Nitrate nutrition increases foliar levels of nitric oxide and waterlogging tolerance in soybean

Leaves are key photosynthetic organs impaired by soil waterlogging. Here, we evaluated the role of nitrate nutrition in alleviating waterlogging-induced stress in leaves of soybean (Glycine max). Plants were cultivated with or without nitrate nutrition over short- (three days) or long-term (40 days) treatments prior to waterlogging exposure. Very small differences in leaf performance were observed between plants supplied with nitrate during the short- and long-term treatments. Both short- and long-term nitrate nutrition resulted in higher activity of nitrate reductase (EC 1.6.6.1) and higher levels of foliar nitric oxide (NO) in plants exposed to waterlogging. Nitrate nutrition did not change the activity of glutamine synthetase (EC 6.3.1.2); however, it increased the levels of amino acids in waterlogged plants. In addition, nitrate nutrition decreased foliar levels of H2O2, $$^{\cdot} {\text{O}}_{2}^{ - }$$ , and malondialdehyde under waterlogging, while increasing the activity of superoxide dismutase (EC 1.15.1.1) and catalase (EC 1.11.1.6). Nitrate nutrition further prevented waterlogging-induced decreases in photosynthetic pigment levels, leaf gas exchange, and foliar biomass accumulation. Using NO scavenger, we demonstrated that nitrate nutrition enhances soybean tolerance to waterlogging stress through NO signaling.