TANNIN CELL CONTENT AND DISTRIBUTION IN PERSIMMON (DIOSPYROS KAKI L.) AS A CAUSE OF RESIDUAL ASTRINGENCY

Non-astringent persimmon cultivars such as ‘Fuyu’ grown in cool climates commonly contain residual astringent polyphenolic compounds at maturity. These compounds are formed within specialised tannin cells during early to mid fruit development, and in warm regions they decline to negligible concentrations between one and two months before harvest. The loss of astringency has been attributed to the dilution of soluble polyphenolics in the expanding flesh, or to heat-induced polymerisation of soluble polyphenolics into an insoluble form. These theories may help in explaining astringency loss in fruit grown under warm climate conditions, but does not adequately describe the late season loss of astringency that occurs in cooler climates, where the mean temperatures during the growing season are <20°C.

Fruit of ‘Fuyu’ and the early ripening bud mutant ‘Matsumoto wase Fuyu’ (‘MWF’) were obtained from mature trees grown at the same site under cool climate conditions. The flowering dates were the same for both cultivars, but ‘MWF’ developed peel coloration two weeks ahead of ‘Fuyu’. Changes in tannin cell size and distribution, and polyphenolic content and composition (total methanol-soluble, and constituent classes proanthocyanidin and protein-binding tannins that confer "bitter" and "Dastringent" flavours) were measured in fruit sampled at two week intervals from flowering to harvest.

Tannin cell density and size showed only minor differences between the two cultivars. Until mid season development, the two cultivars showed similar changes in fruit size, total polyphenolic content and composition. However, towards maturity, total polyphenolic content within the fruit started to decline two weeks earlier in ‘MWF’ than in ‘Fuyu’. The proportion of protein binding to total polyphenolics was similar during this phase, but at harvest ‘MWF’ had insignificant polyphenolics, whereas ‘Fuyu’ had significant residual levels.

The results show that factors other than dilution of tannin cells or heat-mediated tannin polymerisation may cause the loss of astringency under cool climate conditions. This highlights the importance of biochemical changes of polyphenolic consituents within tannin cells during late season fruit development as a mechanism of astringency loss. Because ‘MWF’, as a chimeral mutation of ‘Fuyu’, would be expected to contain only a single gene change in the epidermal histological layer, it is probable that the loss of astringency is under the same genetic control as the initiation of ripening.