PREFORMED ANTIFUNGAL COMPOUNDS OF LEMON FRUIT - CITRAL AND ITS RELATION TO DISEASE RESISTANCE

The young mature-green lemon fruit manifests a significantly lower level of postharvest decay as compared to the older yellow fruit. Inoculation with Penicillium digitatum Sacc. demonstrated that the resistance of young fruit to decay is related to a factor localized in the oil glands of the flavedo. The main antifungal compound of lemon flavedo was identified as the monoterpene aldehyde citral. The flavedo of green lemon contained 1.5-2 times higher levels of citral as compared to the yellow fruit. In parallel with citral decline, the flavedo extracts of yellow lemons exhibited an increased level of the monoterpene ester neryl acetate, which exerted practically no inhibitory activity against P. digitatum and, in concentrations below 500 ppm, even stimulated development of the pathogen. During long-term storage of lemon fruit, citral concentration decreased in parallel with the decline of antifungal activity in the peel and with an increase of decay incidence. It is suggested that the level of citral in the flavedo is related to the resistance of lemon fruit to postharvest decay.