Magnesium application mitigates sunburn damage in citrus fruits by enhancing photosynthetic efficiency and antioxidant defense

The increasing temperature and intense sunlight events, driven by climate change, have exacerbated citrus sunburn, leading to significant fruit damage and quality loss. Magnesium (Mg), a crucial component of chlorophyll in light energy utilization, has a largely unexplored role in mitigating sunburn in citrus fruits. This study investigated the effects of Mg treatments on sunburn incidence, damage severity, photosystem II (PSII) integrity, and oxidative stress in citrus peel, using Satsuma mandarins and navel orange in both field trials and simulated sunburn conditions. Field trial indicated that Mg applications at 150 g MgO tree⁻¹ for Satsuma mandarins and 200 g MgO tree⁻¹ for navel oranges reduced sunburn incidence by 6.6% and 12.4%, respectively, compared to untreated trees. Fruits from these treatments and a control group (0 g MgO tree⁻¹) were collected for simulated sunburn experiments. Mg fertilization (150 g MgO tree⁻¹) significantly increased chlorophyll content in the flavedo by 36.6% in Satsuma mandarins and 22.8% in navel oranges compared to untreated fruits. Both field and controlled experiments consistently showed that Mg supplementation significantly decreased hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) concentrations while enhancing antioxidant enzyme activities, including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT). Additionally, Mg treatments improved photosynthetic performance, regulated energy dissipation, and enhanced light energy utilization, contributing to greater PSII resilience under high light stress. Transcriptomic analysis revealed that Mg applications upregulated genes associated with cellular respiration, H₂O₂ metabolism, and lipid oxidation pathways in Satsuma mandarin flavedo. These findings highlight the role of Mg in strengthening the antioxidant enzymes and improving photosynthetic efficiency, thereby mitigating oxidative damage and lipid peroxidation under sunburn conditions. The results provide valuable insights into the potential of Mg fertilization as a strategy to reduce sunburn related damage and enhance citrus fruit quality in regions prone to high temperature and intense sunlight.