The difference in responses to nitrogen deprivation and re-supply at seedling stage between two barley genotypes differing nitrogen use efficiency

Understanding how crops respond to limited nitrogen supply is essential to develop new ways of manipulating genes for breeding new crop cultivars or lines with high nitrogen use efficiency (NUE). However, little is known about the differences among barley (Hordeum vulgare L.) genotypes in their responses to N starvation and subsequent N re-supply. In this study, two barley genotypes, BI-04 (higher NUE) and BI-45 (lower NUE) were used to investigate N uptake and assimilation at seedling stage in response to N deprivation and re-supply at low (3.75 mM) and normal (7.5 mM) levels. Compared to the continues normal N supply, under N deprivation, both genotypes exhibited less total biomass and N accumulation, but had higher N uptake efficiency, with BI-04 having more biomass, N accumulation and nitrate reductase activity than BI-45. The higher nitrate reductase activity in roots of BI-04 versus BI-45 was associated with up-regulated HvNar1 gene expression under N deprivation condition. NUE of both genotypes was higher under low N re-supply than under normal N re-supply after N deprivation. In addition, glutamine synthetase activity in the two barley roots was higher under low N re-supply than under normal N re-supply, which was associated with the expression of HvGS1_1 and HvGS1_2 genes. Compared to the lower NUE genotype (BI-45), the higher NUE genotype (BI-04) under low N re-supply performed better in response to N stress, and may require relatively less N fertilizer application in production.