Nitrogen increases drought tolerance in maize seedlings.

Drought and nitrogen availability are two important environmental factors that affect plant growth and the global distribution of plants. We examined the effect of nitrogen on PSII in the leaves of maize seedlings under drought stress using three nitrogen concentrations (0.5, 7.5 and 15mM) and three levels of water availability (normal conditions, mild drought and severe drought). Shoot fresh and dry weights and root fresh weight decreased with increasing drought conditions. In maize leaves subjected to drought stress, the chlorophyll a (Chl a) and chlorophyll b (Chl b) contents, net photosynthetic rate, transpiration rate, stomatal conductance, maximum chemical efficiency (Fv/Fm), and photochemical efficiency of PSII (ΦPSII) were significantly reduced. Moderate nitrogen supply relieved the drought stress and enhanced the photosynthetic capacity. Malondialdehyde, H2O2 and O2-• accumulated in maize leaves under drought stress. Superoxide dismutase and ascorbate peroxidase activities increased in maize leaves under mild drought stress, but were significantly reduced under severe drought stress. The NO3- content and nitrate reductase (NR) activity of maize leaves were significantly reduced under drought stress, while moderate nitrogen supply promoted the accumulation of NO3- and an increase in the nitrate reductase activity. The abscisic acid content increased significantly; this increase was positively correlated with the nitrogen concentration under drought stress. Together, these results indicate that moderate nitrogen supply increases plant resistance to drought stress, while high or low nitrogen concentrations increase the sensitivity of maize to drought stress. These findings are important for guiding the agricultural use of nitrogen fertilisers.