Observation on Pericarp Ultrastructure by Scanning Electron Microscope and Its Relation to Keeping Quality of Longan Fruit

Longan (Dimocarpus longan Lour.) fruit is extremely undurable for storage, easy to water loss, browning and decay after havest. When “Fuyan” longans stored for one day at (10±1)℃ and 50% RH, fruit weight loss was 5.84%, pericarp water loss 13.59%, and pericarp began browning. When longans were stored for 2 days, fruit weight loss was 10.37%, pericarp water loss 26.40%, and pericarp browning occurrence was almost complete. Fruit weight loss rate and pericarp water loss rate both positively correlated with pericarp browning index significantly (P＜0.01). Water loss by longan fruit was mainly from the pericarp rather than the aril (pulp). Fruit weight loss and pericarp browning were significantly decreased (P＜0.01) by polyethylene film bag (0.015 mm thick) sealed packing. Pericarp structure of longan fruit was observed by scanning electron microscope. It was found that the pericarp was organised with exocarp, mesocarp and endocarp. Skin layer in the exocarp was thin. Undeveloped subrin and incomplete subrin layer, a lot of micro-cracks with mutual connection, many lenticels with different shapes and sizes appeared on the surface of exocarp. The lenticel ducts connected with the interstices between cells in the mesocarp, which were large and presented in great numbers. Longan fruit stalk was organised by vascular bundle, stone-cell and ventilating tissue, which were large cell interstices and intercellular cavity. These microstructures of longan pericarp were facilitated to postharvest fruit water loss, pericarp browning and pathogens infection, which caused poor storability and low resistant to diseases of fruit. The anatomical reasons for the main color (brown) of longan exocarp surface and easier browning in endocarp than in exocarp of the fruit, the relationship between pericarp structure and fresh-keeping of longan fruit were discussed.