Mitigating Effects Of Exogenous Melatonin, Nitric Oxide And Their Interaction On Alkaline Stress Of Tomato Seedlings

The increasing serious soil salinization has caused extremely serious effect on crop production. To date, there are a large number of reports related with physiology mechanism of salt tolerance; however, it was only recently that the physiological mechanism of plant tolerance to alkaline stress was concerned. Tomato, a very important vegetable, is widely cultivated and consumed in the world. In addition, tomato is a moderate salt-tolerance plant and salinity-alkalinity severely reduces its production. In this study, tomato was used as experimental material to investigate the effect of melatonin(MT) and nitric oxide(NO) as well as their interactive effect on mitigating alkaline stress. The purpose was to explore the mechanism of MT regulating tomato tolerance to alkaline stress, and provide the theoretical foundation for the application of melatonin in improving alkali resistance of plants. The main results were as follows:1. Alkaline stress caused much lower level in maximum photochemistry efficiency of PSII(Fv/Fm), chlorophyll content, the activity of antioxidant enzymes and biomass accumulation of tomato plants, accompanied by the reactive oxygen species(ROS) burst, lipid peroxidation and membrane damage. The exogenous melatonin could increase tomato plants tolerance to sodic alkaline stress, and the effect depended on the concentration. In the experiment, treatment with 0.5 μM melatonin showed the strongest powerful effect on promoting tomato plant growth under alkaline stress and reducing oxidative stress via inducing activitives of antioxidant enzymes and accumulations of ascorbic acid(AsA) and glutathione(GSH), which could be partly responsible for protecting chlorophyll from degradation and keeping higher Fv/Fm. Furthermore, we found that melatonin treatment decreased Na+ content and increased K+ content in tomato leaves under alkaline stress, which showed the ion homeostasis maintenance by melatonin also played important role in enhancing tomato tolerance to alkaline stress.2. Both exogenous melatonin and nitric oxide could significantly mitigate the damage of sodic alkaline stress on tomato seedlings. It was observed that exogenous melatonin treatment elevated NO levels in alkaline–stressed tomato roots. However, exogenous NO had little effects on melatonin levels. Importantly, melatonin–induced tomato tolerance to alkaline stress was accompanied by increasing NO generation. Chemical scavenging of NO reduced melatonin–induced alkaline stress tolerance and defense genes’ expression. However, inhibition of melatonin biosynthesis had little effect on NO–induced alkaline stress tolerance. These results strongly suggest that NO acting as a downstream signal is involved in the melatonin–induced tomato tolerance to alkaline stress.