Melatonin mitigates magnesium deficiency-induced chlorosis through enhancing photosynthetic efficiency and antioxidant capacity in wax gourd (Benincasa hispida).

Magnesium (Mg) deficiency is a major nutritional disorder that impairs chlorophyll biosynthesis, photosynthesis, and redox homeostasis, thereby limiting crop yield and quality. This study investigated the physiological, ultrastructural, and transcriptomic responses of wax gourd (Benincasa hispida) seedlings to Mg deficiency and evaluated the protective effects of exogenous melatonin applied either preventively (MT + LMg) or restoratively (LMg + MT). Mg deficiency induced significant chlorosis, reduced biomass, and photoinhibition, which were reflected by marked declines in chlorophyll content, Fv/Fm, photosynthetic rate, RuBisCO activity, dry weight, and Mg accumulation. Both preventive and restorative melatonin treatments enhanced Mg accumulation and optimized tissue distribution, increased net photosynthetic rate and stomatal conductance, maintained chloroplast ultrastructure, protected photosystem II, and mitigated oxidative stress, as evidenced by increased contents of total Mg (by 62.1% and 29.0%), enhanced net photosynthetic rate (by 61.0% and 55.1%), and reduced levels of malondialdehyde (by 33.3% and 30.4%), compared to the Mg-deficient treatment. Transcriptome and RT-qPCR analyses further revealed that melatonin significantly up-regulated the expressions of genes related to antioxidant enzymes and stress responses. Preventive melatonin treatment triggered an earlier and more pronounced activation of antioxidant and stress response pathways, while restorative treatment primarily contributed to recovery of redox homeostasis and protein protection. Collectively, this study demonstrates that melatonin strengthens plant adaptation to Mg deficiency by improving Mg accumulation, protecting photosynthetic apparatus, and systemically activating antioxidant and stress-responsive transcriptional networks. These findings highlight the potential of melatonin as a biostimulant to mitigate nutrient stress and improve crop resilience in Mg-limited agroecosystems.