Comparative Study of Water and Fertilizer Regimes on Water Potential, Phenolic Substances, and Photosynthetic Characteristics of Pistacia weinmannifolia

Effective water and fertilizer management is crucial for the forestry production of Pistacia weinmannifolia. This experiment employed an orthogonal design to measure the water potential, anthocyanins, chlorophyll, and photosynthetic parameters of Pistacia weinmannifolia under different water and fertilizer regimes. The effects of different water and fertilizer regimes on the water potential, phenolic compounds, and photosynthetic characteristics of Pistacia weinmannifolia were analyzed. A comprehensive analysis method was used to evaluate and establish the best water and fertilizer regimes system. The results showed that the water and fertilizer regimes increased the water potential, anthocyanins, chlorophyll content, flavonoids, and photosynthesis (p < 0.05). During the mid-growth stage and late mid-stage growth of Pistacia weinmannifolia, the fertilizers with the most significant effects on water potential, chlorophyll, and anthocyanins were nitrogen (N) and phosphorus (P). The supply of a certain amount of N and P had positive effect on water potential, chlorophyll, and anthocyanins. Increasing N content was more effective in improving carboxylation efficiency than increasing P content. The effect of N content on photosynthetic efficiency was greater than that of P content Analyses using the TOPSIS model demonstrate that Pistacia weinmannifolia exhibits superior comprehensive efficiency in water potential, chlorophyll, flavonoid, and anthocyanin content. When applying 0.54 g·plant−1 of pure P and 0.67 g·plant−1 of pure N, with the relative soil moisture content maintained at 85%, the optimal comprehensive benefit for photosynthetic indicators is achieved with 0.34 g·plant−1 of pure P and 0.77 g·plant−1 of pure N, while maintaining the relative soil moisture content at 46.66%. These findings indicate that the water–fertilizer coupling treatment group exhibited improved growth status and photosynthesis. Therefore, the cultivation of Pistacia weinmannifolia should prioritize maintaining a balanced water–fertilizer ratio to optimize resource utilization.