Autotoxicity in Cucumis melo L. and its alleviation by exogenous silicon: Physiological and biochemical mechanisms

Melon, a globally important horticultural crop, faces increasing continuous cropping obstacles (CCOs) due to cultivation intensification, with autotoxicity being a primary cause. Autotoxin accumulation severely impacts plant growth, reducing yield and quality. Exogenous silicon (Si) plays an important role in improving plant stress adaptation and is an environmentally friendly element with broad application prospects. This study investigated the alleviative effects and underlying mechanisms of Si on melon autotoxicity through integrated morphological, physiological, biochemical, and gene expression analyses. The results demonstrated that autotoxicity severely inhibited plant growth, manifesting as 18-30 % reductions in root length, 12-17 % decreases in plant height, and a 16.03 % decline in aboveground fresh weight. Concurrently, oxidative stress was markedly induced, exemplified by a 163.43 % increase in leaf malondialdehyde content. The application of Si effectively mitigated these perturbations, resulting in enhancements of up to 38.10 % in root length and 38.26 % in root dry weight. It also reduced oxidative damage, decreasing malondialdehyde and O₂^- content by over 30 % and 23.40 %, respectively. The antioxidant system was significantly bolstered, with superoxide dismutase and peroxidase activities elevated by up to 102.91 % and 56.38 %. Furthermore, photosynthetic performance was substantially restored, as indicated by increases of up to 56.94 % in chlorophyll a and 71.75 % in total chlorophyll content. At the molecular level, Si application upregulated the expression of most Si transporter genes that were inhibited under autotoxicity. Correlation analysis demonstrated significant negative relationships between growth parameters and oxidative stress markers. These findings contribute to understanding the mechanisms of Si in alleviating autotoxicity stress and provide valuable insights for overcoming CCOs in melon production.