Fruit drop at the junction between the calyx and fruit of longkong (Lansium domesticum Corr.) does not depend on ethylene or the induction of cell wall degrading enzymes

Fruit drop in longkong (Lansium domesticum Corr.) is a serious postharvest problem with total loss of fruit not being uncommon. Fruit drop at separation zone 1 (SZ1, between the peduncle and the calyx) was known to be due to ethylene exposure. In contrast, fruit drop at separation zone 2 (SZ2, between the calyx and the fruit) was the result of declining break strength (BS) of the zone during fruit color development (9-15 weeks after full bloom: WAFB) and the force exerted to the fruit during harvesting and handling. To better understand fruit drop in longkong, particularly at SZ2 during harvesting and handling, the role of ethylene and cell wall degrading enzymes at the two sites were investigated. The results showed that, during fruit coloration, ethylene production declined and all cell wall enzyme activities decreased at both sites. With ethylene treatment on detached bunches obtained at 13 WAFB, the BS at SZ1 was dramatically reduced and the activities of polygalacturonase (PG) and pectin methylesterase (PME) increased significantly. In contrast, a slight reduction in BS at SZ2 was observed. PG and β-1,4-glucanase activities were found to increase three and six days after treatment with ethylene, respectively. Application of 1-methylcyclopropene (1-MCP) prevented both the reduction of BS at SZ1 and the increase in the activities of cell wall degrading enzymes. However, no significant changes in BS and in the activities of cell wall degrading enzymes were observed at SZ2 as compared to the control. These results suggest that the decline in BS at SZ2 was not under the control of ethylene and cell wall degrading enzymes. It was possible that an unequal growth of cells in the SZ2 and adjacent cells in the fruit was responsible for the declining BS and subsequent fruit drop.