Nitric oxide regulates watermelon ( responses to drought stress.

The role of exogenous nitric oxide (NO) application in alleviating drought stress responses by enhancing the antioxidant activities in plants is well established for several species. However, none of the studies reported its role in protecting the watermelon genotypes from drought stress. In this study, we aimed to observe the effect of NO application on the physiological and biochemical responses of the two watermelon ( var. ) genotypes grown under drought stress conditions by treating the plants with 15% polyethylene glycol 6000 (PEG 6000) and 100 µM sodium nitroprusside (SNP), which is a NO donor in Hoagland solution. Among the two genotypes, one genotype, KAR 98 was drought tolerant; while another, KAR 147 was drought sensitive. Drought stress showed a decrease in the growth parameters of both the genotypes; however, as expected it was higher in the susceptible genotype, KAR 147. NO application could not prevent the reductions in the growth parameters; however, it reduced the increment in malondialdehyde (MDA) content caused by the drought stress in both watermelon genotypes. Moreover, while drought stress condition reduced the ascorbate peroxidase (APX), catalase (CAT), glutathione reductase (GR), and peroxidase (POX) activities in both genotypes, NO + PEG application increased the APX activity in the tolerant genotype, KAR 98. Though the obtained results does not show the direct involvement of NO in increasing drought tolerance of watermelon plants, the increase in the APX antioxidant enzyme activity on NO application under drought stress confirmed its role in protecting the watermelon genotypes from the oxidative damage caused by the drought stress. Moreover, it can be concluded that the effect of NO application on watermelons' responses towards drought stress condition may vary according to the specific genotypes. As to date none of the studies reported the effect of NO application on the antioxidant activity of watermelon genotypes under drought stress, the present study may provide information about the mechanisms that can be focused to improve drought stress tolerance of watermelon genotypes.