Postharvest senescent dark spot development mechanism of Musa acuminata ("Khai" banana) peel associated with chlorophyll degradation and stomata cell death

Abstract

The occurrence of the postharvest physiological disorder of dark spots on the peel of the ripened "Khai" banana has led to a reduction in its commercial value. The objective of the present study was to investigate the development mechanisms of senescence dark spots of the "Khai" (Musa AA group) banana peel in relation to chlorophyll degradation and stomata cell death. Freshly harvested bananas (commercial mature green stage) were let to ripened at 25 ± 2°C (90%-95% RH). Peel color, senescent spots, DNA degradation, chlorophyll content, chlorophyll-degrading enzyme activities were assessed. The senescent dark spots developed on the ripened bananas right after 6 days of storage, which coincided with remarkably increased DNA degradation, and a rapid decreased of hue angle value and total chlorophyll content which indicated the chlorophyll degradation. The activities of chlorophyllase, chlorophyll-degrading peroxidase and pheophytinase increased gradually to the highest point where the chlorophyll content drastically reduced and the appearance of the dark spots was first recorded after 6 days of storage. These dark spots were observed to be surrounded with a bright luminescent ring of hypermodified fluorescent chlorophyll catabolites (FCCs), the product of chlorophyll breakdown. Additionally, the scanning electron microscope (SEM) revealed that the dark spots were found to have originated from the collapsed cells around the stomata of the ripened banana peel whereby the chlorophyll was entirely diminished. PRACTICAL APPLICATIONS: This research revealed the senescent dark spot development mechanisms of the "Khai" banana peel. The dark spot development symptom on the banana peel surface was caused by the senescence and cell death of the relevant stomata, further associated with chlorophyll degradation. Therefore, any further research into minimizing the dark spot symptom must focus on preventing or delaying stomata senescence and cell death.

Keywords: chlorophyll degradation; fluorescent chlorophyll catabolites; senescent dark spot; stomata cell death.