Seedlessness Induced by Gibberellic Acid in Large-sized Berry Grapes

This study was conducted to elucidate the mechanism of seedlessness in large-sized berry grapes induced by gibberellic acid (GA) treatment at the stage of full bloom to three days after full bloom. We selected ‘Kyoho’, ‘Pione’, ‘Honey Venus’, ‘Shine Muscat’, and ‘Fujiminori’ as large-sized grapes, and some clusters of these cultivars were treated with GA at the full-bloom stage, but some others were kept without GA treatment. Then, the percentages of seeded berries in a fruit cluster were investigated among GA-treated or GA-untreated clusters. As a result, ‘Kyoho’ and ‘Pione’ had seeded berries with low percentages even in GA-untreated clusters, whereas ‘Honey Venus’, ‘Shine Muscat’, and ‘Fujiminori’ had seeded berries with 60.3 to 71.6% in a cluster when GA was not applied. ‘Kyoho’ and ‘Pione’ may have a putative factor that prevents fertilization. In addition, GA treatment of ‘Honey Venus’, ‘Shine Muscat’, and ‘Fujiminori’ in the full-bloom stage induced seedlessness, even though the seeded percentage was cultivar-dependent with values from 40.8 to 0.0%. This shows that GA treatment prevents embryo development after fertilization among these cultivars. Furthermore, when gibberellin activities of florets before full bloom on some selected dates were evaluated with a bioassay in ‘Pione’, as a difficult fertilizable cultivar, and in ‘Honey Venus’, as an easy fertilizable cultivar, gibberellin activities of florets in ‘Pione’ were always higher than in ‘Honey Venus’. This may indicate that the inhibition of embryo sac formation led to difficult fertilizable properties of ‘Pione’ is induced with higher activities of gibberellin. Consequently, we could reveal two types of mechanism for seedlessness in large-sized berry grapes with GA treatment at full bloom. One type is caused by difficult fertilizable properties at flowering due to disturbance of the formation of the embryo sac, as shown in ‘Kyoho’ and ‘Pione’. In this type, GA only promotes fruit set of unfertilized berries. The other type is caused by inhibition of embryo development with GA after fertilization, as shown in ‘Honey Venus’, ‘Shine Muscat’, and ‘Fujiminori’.