Exogenous Application of Melatonin Improves the Growth and Productivity of Two Broccoli (Brassica oleracea L.) Cultivars Under Salt Stress

Salt accumulation in irrigated soils is a worldwide hazard to plants, which adversely reduces crop productivity. Broccoli is considered moderately tolerant to salinity. Melatonin is a multifunctional organic biomolecule with significant importance in plants against abiotic stresses. In this experiment, we studied the role of exogenously applied melatonin on broccoli cultivars under salinity. For this experiment, two popular cultivars (Marathon and Greenport) were selected. Three levels of melatonin (control, 50, & 100 mu M) were sprayed on broccoli cultivars exposed to salt stress (0, 40, 80, & 120 mM) and under non-stressed conditions. The results showed that exogenous melatonin application improved morphological characteristics of broccoli, i.e., plant height, the number of leaves, head weight, head diameter, root length, shoot length, leaf relative water content, the number of florets, shoot fresh weight, root fresh, and dry weight. The application of 50-mu M melatonin as foliar spray increased TSS, Acidity, Vitamin C, beta-carotene, and total phenolics in the broccoli head. Additionally, the foliar spray of melatonin enhanced chlorophyll (a & b) in the head and leaves and reduced antioxidant damage under salinity by enhancing the activity of antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT). Lower malondialdehyde (MDA), hydrogen peroxide (H2O2)(,) and higher Proline and glycine betaine (GB) contents were observed in exogenously applied melatonin than in controlled plants under salinity stress in both cultivars of the broccoli. The exposure to salinity (NaCl) determined an increase in Na+ and K+ concentrations in plant tissues. However, exogenously applied melatonin reduced the uptake of Na+ from the soil. Among the cultivars, Marathon showed more tolerance against salt stress than Greenport. Collectively, our data provide evidence that the exogenous application of melatonin may ameliorate the adverse effects of salinity on broccoli plants.