Nitrogen assimilation under osmotic stress in maize (Zea mays L.) seedlings

Nitrogen (N) is one of the most important nutrients affecting maize productivity. The effects of osmotic stress on nitrogen assimilation still remain unclear. The aim of this study is to characterize the physiological and biochemical responses to the N level and examine the expression of the genes involved in the N assimilation pathway under osmotic stress. Maize seedlings were supplied with three N levels; low N (LN, 0.5 mM), moderate N (MN, 10 mM) and high N (HN, 20 mM) with or without 10% of PEG(6000). Results showed that osmotic stress reduced photosynthesis, and transpiration rate as well as stomatal conductance while increased N assimilation at HN. The activities of NR and GPT enzymes were negatively correlated with the N level. However, a positive correlation was observed between the activities of other N assimilation related enzymes and HN level under osmotic stress. The relative expression of genes involved directly in nitrate transport, for example, ZmNRT1.2, ZmNRT2.2, and ZmNRT2.3 were higher at LN level. At LN application, the genes involved in the N assimilation pathway, like ZmGln4, ZmGln5, ZmGs1.3, ZmGs1.4, and ZmSupp showed higher expression levels under osmotic stress. Overall, the expression level of osmotic-stress related genes was decreased at HN level. Taken together, we concluded that though the mRNA levels and enzymatic activities of N assimilation related enzymes were varied and not typically correlated at various nitrogen levels under osmotic stress. However, the expression level of major N assimilation related genes could be used as biomarkers for the identification of maize genotypes or mutants with high nitrogen assimilation under osmotic stress.

Automated summary

This study revealed that under osmotic stress, nitrogen assimilation was increased at high nitrogen levels, while activities of related enzymes were negatively affected by nitrogen levels. Genes involved in the nitrogen assimilation pathway showed higher expression levels at low nitrogen levels under osmotic stress.