Effect of 4-Methoxycinnamaldehyde on Postharvest Resistance to Blue Mold in Citrus Fruit

In order to explore the inhibitory effect of 4-methoxycinnamaldehyde (4-MCA) treatment on postharvest blue mold and organic acid metabolism in citrus fruit, ‘Newhall’ navel oranges were soaked in 1 000 mg/L 4-MCA or distilled water as a control, artificially wounded, inoculated with Penicillium italicum and stored at (27 ± 1.0) ℃ and relative humidity of 90%–95% for up to 96 h. Lesion diameter (LD), disease index (DI), pH, titrable acid content, organic acid contents, and the activity and gene expression of organic acid-related enzymes were measured every 24 h. Results indicated that 4-MCA treatment effectively delayed the decline in pH and the increase in titratable acid content during the whole period of P. italicum infection, significantly reduced LD and DI, and inhibited the incidence of blue mold. Compared with the control group, 4-MCA treatment inhibited the increase in the activities and gene expression of phosphoenolpyruvate carboxylase, malate dehydrogenase, malic enzyme, and citrate synthase. Meanwhile, 4-MCA treatment increased the activities and gene expression of cis-aconitase and isocitrate dehydrogenase, thereby inhibiting the accumulation of organic acids in orange peels. The above results indicate that 4-MCA treatment can enhance the resistance to P. italicum infection in harvested ‘Newhall’ navel oranges by regulating organic acid metabolism, providing new insights and a theoretical basis for the prevention and management of fungal diseases in citrus fruit.