Inhibitory mechanisms of cinnamic acid on the growth of Geotrichum citri-aurantii

Geotrichum citri-aurantii is the leading causes for sour rot diseases of postharvest citrus fruit, which results in a big loss of commercial value of citrus fruit. Cinnamic acid (CA) is widely used for the preservation of agricultural products due to its antiseptic and bactericidal effect. The effects of CA on the growth of G. citri-aurantii and its potential inhibitory mechanisms were investigated in this study. The results showed a complete inhibition in the growth of G. citri-aurantii caused by 400 mg L−1 CA in vitro, however, in vivo, the disease incidence in the citrus fruit decreased to 38.31% after the application of 4000 mg L−1. Scanning electron microscopy results showed that CA caused a shrinking and distortion of G. citri-aurantii hyphae, and transmission electron microscopy analysis revealed that the intracellular components were greatly leaked and degraded. When treated with 800 mg L−1 CA, 37.72% of G. citri-aurantii spores showed a destruction of plasma membrane integrity. The leakage of electrolyte and the intercellular constituents exosmosis results showed that CA led to a significant increase of the cell membrane permeability. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and the protein test kit assay showed that the number of the protein bands reduced after 3 h of CA treatment compared with the control, suggesting that small-molecular and macromolecular substances had been leaked out of the cells. Soluble protein contents of the fungal cells treated with CA significantly dropped. All these results indicated that CA was effective in protecting citrus fruit from postharvest sour rot by damaging the cell membrane of G. citri-aurantii.