Melatonin Alleviates Cinnamic Acid Autotoxicity in Melon Seedlings by Modulating Physiological and Biochemical Processes

Autotoxicity, primarily mediated by cinnamic acid (CA), is a major contributor to continuous cropping obstacles in melon (Cucumis melo L. cv. Xin Yinhui), severely inhibiting plant growth. Melatonin (MT), a pleiotropic signaling molecule, enhances plant stress tolerance. This study investigated the protective role of exogenous MT against CA-induced autotoxicity in melon seedlings by comprehensively assessing growth, photosynthesis, reactive oxygen species (ROS) accumulation, and antioxidant enzyme activities. Exposure to 0.4 mM CA severely inhibited seedling growth, reducing the relative growth rate of plant height, leaf area, and fresh weight by up to 67.71%, 59.60%, and 74.47%, respectively, and causing significant root damage. CA severely impaired photosynthesis, depressing the net photosynthetic rate (Pn) and the maximum photochemical efficiency of PSII by 69.52% and 5.27%, respectively. CA also induced severe oxidative stress, as evidenced by substantial ROS accumulation and marked decreases (21.78%, 40.45%, and 47.41%, respectively) in the activities of key antioxidant enzymes: superoxide dismutase, catalase, and ascorbate peroxidase. Exogenous application of 4 μM MT effectively mitigated these stresses. MT treatment promoted root recovery, increased leaf area and fresh weight by 40.24% and 88.25% compared to the CA-stressed group, and alleviated oxidative damage by directly scavenging ROS and restoring root antioxidant enzyme activities to near-control levels. Furthermore, MT enhanced the quantum efficiency of PSII reaction centers and electron transport, culminating in a substantial recovery of Pn. Collectively, our findings demonstrate that exogenous MT comprehensively alleviates CA-induced autotoxicity in melon seedlings through coordinately improving growth, antioxidant capacity, and photosynthetic function.