Background

Nutrition with ammonium (NH₄⁺) can enhance the drought tolerance of rice seedlings in comparison to nutrition with nitrate (NO₃⁻). However, there are still no detailed studies investigating the response of nitric oxide (NO) to the different nitrogen nutrition and water regimes. To study the intrinsic mechanism underpinning this relationship, the time-dependent production of NO and its protective role in the antioxidant defense system of NH₄⁺- or NO₃⁻-supplied rice seedlings were studied under water stress.

Results

An early NO burst was induced by 3h of water stress in the roots of seedlings subjected to NH₄⁺ treatment, but this phenomenon was not observed under NO₃⁻ treatment. Root oxidative damage induced by water stress was significantly higher for treatment with NO₃⁻ than with NH₄⁺ due to reactive oxygen species (ROS) accumulation in the former. Inducing NO production by applying the NO donor 3 h after N₃⁻ treatment alleviated the oxidative damage, while inhibiting the early NO burst by applying the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) increased root oxidative damage in NH₄⁺ treatment. Application of the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester(L-NAME) completely suppressed NO synthesis in roots 3 h after NH₄⁺ treatment and aggravated water stress-induced oxidative damage. Therefore, the aggravation of oxidative damage by L-NAME might have resulted from changes in the NOS-mediated early NO burst. Water stress also increased the activity of root antioxidant enzymes (catalase, superoxide dismutase, and ascorbate peroxidase). These were further induced by the NO donor but repressed by the NO scavenger and NOS inhibitor in NH₄⁺-treated roots.

Conclusion

These findings demonstrate that the NOS-mediated early NO burst plays an important role in alleviating oxidative damage induced by water stress by enhancing the antioxidant defenses in roots supplemented with NH₄⁺.