Abstract
Two of the major challenges in rice cultivation are protection against pests and diseases and the management of nitrogen inputs. In addition, improperly managed nitrogen inputs can increase the susceptibility of rice to blast disease, caused by the pathogen Magnaporthe oryzae. This increase in susceptibility at the level of the infected plant is called Nitrogen Induced Susceptibility (NIS). Preliminary studies suggest a possible relationship between mechanisms of nitrogen utilization and NIS. The main objective of our work was to explore the diversity of N use and uptake in a rice panel adapted to European cultivation and to relate it to induction of susceptibility to blast disease. Nitrogen use and uptake were characterized physiologically and morphologically in the field and under controlled conditions. It appeared that there was genotype-related diversity in the amounts of nitrogen uptake after flowering. A high capacity for post-flower uptake was associated with (i) a small individual surface area of the flag leaf and (ii) a slowing of senescence. The major result of this work is the correlation that could be established between the post-flower uptake capacity (continuous and late phenomenon, impacted by a large number of factors) and the NIS (one-time phenomenon measured very early in the cycle). We sought the causes of this relationship using genetic (GWAS) and metabolic approaches. Several elements suggest that the early establishment of sink forces differs according to the plant's post-floral uptake capacity; a genotype favoring post-floral uptake would modulate very early its tillering capacities, inducing metabolite mobilization. This would be accentuated by the early addition of nitrogen, which, concomitantly, would make the plant more susceptible to blast disease in the event of an early attack. Thus, this thesis project provides the keys to identifying nitrogen use and uptake patterns in rice that, on the one hand, allow us to predict genetic predisposition to NIS; on the other hand, allow us to optimize fertilization by applying nitrogen synchronously with the phases when the plant is most receptive, leading to better nitrogen use efficiency (NUE).