ř，表明土壤水分显着影响杨树的蒸腾耗水量。根据临界值将土壤水划分为六个阈值等级，以将光合作用效率维持在不同水平，并且将W _r的64–71％划分为高生产力和高效率。在此范围内，杨树具有较高的光合作用能力和高效的生理耗水特性。幼树具有耐水特性，不耐干旱。种植杨树时应充分重视黄河三角洲的土壤水分环境。