Effectiveness and mode of action of gibberellic acid on grape berry thinning
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Date
1994
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AMERICAN SOCIETY ENOLOGY & VITICULTURE
Abstract
Four trials were conducted to determine the influence of gibberellic acid on fruit set and cluster compactness of Flame Seedless and Thompson Seedless grapes. Spraying GA(3) at bloom time to the canopy of Flame Seedless (8 mg/L) and Thompson Seedless (15 mg/L) resulted in loose clusters due to rachis elongation. Spraying before bloom increased number of berries in spite of decreased pollen germination. In Flame Seedless, shoot pinching alone seven days before full bloom (DBFB) increased berry number per cluster. Shoot pinching combined with GA(3) to clusters at bloom time increased the number of berries in Thompson Seedless. Defoliation followed by GA(3) to the entire shoot, apex, or cluster decreased the berry set of both cultivars. In Thompson Seedless, GA(3) applied to the terminal half of the cluster or shoulder produced berry thinning in the basal non-treated half, while treatment to the cluster base reduced set on the top. GA(3)-treated shoulder halves increased the number of berries. The results support the idea that GA(3) acts as a thinning agent by inducing rachis elongation and stimulating competition among organs which reduces set when nutrients become limiting for berry growth.