Water and
nitrogen stresses are major constraints for agricultural and forest productivity. Although the effects of water scarcity or
nitrogen stress on plant growth, physiology, and yield have been widely studied, few studies have assessed the combined effects of both stresses. In the present study, we investigated the effects of different
nitrogen forms (NO3-N, NH4+-N, and a combination of NO3-N + NH4+-N) on
antioxidant enzyme activity, osmotic regulatory substances, and
nitrogen assimilation in Chinese fir (Cunninghamia lanceolata) plantlets under drought stress (induced by 10%
polyethylene glycol). We found that different N ionic forms had different effects on drought-stressed plantlets.
Nitrogen supply greatly increased the activities of
superoxide dismutase (SOD),
peroxidase (POD) and
polyphenol oxidase (PPO) when plantlets were exposed to
water stress. The
malondialdehyde (MDA) contents significantly decreased under the NH4+ + water stress treatment. The
proline (Pr) contents significantly increased in both the NO3-N and NH4+-N + water stress treatment. The
nitrate reductase (NR) increased by 7.1% in the NO3- + water stress treatment, and the
glutamine synthetase (GS), and the
glutamate synthase (GOGAT) activity increased in all the
nitrogen +
water stress treatments. These results suggested that
nitrogen supply could alleviate the adverse effects of drought stress on plants by enhancing
antioxidant defense and improving
nitrogen assimilation, while the effects on plant tolerance to drought stress varied with
nitrogen ionic forms.