Nitrogen use efficiency (NUE) in rice links to NH4 + toxicity and futile NH4 + cycling in roots

Rice is known as an ammonium (NH4 (+))-tolerant species. Nevertheless, rice can suffer NH4 (+) toxicity, and excessive use of nitrogen (N) fertilizer has raised NH4 (+) in many paddy soils to levels that reduce vegetative biomass and yield. Examining whether thresholds of NH4 (+) toxicity in rice are related to nitrogen-use efficiency (NUE) is the aim of this study. A high-NUE (Wuyunjing 23, W23) and a low-NUE (Guidan 4, GD) rice cultivar were cultivated hydroponically, and growth, root morphology, total N and NH4 (+) concentration, root oxygen consumption, and transmembrane NH4 (+) fluxes in the root meristem and elongation zones were determined. We show that W23 possesses greater capacity to resist NH4 (+) toxicity, while GD is more susceptible. We furthermore show that tissue NH4 (+) accumulation and futile NH4 (+) cycling across the root-cell plasma membrane, previously linked to inhibited plant development under elevated NH4 (+), are more pronounced in GD. NH4 (+) efflux in the root elongation zone, measured by SIET, was nearly sevenfold greater in GD than in W23, and this was coupled to strongly stimulated root respiration. In both cultivars, root growth was affected more severely by high NH4 (+) than shoot growth. High NH4 (+) mainly inhibited the development of total root length and root area, while the formation of lateral roots was unaffected. It is concluded that the larger degree of seedling growth inhibition in low- vs. high-NUE rice genotypes is associated with significantly enhanced NH4 (+) cycling and tissue accumulation in the elongation zone of the root.