Chlorophyll-catalyzed photooxidation of limonene determines the interaction of citrus fruit with a wound pathogen

Abstract

Citrus exocarp (flavedo) contains numerous cavities filled with essential oil comprising predominantly the monoterpene hydrocarbon limonene. Limonene discharge from ruptured cavities is known to stimulate the wound pathogen Penicillium digitatum, the major cause of citrus postharvest losses. Limonene content reaches maximum in full-sized green-colored (mature-green) citrus fruit. However, in spite of their high limonene content, the mature-green fruit are less prone to decay than the fully colored ones. Mechanisms counteracting the decay-promoting effect of limonene in mature-green citrus were investigated in this work. Two antifungal spots were detected by bioautography assay in the flavedo extract of mature-green lemon, but not of the yellow one. Limonene hydroperoxides (LHP) were identified in these spots. The LHP were absent in intact oil cavities. They were generated by the interaction of the cavities' content with the ingredients of green flavedo, but only under light. Activity-driven fractioning of the deterpenated green flavedo extract revealed chlorophyll and phaeophytin as photosensitizers catalyzing the LHP generation. We suggest that the light-dependent conversion of limonene into potent antifungal and defense-activating LHP may neutralize and even reverse the disease-promoting effect of limonene. Aqueous 1% LHP emulsions showed efficacy comparable with synthetic fungicides in controlling decay of artificially pathogen-inoculated agricultural produce.