Concentration–Dependent Effects of Exogenous α-Ketoglutarate on PSII Photochemical Performance and C–N Metabolism in Carya cathayensis under High Urea Application

Chinese hickory (Carya cathayensis Sarg.) is a significant woody oil tree species indigenous to China; nevertheless, excessive nitrogen (N) application has emerged as a primary limiting factor for sustainable production, as it induces leaf damage, metabolic imbalance, and premature senescence. α-Ketoglutarate (α-KG), an intermediate of the tricarboxylic acid (TCA) cycle, plays a crucial role in coordinating carbon (C) and N metabolism by providing carbon skeletons for N assimilation. In this study, one-year-old C. cathayensis seedlings were subjected to the following treatments: water control (CK), α-KG alone (40 mg/L, KG3), high urea (0.8%, U), and sequential applications of urea combined with 10, 20, or 40 mg/L α-KG (UKG1–UKG3). Photosynthetic characteristics, antioxidant responses, C–N metabolic parameters, and mineral element contents were evaluated at 0, 4, 8, and 12 weeks after treatment. High-concentration urea application led to visible leaf scorching and curling. At week 8, the urea-only treatment showed no significant difference from the water control in the six OJIP parameters; however, exogenous α-KG application significantly influenced the PSII fluorescence parameters of urea-treated seedlings. Compared with U, UKG3 markedly enhanced PSII performance, with the PIABS value rising to 0.614 ± 0.093 (approximately 6.4 times that of U). Although urea treatment increased chlorophy ll content and net photosynthetic rate (Pn), these responses were not accompanied by a significant improvement in PSII photochemical efficiency relative to CK, indicating an imbalance between carbon assimilation and photosynthetic electron utilization under high urea supply. α-KG supplementation also improved nitrogen assimilation-related enzyme activities. GS activity under UKG treatments reached a level comparable to that of KG3. NR activity under UKG treatments decreased compared with both U and KG3 but remained higher than that of CK. α-KG alone enhanced photosynthetic performance, carbon accumulation, GS/Fd-GOGAT activities, and leaf Fe concentration without causing visible stress symptoms. Overall, exogenous α-KG supplementation partially alleviated high-urea-induced physiological disturbances in C. cathayensis seedlings, which was associated with improved PSII function and a more balanced regulation of C–N metabolism. These findings provide physiological evidence supporting the potential application of α-KG in optimizing nitrogen management strategies for C. cathayensis cultivation.