Melatonin seed priming improves growth and physio-biochemical aspects of Zinnia elegans under salt stress

Salt stress is a major abiotic aspect that inhibits plant development, growth, and productivity of ornamental plants. In recent years, there has been a lot of interest in seed priming and biological treatments for salt stress mitigation in plants. Melatonin (MLT) is a versatile signaling molecule detected in higher plants. Its function is involved in various signaling and physiological processes, particularly under severe environmental conditions. The aim of this study was to assess the performance of different levels of MLT (0, 1, 10, 20 mu M) primed zinnia plants (Zinnia elegans cv. 'Super Yoga') under salt stress (100 mM NaCl). Salt stress exposure reduced growth and physiological variables and increased oxidative stress, recorded as a rise in stress markers. The results showed that seed priming with MLT improved morphological characteristics of zinnia, such as shoot and root length, shoot diameter, leaf length, root collar diameter, number of roots, root fresh, and dry weight. MLT based seed priming also increased photosynthesis pigments and reduced salt induced oxidative damage by enhancing the activity of antioxidant enzymes (i.e. superoxide dismutase-SOD, peroxidase-POD, and catalase-CAT). The reduction in oxidative stress was recorded as decrease in electrolyte leakage (EL), hydrogen peroxide (H2O2), and malondialdehyde (MDA). Primed plants also showed significant increase in the osmoprotectants proline and glycine betaine, as compared to the non-primed salt stress plants. Exposure of plants to salt stress lead to an increase in sodium (Na+) and potassium (K+) contents in plant tissues in the non-primed plants. However, priming with MLT reduced the uptake of Na+ and improved the uptake of K+ in the leaves of zinnia. Overall, these results provided evidence that MLT seed priming may ameliorate the ill effects of salt-induced oxidative stress in zinnia plants.