UV-B Light Elicits Mitochondrial Hyperfusion and Modulation of Antioxidant Systems in Flavedo of Postharvest Lemons

Abstract

In a previous report, we demonstrated that UV-B radiation (UVBr) effectively prevents major postharvest fungal diseases in lemons, offering a residue-free alternative to chemical treatments. The present study aimed to investigate anatomical and biochemical changes induced by UVBr in lemon flavedo that may enhance intrinsic fruit defenses. Lemons (Citrus limon (L.) Burm. var. Eureka) were harvested, exposed to 0.1 J/cm2 UVBr, and evaluated at 1 and 7 days post-irradiation (dpi), considering previously standardized conditions. At 1 dpi, the flavedo tissue of UVBr treated lemons exhibited structural and morphological characteristics similar to the controls, whereas at 7 dpi deposits of filamentous substances were observed within and around the cells. Additionally, at both assayed times, irradiated flavedo cells showed a higher number of mitochondria, peroxisomes, glyoxysomes, mixed-type chromoplasts, and amyloplasts, compared to untreated samples. Mitochondria of irradiated lemons cells exhibited changes in shapes and sizes, which constitute an indicative of mitochondrial hyperfusion. UVBr also altered the antioxidant system responses in flavedo cells. At 1 dpi, NADPH oxidase activity increased, while superoxide dismutase (SOD) and catalase activities decreased. At 7 dpi, peroxidase and SOD activities increased, whereas ascorbate peroxidase activity decreased. Ascorbic acid content decreased by 34% at 1 dpi, a difference that was not observed at 7 dpi. Reduced-glutathione content increased by approximately 28% after irradiation, at both assessed times. Together, UVBr triggers modifications in the cellular ultrastructure and antioxidant system of lemon flavedo, which may contribute to strength postharvest fruit defenses against pathogens. Findings highlight UVBr as a promising tool to apply before storage and commercialization of fruits.