Effect of Potassium Gibberellate on Fruit-Set and Development of Thompson Seedless and Zinfandel Grapes

Abstract

KGA₃ applied to Thompson Seedless grapes when the flowers ranged from 30 to 80% bloom tended to increase the percentage of abscised ovaries, most of which were 1 mm or less in width. The percentage of berries set was decreased by KGA₃ at 25 ppm. Berry weight was increased by the KGA₃ treatments, but cluster weight was decreased at 25 ppm. Percent looseness was also increased by KGA₃ at 25 ppm.

Application of KGA₃ to Zinfandel grapes at pre-bloom caused a decrease in percentage of abscised ovaries, most 1 mm or less in width. There was a large increase in the percentage of ovaries that remained attached to the clusters, and a decrease in percentage of berries set. Weight per cluster and weight per berry both decreased. Elongation of clusters increased with higher concentrations of KGA₃. Clusters dipped into KGA₃ at 10, 25, and 50 ppm became straggly. The °Brix increased in fruits treated with KGA₃ at 50 ppm. Total titratable acidity did not differ significantly among treatments.

The frequency distributions for Thompson Seedless and Zinfandel berry sizes were maximum for the class interval 12.0 to 13.9 mm wide.