Regulation of antioxidant defense and stress physiology by foliar chemical elements and osmoprotectants for enhanced fruit development in mandarin under heat and high irradiance.

Citrus fruits are frequently affected by high temperatures and severe solar radiation, particularly under arid and semi-arid environments, which causes extensive fruit sunburn and leads to significant losses in both yield and commercial quality. Developing sustainable, low cost, and practical strategies to mitigate sunburn damage while maintaining optimal fruit productivity is becoming increasingly crucial in the context of climate-driven environmental stresses. Therefore, the present study was conducted during the 2022 and 2023 growing seasons to evaluate the comparative efficacy of foliar sprays of potassium silicate (K2SiO3); 2.5% and 5.0%), zinc sulfate (ZnSO4; 0.5% and 1.0%), and a commercial sugar alcohol formulation (Sorbitol 70/70; 0.25% and 0.5%, v/v) in alleviating sunburn incidence and enhancing the agro-climatic performance of five-year-old 'Murcott' tangerine (Citrus reticulata Blanco) trees cultivated under arid conditions in sandy soil using a drip irrigation system. Foliar applications were performed three times from June to August (at monthly intervals) coinciding with macroclimatic daily ambient extremes. The results demonstrated that foliar sprays of 1.0% ZnSO4 and 0.5% (v/v) sorbitol exerted the most stable, cross seasonal multivariate performance, significantly improving individual fruit weight, peel thickness, peel firmness, total soluble solids (TSS, °Brix), and overall tree productivity compared to the untreated water-sprayed control plots, while remarkably reducing sunburn incidence. Concurrently, total fruit yield was significantly increased by up to 20.15% and 14.34% during the respective seasons, while the absolute percentage of sunburned fruits per tree was reduced by 50.14% and 44.02%, respectively. At the biochemical level, these high performance treatments were strongly associated with robust increases in catalase (CAT) and ascorbate peroxidase (APX) activities, alongside higher leaf proline and fruit juice ascorbic acid accumulation. Exploratory principal component analysis (PCA) and hierarchical clustering analysis (HCA) constructed at the individual biological replicate level further validated these patterns, revealing a stable multivariate association where elevated antioxidant profiles were consistently positioned alongside improved fruit quality and yield traits, operating in explicit opposition to sunburn indicators across individual and combined seasons. These findings suggest that the observed physiological and biochemical responses provided supportive evidence of treatment-associated stress adaptation under the prevailing heat and high-irradiance exposures. Consequently, foliar interventions utilizing 1.0% ZnSO4 or 0.5% (v/v) sorbitol present a highly practical, economically viable, and environmentally sustainable orchard management strategy for securing citrus productivity and premium fruit quality in arid, climate-challenged growing regions.