Melatonin-mediated physio-biochemical, and ultrastructural modifications in tomato plants to foster drought stress tolerance

Drought stress is the most critical threat to cultivated crops including tomato plants and poses a potential challenge to world food security. Melatonin (MEL) is a multifaceted signaling pleiotropic molecule involved in diverse plant processes. In this study, we explored the potential of foliar-applied MEL (50 or 100 mu M) on two varieties (K-21 and S-22) of tomato (Solanum lycopersicum L.) under different drought levels. Drought stress was imposed using a reduced irrigation strategy based on re-watering at 60% and 30% of the field capacity. Drought stress resulted in a significant decrease in overall growth parameters, negatively affecting the photosynthetic parameters, disturbing the stomatal movement, and damaging the ultrastructure of the chloroplast. Moreover, severe drought stress level (30% FC) caused a significant decrease in rubisco activity by 35.46%, Fv/Fm by 36.07%, SPAD chlorophyl value by 48.85%, along with upsurge in malondialdehyde content by 153.23%, electrolyte leakage by 42.18%, and reactive oxygen species (ROS: H2O2 by 63.73% and O-2(center dot-) by 48.89% in K-21 variety, while in S-22 the values of rubisco activity, Fv/Fm and SPAD chlorophyl value were decreased by 41.20%; 47.06%; 60.33%; while the malondialdehyde content, electrolyte leakage, H2O2 and O-2(center dot-) were increased by 162.69; 60.67%; 73.39 and 59.96% respectively. Interestingly, foliar application of MEL (50 mu M) alleviated the drought-induced damage (30% FC) by improved the activities of antioxidant enzymes (superoxide dismutase activity by 81.87 and 66.66%, peroxidase activity by 72.06 and 55.62% and catalase activity by 62.85 and 54.25%), enhanced proline content by 56.72 and 44.27%, protected the ultra-structure of chloroplast, maintained proper movement of stomata and substantially reduced the oxidative stress in K-21 and S-22 variety respectively. In conclusion, when tomato plants were subjected to drought stress, MEL treatment counteracted drought-induced negative impact by improving the growth attributes, photosynthetic efficiency, protects the ultrastructure of chloroplast and root tissues, improves the cell viability as well as boosts the antioxidant machinery, which ultimately promotes the growth, yield of the tomato plants. Notably, the impact of drought stress was more prominent in S-22 than K-21, however the counteracting effects of MEL under drought stress was pronounced in K-21 than S-22.