Effects of nitrogen addition on the growth and photosynthetic physiology of Gleditsia sinensis Lam. seedlings under shading conditions.

Light and atmospheric nitrogen (N) deposition are key environmental factors that significantly influence plant growth and forest regeneration. As a characteristic economic tree species in South China, Gleditsia sinensis Lam. has value for both commercial applications and ecological restoration; however, research on the effects of shading and nitrogen addition on its growth physiology has so far been limited. Therefore, this study examined the combined effects of different light environments (full light, S0; light shade, 70% light, S1; moderate shading, 40% light, S2; severe shading, 10% light, S3) and nitrogen addition levels (0 g/plant, N0; 1 g/plant, N1; 3 g/plant, N2; 5 g/plant, N3) on the leaf physiology and growth of different G. sinensis seedling families (Huishui1 and Huishui4). The results show that moderate shading positively affected the growth and development of G. sinensis, including producing increases in plant height, ground diameter, total biomass, and leaf area. At the same time, increased photosynthetic pigment content and antioxidant enzyme activity were found, indicating that G. sinensis optimizes its photosynthetic mechanisms and strengthens its antioxidant defenses under low-light conditions to cope with environmental stress. Specifically, under S2 conditions, nitrogen treatment increased the photochemical efficiency and electron transfer rate of both Huishui1 and Huishui4, suggesting that nitrogen addition could enhance the conversion efficiency of light energy. Nitrogen supplementation also significantly increased the photosynthetic pigment content, antioxidant enzyme (superoxide dismutase, SOD; peroxidase, POD; and catalase) activities, and carbon and nitrogen metabolizing substances (proline, Pro and soluble sugars, SS) in G. sinensis seedlings under low-light conditions, while reducing the chlorophyll ratio and malondialdehyde (MDA) content. Furthermore, based on a random forest model, it was determined that Pro content and SOD activity under shade and nitrogen addition treatment were the key factors affecting the growth of Huishui1. In Huishui4, photosynthetic pigment content, POD activity, SS content, and MDA content emerged as the key factors regulating seedling growth. The study revealed the physiological differences and distinct regulatory mechanisms employed by different G. sinensis families to adapt to low-light environments. Overall, the S2N2 treatment yielded the most significant physiological effects, improving the adaptation of G. sinensis to varying light environments by increasing photosynthetic pigment content, strengthening antioxidant defense mechanisms, maintaining carbon and nitrogen metabolic homeostasis, and improving light energy conversion efficiency and photoprotection. In addition, Huishui4 exhibited better shade tolerance than Huishui1.