581 Effect of CPPU on Fruit Development in Seedless and Seeded Grape Cultivars

The effect of CPPU [forchlorfenuron; N-(2-chloro-4-pyridinyl)-N-phenylurea] on berry development of selected seedless and seeded grape cultivars was evaluated under field conditions. A concentration response curve was initially established by spraying clusters of `Himrod' at a mean berry diameter of 5 mm with 0, 5, 10, and 15 ppm CPPU. Berry enlargement was monitored (16, 30, 44, and 59 days after treatment) during development and cluster weight, berry number/cluster, weight, firmness and °Brix were determined at harvest. Berry size was dramatically (2.3 vs. about 3.6 g/berry) increased at harvest by all concentrations of CPPU; the response being linearly related to concentration ( r 2 = 0.89). Cluster weight and compactness (rated) and berry firmness were significantly increased. There was no significant effect on berry number (79 to 86/cluster). °Brix and rachis necrosis (at harvest) as well as berry abscission after 30 days of refrigerated storage were significantly reduced. Effect of time of CPPU application (0, 5, and 10 ppm) was established by treatment of clusters at mean berry diameters of 4, 5, 7, and 9 mm. Response was indexed by following berry enlargement at 14, 28, 42, and 56 (maturity) days after treatment. Maximum berry enlargement for both 5 and 10 ppm was obtained from applications at 4 to 7 mm in diameter. Relative responsiveness of seedless and seeded cultivars was compared by application of CPPU at 0, 5, 10, and 15 ppm to clusters (4–6 mm berry diameter) of seedless `Himrod', `Vanessa' and `Lakemont' and seeded `Concord' and `Niagara'. Bioresponse was determined by a time course of berry enlargement and berry and cluster weight, number of berries/cluster and rating cluster compactness at maturity. CPPU at all concentrations increased seedless berry diameter significantly from the first measurement at 14 days through 56 days after maturity. Berry and cluster weight and cluster compactness were increased in the seedless cultivars, although `Lakemont' appeared less responsive than `Himrod' and `Vanessa'. CPPU did not change (`Lakemont') or decreased (`Himrod', `Vanessa') berries/cluster. In contrast, the only effect of CPPU on the seeded cultivars was an initial increase in berry diameter 14 days after application.