Seedlings of Poncirus trifoliata exhibit tissue-specific detoxification in response to NH 4 + toxicity.

Ammonium nitrogen (NH 4 + -N) is essential for fruit tree growth, but the impact of excess NH 4 + -N from fertilizer on evergreen citrus trees is unclear. In a climate chamber, 8-month-old citrus plants were exposed to five different hydroponic NH 4 + -N concentrations (0, 5, 10, 15 and 20 mm) for 1 month to study effects of NH 4 + -N on growth characteristics, N uptake, metabolism, antioxidant enzymes and osmotic regulatory substances. Application of 10 mm NH 4 + -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 NH 4 + -N assimilation in leaves, while the primary pathway involved in roots was GS-GOGAT. Under comparatively high NH 4 + addition, roots were the main organs involved in NH 4 + utilization in citrus seedlings. Our results demonstrated that variations in NH 4 + concentration and enzyme activity in various organs are associated with more effective N metabolism in roots than in leaves to prevent NH 4 + 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.