Fig. 7

Schematic illustration of a proposed model for the different responses of early NO production and its effects on the defense response of rice to water stress. In the roots of NH₄⁺-supplied rice, the NOS-mediated early NO burst (3 h) significantly enhanced plant antioxidant defense by reducing ROS accumulation and enhancing antioxidant enzyme activity; the relative lower NO production after 24 h of water stress in comparison to NO₃⁻-supplied rice also helped maintaining the redox balance in root cells, thus enhancing their drought tolerance. In the roots of NO₃⁻-supplied rice, ROS accumulation and oxidative damage induced by 3 h of water stress were significantly higher than that in NH₄⁺-supplied rice. High NO accumulation in the NO₃⁻-treated roots likely caused the nitrosative stress at 24 h of water stress. A combined effect of oxidative and nitrification stresses might have led to the weak resistance to water stress in NO₃⁻-supplied rice. NR, nitrate reductase. Red arrows represent increase; green arrows represent decrease. Black solid arrows represent defined pathways, dotted arrows represent undefined pathway