Response of plant physiological parameters to soil water availability during prolonged droughtis affected by soil texture

Soil water deficit is increasingly threatening the sustainable vegetation restoration and ecological construction on the Loess Plateau of China due to the climate warming and human activities. To determine the response thresholds of Amygdalus pedunculata(AP) and Salix psammophila(SP)to soil water availability under different textural soils, we measured the changes in net photosynthetic rate(Pn), stomatal conductance(Gs), intercellular CO2concentration(Ci), leaf water potential(ψw), water use efficiency(WUE) and daily transpiration rate(Td) of the two plant species during soil water content(SWC) decreased from 100%field capacity(FC) to 20%FC in the sandy and loamy soils on the Loess Plateau in the growing season from June to August in 2018. Results showed that Pn, Gs, WUE and Tdof AP and SP remained relatively constant at the beginning of soil water deficit but decreased rapidly as plant available soil water content(PASWC) fell below the threshold values in both the sandy and loamy soils. The PASWC thresholds corresponding to Pn, Gsand Ciof AP in the loamy soil(0.61, 0.62 and 0.70, respectively) were lower than those in the sandy soil(0.70, 0.63 and 0.75, respectively), whereas the PASWC thresholds corresponding to Pn, Gsand Ciof SP in the loamy soil(0.63, 0.68 and 0.78, respectively) were higher than those in the sandy soil(0.58, 0.62 and 0.66, respectively). In addition, the PASWC thresholds in relation to Tdand WUE of AP(0.60 and 0.58, respectively) and SP(0.62 and 0.60, respectively) in the loamy soil were higher than the corresponding PASWC thresholds of AP(0.58 and 0.52, respectively) and SP(0.55 and 0.56, respectively) in the sandy soil. Furthermore, the PASWC thresholds for the instantaneous gas exchange parameters(e.g., Pnand Gs) at the transient scale were higher than the thresholds for the parameters(e.g., Td) at the daily scale. Our study demonstrates that different plant species and/or different physiological parameters exhibit different thresholds of PASWC and that the thresholds are affected by soil texture. The result can provide guidance for the rational allocation and sustainable management of reforestation species under different soil conditions in the loess regions.