Physiology and genetics of nitrogen use efficiency under saline conditions in rice (Oryza sativa L.) by Paan Thi Hong Nhung

Approximately 75% of the total area of harvested rice in the world is cultivated in the South and South-East Asian countries. However, these regions are facing both over-use of N fertilizers and salt intrusion from the sea into coastal area and river deltas. Therefore, improvement of nitrogen use efficiency (NUE) under saline conditions is crucial for both food security and sustainable agriculture.

Our research was carried out to quantify the effect of interaction of nitrogen and NaCl on both physiology and genetics of NUE in rice. The results showed that the effect of N on growth and development was counteracted by salt. Reducing N levels resulted in increasing agronomical NUE (agNUE) and its components – absorption NUE (aNUE) and physiological NUE (pNUE) but the increase differed depending on NUE components and salinity level. By increasing NaCl, the positive effect of lowering N applied on both aNUE and agNUE was stronger than in non-saline condition; however, the effect on pNUE dismissed and had no influence under the highest NaCl concentration. Under saline condition, rice showed the highest growth under reduced N levels (1/2 - 1/4N) with the highest values of number of tillers, number of crown roots, shoot dry weight, and shoot N content. These parameters reached a peak at 1N level under non-saline condition. Thus, reducing N level is beneficial for rice under saline conditions.

By using genome-wide association study, we identified 55 QTLs associated with one of the NUE-related traits – shoot dry weight (DW), root DW, whole plant DW, and the relative plant DW between the N * NaCl treatments. Among these, 27 QTLs co-localized with previously identified QTLs for DW-related traits while the other 28 were newly detected. Ten candidate genes for eight important QTLs affecting the tested traits were identified by combining gene-based association study plus functional annotation and haplotype analyses. The identified QTLs and genes provide useful materials and genetic information for future functional characterization and genetic improvement of NUE in rice, especially under salt conditions.