Early Detection of Alternaria alternata Infection in Fresh Pepper Fruit Based on Chlorophyll Fluorescence Properties and Membrane Leakage

Timely identification of Alternaria alternata infection in postharvest green peppers is crucial before overt symptoms. This study evaluated peppers' appearance, chlorophyll, chlorophyll fluorescence parameters, and membrane leakage after A. alternata infection. Fatty acids of thylakoid membrane in peppers were determined by gas chromatography-mass spectrometry (GC-MS). Diseased fruit showed early declines in appearance, chlorophyll, fluorescence parameters, and membrane integrity, with minimum fluorescence (F0), electrical conductivity, and the absorbance of nucleic acid at 260-nm wavelength (OD260) distinguishing them from the controls within 1 d. These changes preceded sensory quality and other chlorophyll metrics. Diseased areas exhibited significant differences than healthy areas. Total fatty acid contents in the controls increased, while those in diseased fruit decreased. Linoleic acid had a significant correlation with membrane leakage, fluorescence, and chlorophyll, respectively. The degradation of unsaturated fatty acids led to significant changes in chlorophyll fluorescence parameter and membrane leakage. F0, electrical conductivity, and OD260 were all suitable for early identification of A. alternata infection in postharvest peppers.