Effect of the Cytokinin Forchlorfenuron on Tannin Content of Thompson Seedless Table Grapes

Abstract

The cytokinin-like compound forchlorfenuron (CPPU) is commonly applied in many vineyards worldwide to achieve optimal berry size. It was noticed that berries treated by CPPU may be more astringent and it was therefore hypothesized that CPPU-treated berries produce more tannins or delay tannin decomposition during ripening. CPPU was applied to Thompson Seedless grapes (Vitis vinifera L.) after fruit set in three different vineyards and over three seasons. As previously reported, CPPU affected berry size and delayed maturation. Application of CPPU at 6 mm berry diameter (early treatment) had greater influence on increasing berry size, while application at 10 mm berry diameter (late treatment) had greater influence on delaying berry maturation. Berry ripening was also measured by following changes in berry autofluorescence, and it was shown that CPPU caused higher chlorophyll-related autofluorescence throughout the ripening period. The CPPU treatments consistently elevated the level of condensed tannins with up to a four-fold increase as measured by the protein precipitation assay and total phenol content. The level of total tannins in either control or CPPU-treated fruit did not seem to decrease during ripening. These results were in agreement with a sensory assessment in which CPPU-treated berries were more astringent than untreated berries. As reported here for the first time, the fact that CPPU can increase the level of tannins in Thompson Seedless must be taken into account from the perspective of berry taste.