The role of melatonin in improving photosynthesis and salt tolerance of Dracocephalum kotschyi Boiss.

In this study, the effect of various levels of salinity on Dracocephalum kotschyi Boiss. (Fereidunshahr population) and the role of melatonin in improving photosynthesis and the acquisition of salt tolerance was investigated using two independent experiments. In the first experiment, the effect of different concentrations of sodium chloride (0, 25, 50, 75 and 100 mM) on the growth and photosynthetic traits of 60-day-old plants was evaluated. The results showed that with increasing salt concentration the tolerance index and the amount of photosynthetic pigments, Fv/Fm, and stomatal conductance decreased, but the level of lipid peroxidation and catalase activity increased in the leaves. In the second experiment, the effect of root pretreatment with 100 µM melatonin was investigated on salt tolerance (100 mM sodium chloride) of D. kotschyi seedlings. The results showed that pretreatment with melatonin increased fresh and dry weight of aerial part, chlorophyll a and b, total chlorophyll and carotenoids, Fv/Fm, stomatal conductance and relative water content in leaves under salt stress. The positive effect of melatonin on photosynthetic parameters was closely related to its role in increasing ascorbate peroxidase enzyme activity, reducing malondialdehyde content and improving Na+, K+ and Ca2+ homeostasis in leaves, and these effects were weakened by the application of p-CPA as an inhibitor of melatonin synthesis. Based on these results, the application of external melatonin, probably by affecting the endogenous melatonin synthesis in plants, is able to moderate the osmotic stress, ionic toxicity and oxidative stress caused by salinity in D. kotschyi plants, and it is possible that melatonin treatment can use to improve phosynthesis and the level of plant's tolerance to salinity.