Nitric oxide is involved in dehydration/drought tolerance in Poncirus trifoliata seedlings through regulation of antioxidant systems and stomatal response

Nitric oxide (NO) is a component of the repertoire of signals implicated in plant responses to environmental stimuli. In the present study, we investigated the effects of exogenous application of NO-releasing donor sodium nitroprusside (SNP) and nitric oxide synthase inhibitor N-G-nitro-L-arginine-methyl ester (L-NAME) on dehydration and drought tolerance of Poncirus trifoliata. The endogenous NO level was enhanced by SNP pretreatment, but decreased by L-NAME, in the hydroponic or potted plants with or without stresses. Under dehydration, leaves from the SNP-treated hydroponic seedlings displayed less water loss, lower electrolyte leakage and reactive oxygen species accumulation, higher antioxidant enzyme activities and smaller stomatal apertures as compared with the control (treated with water). In addition, pretreatment of the potted plants with SNP resulted in lower electrolyte leakage, higher chlorophyll content, smaller stomatal conductance and larger photosynthetic rate relative to the control. By contrast, the inhibitor treatment changed these physiological attributes or phenotypes in an opposite way. These results indicate that NO in the form of SNP enhanced dehydration and drought tolerance, whereas the inhibitor makes the leaves or plants more sensitive to the stresses. The stress tolerance by NO might be ascribed to a combinatory effect of modulation of stomatal response and activation of the antioxidant enzymes. Taken together, NO is involved in dehydration and drought tolerance of P. trifoliata, implying that manipulation of this signal molecule may provide a practical approach to combat the environmental stresses.