The loess plateau region is an extremely adverse eco-environment in China, and drought and water shortage are the most critical ecological factors restricting the development of local economy, vegetation restoration and the production of agriculture and forestry. With increasing water-resource crises and drying hazards, some tree species during the process of farmland to forest project grew slowly and even to dead on the parts of loess plateau; thus, the selection of tree species is critical for vegetation restoration, and the development of soil and water conservation plantation composed of shrubs, grasses and arbors is significant for accelerating afforestation and soil and water conservation.However, complications such as single tree species and unreasonable tree species selection and arrangement in some areas have hindered the farmland to forest conversion and artificial vegetation construction in the loess plateau region. These complications reflect the fact that little is known about the local shrub species, and there is a lack of researches concerning the relationship between plants and water.
In this study, Li-Cor6400 portable photosynthesis system was used to investigate the threshold effect of net photosynthetic rate (P_n), transpiration rate (T_r), water use efficiency (WUE), stomata conductance (G_s), intercellular CO₂ concentration (C_i) and stomata limitation (L_s) in leaves of three-year-old naturally regenerated Rosa xanthina L. shrubs in the loess hilly region, to soil water content obtained through artificial water supply and natural consumption, and its photosynthetic productivity was classified. The results showed that the diurnal changes of P_n and T_r showed an evident double-peaked curve when the relative water content (RWC) was in the range of 33.8%-81.3%; the reduction of P_n was mainly caused by stomatal limitation in the morning (before 11:00) and non-stomatal limitation in the afternoon (after 13:00). The time when the first peak value of P_n and T_r appeared was delayed with increasing RWCs. The daily average P_n maintained high levels in the range of about 33.8%-81.3%, and the maximum daily average P_n (8.5±1.8) mmol·m^-2·s^-1 was observed at an RWC of 71.7%. Significant differences in the response of WUE to RWC were observed, and when the RWC was greater than 47.1%, the diurnal changes of WUE showed an inconspicuous single-peaked curve at 13:00 in the afternoon, with a minor variation scope before and after 13:00. When the RWC was 47.1%, the WUE was highest with a daily average rate of (2.72±0.59) μmol·mol^-1. According to the quantitative relations between the P_n, T_r, WUE and RWC, and ecological and economic benefits, the suitable RWC for R. xanthina L. growth ranges from 33.8% to 71.7%. Based on photosynthetic and physiological parameters, the soil water availability and productivity of R. xanthina L. was classified and evaluated. More than 90.4% or less than 33.8% of RWC exhibited medium yield and efficiency; and an RWC ranging from 71.7% to 81.3% and 47.1% to 71.7% was classified as high yield and medium efficiency and high yield and high efficiency, respectively. The optimum high productivity and efficiency was observed at an RWC of approximately 57.4%. An RWC of 18.8% to 26.5% was classified as low yield and low efficiency.