Constitutive and induced resistance of citrus fruit against pathogens

摘要

The mechanisms of resistance of citrus fruit against pathogens were investigated. Oil cavities of citrus flavedo (exocarp) constitutively contain a range of materials. many of which (citral, perillaldehyde, perillalcohol, carveol, citronellal, hexanal etc.) possess antifungal activity. Mechanical wounding releases the contents of the cavities, which may come into contact with penetrating wound pathogen, such as Penicillium digitatum. In young mature-green citrus fruit, the complex of constitutive antifungal compounds acts as a first line of defence against pathogens. Depending on the physiological age of the fruit the materials released from the oil cavity may either inhibit or encourage pathogen development. The effect of several postharvest treatments on citrus decay(growth regulators, ethylene, heat) might be related to modulation of the ageing-associated decline of constitutive antifungal materials. In addition to the direct effect on the pathogen, the release of the contents of the destroyed oil cavities was shown to act as a primary signal triggering the induced defensive reactions in the flavedo and albedo (mesocarp) of mature-green citrus fruit. The induced resistance mechanisms (phytoalexins accumulation, cell wall reinforcement) could be elicited also by pathogen challenge followed by heat treatment, by biocontrol yeast agents, and by ultraviolet illumination. The accumulation of phytoalexins, scoparone and scopoletin, is a convenient biochemical marker of induced defensive reactions in citrus. Although PR-proteins (chitinases) were detected as constituents of the orange peel, their level was markedly affected by pathogen inoculation and subsequent heat treatment. The evolutionary advantages and practical implications of these mechanisms will be discussed.