Responses of different physiological parameter thresholds to soil water availability in four plant species during prolonged drought

Large-scale vegetation restoration on the Loess Plateau in China has been performed by the central government in recent decades; however, the planting of incompatible vegetation during these efforts has resulted in serious environmental problems, such as dry soil layers, that are widespread and difficult to ameliorate. To determine the proper evaluation indices for plant available soil water content (PASWC) in commonly reforested plants and crops, we examined the physiological responses of Robinia pseudoacacia (tree), Amorpha fruticosa (shrub), Medicago saliva (perennial leguminous herb) and Zea mays (crop) to prolonged drought; these plants were planted widely in the semiarid Loess Plateau region of China. Leaf water status, gas exchange and fluorescence parameters did not show marked changes at the beginning of the prolonged drought but changed rapidly as PASWC continued to decrease. These data were fitted with a sigmoid function (P < 0.0001). In addition, different physiological parameters showed different PASWC thresholds; the fluorescence parameters exhibited the lowest PASWC threshold among the four species, with an upper threshold that was less than 50% of the PASWC for all but Z. mays. In this study, the photosynthesis rate was a better indicator of the PASWC, and the upper and lower thresholds of PASWC of the normalized photosynthesis rate were 64.1% and 47.6%, 83.0% and 44.1%, 82.7% and 35.2%, 82.9% and 39.3% for R. pseudoacacia, A. fruticosa, M. sativa and Z. mays, respectively. The current study also suggests that R. pseudoacacia is a suitable afforestation species in areas with higher levels of rainfall. These results provide important information for determining the PASWC and the supply capacity of soil water on the Loess Plateau.