Ammonium nutrition inhibits plant growth and nitrogen uptake in citrus seedlings

Nitrogen (N) is an essential mineral nutrient for plant growth. Appropriate N fertilization may enhance the efficiency of N use and improve crop yield. However, the effects of N form and level on citrus plants are not well understood. We hypothesized that citrus plants are ammonium (as NH4+-N or AN) partial or insensitive based on the characteristics of preferential growth in the acidic red soil of southern China, where AN is the dominant N form in the soil. Citrus (Citrus reticulata Blanco cv. Ponkan) seedlings were cultivated in sand in the presence of three N levels (1, 4 and 8 mmol L-1) supplied in two N forms (AN; nitrate, as NO3-N or NN). Biomass, root morphology, leaf characteristics, N uptake, and assimilation were determined. Compared with NN treatment, plant growth characteristics, including biomass, growth rate, root and leaf characteristics were all markedly inhibited under AN conditions, resulting in malondialdehyde (MDA) accumulation throughout the plants, except the specific leaf area. Under AN conditions, higher N concentration and free amino acid (FAA) content were observed, while N accumulation, soluble protein content, and the activities of nitrate reductase (NR), glutamine synthetase (GS) and glutamate synthase (GOGAT) in root and leaf were reduced, except for GOGAT in root. Moreover, the trend in these changes was amplified by increasing N supply, and significant interactions between N form and level were also found. Taken together, our results demonstrate that citrus plants are highly ammonium sensitive, the findings provide a theoretical basis for the optimized N management in the high-quality production of citrus.