Differences in Softening of ‘Shigyoku’ and ‘Heukboseok’ Grapes during Harvest Period Appears to be Related to Differences of Pedicel Vascular Bundle

Abstract

The objective of this study was to determine the differences in fruit growth, fruit quality, and particularly the pedicel vascular bundles of 'Shigyoku' and 'Heukboseok' grapes, which appeared to be different in softening at harvest. 'Shigyoku' grape matured faster (by about 20 days) than 'Heukboseok' grape with slight fruit enlargement after veraison. However, fruit of 'Heukboseok' grapes showed remarkable enlargement in both the primary and secondary fruit enlargement periods. Hypodermal cell layers were not different after veraison in 'Shigyoku' grape, but degradation of the hypodermis cell wall continued in 'Heukboseok' grape, resulting in a gradual decline in firmness. The numbers of hypodermal cell layers in 'Shigyoku' and 'Heukboseok' grapes were 14.2 and 9.0, respectively. The average content of soluble solids in 'Shigyoku' grape (19.5°Brix) was significantly (p ≤ 0.01) higher than that of 'Heukboseok' grape (17.0°Brix). Xylem of the pedicel did not differ between the two varieties. However, average phloem area after veraison of 'Shigyoku' grape (19044.8 µm 2) was about 1.8 fold greater than that of 'Heukboseok' grape (10509.4 µm 2), based on the number of cells constituting the phloem. The cell number and area of the phloem might affect the accumulation of sugars, the main constituents of the cell wall, thus maintaining the firmness of grapes until late maturity. Therefore, the increased softening of 'Heukboseok' grapes at harvest might due to their phloem structure.