Exogenous melatonin and grafting improved the growth and physiological characteristics of Citrullus lanatus seedlings under cadmium stress

Citrullus lanatus is an important vegetable crop, but it is heavily polluted by cadmium. In this study, we used C. Lanatus as experimental material to investigate effects of different concentrations (0, 50, 100, 200, 400 µmol.L−1) of exogenous melatonin, and grafting on the physiological growth index and anatomical structure of seedlings were studied by simulating Cd2+ (180 mg L−1) stress environment. The results showed that exogenous melatonin could reduce the injury degree of C. Lanatus seedlings under cadmium stress, and the best effect was obtained when the concentration is 100 µmol.L−1. Compared with the control treatment (0 µmol.L−1 melatonin), the number of leaves treated with 100 µmol.L−1 melatonin increased by 32.61%, the density of mesophyll tissue decreased by 16.47%, the relative content of chlorophyll in seedling leaves increased by 42.90%, and the activity of superoxide dismutase (SOD) and the content of soluble protein increased by 28.64% and 60.11%, respectively. In addition, grafting could effectively alleviate the damage of cadmium stress to C. lanatus seedlings, and the effect of 100 µmol.L−1 melatonin and grafting interaction on seedlings was higher than that of single melatonin or grafting treatment. Under cadmium stress, compared with the self-rooted seedlings of melatonin (0 µmol.L−1), the stem diameter and the diameter of main vein increased by 55.56% and 34.12%, and the water content of leaf tissue increased by 93.55%. In addition, the relative conductivity and MDA content decreased by 54.45% and 53.07%, respectively. Therefore, there was a significant interaction between exogenous melatonin and grafting in alleviating the injury of C. lanatus seedlings under cadmium stress. Our research explored the optimal concentration of exogenous melatonin to alleviate the injury of C. lanatus seedlings under cadmium stress that provided theoretical basis for revealing the mechanism of exogenous melatonin to improve the resistance of C. lanatus to cadmium stress.