Optimizing chloride and ammonium nutrition in Poncirus trifoliata: Balancing growth enhancement and toxicity mitigation

Due to the similar phytotoxicity symptoms induced by chloride (Cl⁻) and ammonium (NH 4 ⁺) in citrus, which complicate diagnosis and nutrient management, this study examined the individual and combined effects of NH 4 ⁺ (4, 8, 16 mM) and Cl⁻ (0, 8, 32 mM) on the growth, nitrogen metabolism, and antioxidant responses of Poncirus trifoliata seedlings. Moderate concentrations of 8 mM Cl⁻ and 4 mM NH 4 ⁺ (Cl 8 N 4 ) significantly increased growth parameters, including plant height, biomass, and leaf chlorophyll content, while reducing oxidative stress indicators such as malondialdehyde (MDA) and proline in leaves. In contrast, higher ion concentrations led to leaf chlorosis, membrane damage, and impaired nitrogen metabolism, with NH 4 ⁺ exhibiting a stronger toxic effect than Cl⁻. Nitrogen metabolism analysis demonstrated that the combined treatment Cl 8 N 4 optimized the levels of soluble sugar, glutamate, and free amino acids, while maintaining the activity of key enzymes such as nitrate reductase, glutamine synthetase, and glutamate synthase. Additionally, Cl 8 N 4 enhanced photosynthetic efficiency (Fv/Fm), reduced MDA levels, and boosted antioxidant enzyme activity, thereby minimizing oxidative damage. In contrast, other combined treatments significantly inhibited plant growth and disrupted metabolic functions. Redundancy analysis further confirms the beneficial role of the Cl 8 N 4 combination in enhancing plant adaptation to ionic stress. These findings highlight the importance of balanced NH 4 ⁺ and Cl⁻ levels in promoting citrus growth and stress resilience. Accordingly, chloride containing fertilizers may be beneficial in citrus cultivation, but their application alongside excessive NH 4 ⁺ should be avoided due to the risk of increased toxicity. • Poncirus trifoliata is more susceptible to NH 4 ⁺ toxicity than to Cl⁻ stress. • 4 mM NH 4 ⁺ and 8 mM Cl⁻ promote the growth and development of Poncirus trifoliata. • Cl⁻ fertilizers enhance citrus growth, while avoiding excessive application of NH 4 ⁺ to prevent toxicity.