Evaluation of Nitrogen Use Efficiency (NUE) of rice (O. sativa) under saline conditions

Rice (Oryza sativa L.) is a major food supply in the world and many challenges for its cultivation sustainability have to be faced. One of these challenges consists in an efficient management of the nitrogen fertilization. Indeed, nitrogen is one of the most important elements for the plant growth and development. However, rice cropping conditions can be favorable to nitrogen losses leading to problems like eutrophication. Developing varieties reaching higher nitrogen use efficiency could represent a valuable improvement for the crop yield and economically, but also a sustainable improvement. Another challenge is to deal with the increasing risk of abiotic stress due to climate change, such as salinity which is more frequently present in Vietnamese deltas. This study consisted in an analysis of the interactions between three fixed parameters: the nitrogen fertilization applied, the salinity level and the rice genotype. Two first experiments were conducted in hydroponics during the early vegetative phase with two nitrogen concentrations (standard N and 0.125N) and three increasing salt levels (no salt, moderate and strong salinity) for three genotypes Cuom: Vietnamese salt tolerant; Pokkali: salt tolerant reference; Ngoi: Vietnamese salt sensitive). The study was completed with an experiment in pots in Vietnam to validate the findings of the hydroponics analysis. This last experiment was conducted until the harvest but was focused on the active tillering. Several conclusions can be highlighted with this work. The first one was the absence of salt sensitivity before the seedling emergence. The second conclusion showed that the N toxicity seemed to increase with the salinity. Finally, the salt tolerance had an influence on the Nitrogen Use Efficiency (NUE). A high salt-tolerance cultivar, such as Cuom, presented a high absorption (aNUE) and agronomical NUE (agNUE) coupled to a low physiological one (pNUE), while the opposite NUE levels were more related to salt-sensitive cultivar. The Vietnamese in-situ experiment did not invalidate the conclusions but can only confirm the negative influence of salt stress on rice and the basic nitrogen effects in absence of salt. Nitrogen Use Efficiency is a valuable indicator to be considered in future studies regarding the salinity tolerance of rice.