Seedlings of Poncirus trifoliata exhibit tissue‐specific detoxification in response to NH₄⁺ toxicity

Abstract

Ammonium nitrogen (NH4+‐N) is essential for fruit tree growth, but the impact of excess NH4+‐N from fertilizer on evergreen citrus trees is unclear. In a climate chamber, 8‐month‐old citrus plants were exposed to five different hydroponic NH4+‐N concentrations (0, 5, 10, 15 and 20 mm) for 1 month to study effects of NH4+‐N on growth characteristics, N uptake, metabolism, antioxidant enzymes and osmotic regulatory substances. Application of 10 mm NH4+‐N adversely affected root plasma membrane integrity, root physiological functions, and plant biomass. MDA, CAT, POD, APX and SOD content were significantly correlated with leaf N metabolic enzyme activity (GOGAT, GDH, GS and NR). GDH was the primary enzyme involved in NH4+‐N assimilation in leaves, while the primary pathway involved in roots was GS‐GOGAT. Under comparatively high NH4+ addition, roots were the main organs involved in NH4+ utilization in citrus seedlings. Our results demonstrated that variations in NH4+ concentration and enzyme activity in various organs are associated with more effective N metabolism in roots than in leaves to prevent NH4+ toxicity in evergreen woody citrus plants. These results provide insight into the N forms used by citrus plants that are important for N fertilizer management.