Physiological and Biochemical Responses of Dargazi Seedling and Pyrodwarf Clonal Pear Rootstocks to Melatonin Application under Drought Stress Conditions

In this research, the efficiency of melatonin for reducing the adverse effects of drought stress on two commercial pear rootstocks (Dergzi seedling and Pyrodwarf clonal rootstocks) was evaluated as factorial experiment in randomized complete block design with three replications for one year in the research greenhouse of Tarbiat Modares University, Tehran, Iran. For this purpose, 10 days before the application of drought stress, melatonin with a concentration of 100 μM was applied in the root environment of plants. Experimental factors included: rootstock at two levels (Dargazi seedling and Pysodwarf clonal rootstocks), melatonin at two levels (without melatonin and 100 μM melatonin), drought stress (irrigation in 10 days interval and withholding water for 40 days) and duration (time) of dorugh stress at four levels (10, 20, 30 and 40 days after drought stress application). The results for chlorophyll content and maximum quantum efficiency of photosystem II showed that melatonin had significant effect on reducing the adverse effect of drought stress in both rootstocks. As the duration of drought stress increased, the rate of leaf chlorophyll degradation was less in the rootstocks treated with melatonin. The interaction effects of time × drought stress, drought stress × melatonin and melatonin × time on all studied traits were significant at the 5% and 1% probability levels. Melatonin pretreatment significantly prevented ion leakage and increased cell membrane stability index in both Dergzi and Pyrodwarf rootstocks. Also, melatonin pretreatment increased total phenol in the two rootstocks and improved plant tolerance to drought stress. The total phenol in plants under severe drought stress increased by 123 and 100 percent in comparison with Daragazi seedling and Pyrodwarf rootstock, respectively. Application of melatonin also contributed to higher absorbtion of nutrient elements of sodium, potassium and phosphorus under drought stress conditions.