Melatonin Influences Stomatal Behavior, Root Morphology, Cell Viability, Photosynthetic Responses, Fruit Yield, and Fruit Quality of Tomato Plants Exposed to Salt Stress

Melatonin (Mel) is a multifunctional signaling molecule, quantified in a wide range of higher plants. It acts as a pleiotropic molecule and is involved in numerous signaling and physiological processes, particularly under harsh environmental conditions in plants. In the present study, the defensive role of Mel was explored in salt (NaCl)-stressed tomato (Solanum lycopersicum) plants. Mel (50 or 100 µM) was supplied to the foliage of tomato plants at 30 days after transplantation (DAT), grown without stress, and in the presence of NaCl (100 or 150 mM) stress. Salt stress negatively affected photosynthetic efficiency, cell viability, plant growth, fruit quality, and impaired root morphology, and stomatal physiology, but induced the accumulation of reactive oxygen species ((H2O2 and O2·−), malondialdehyde (MDA) content, and electrolyte leakage. Interestingly, foliar supplementation of Mel to stressed and non-stressed plants lowered MDA levels, reduced oxidative stress and promoted photosynthesis, improved cell viability, stomatal behavior, and plant growth, and boosted the activity of catalase (CAT), peroxidase (POX), and superoxide dismutase (SOD), and elevated proline content which bestowed endurance and reduced the salt-induced oxidative damage.