Effects of Shoot Vigour on the Induction of Seedless Berries by GA Application in Muscat Bailey A Grape

Abstract

The purpose of this study was to confirm the effect of shoot vigour on the induction of seedless berries by the pre- and post-bloom gibberellin (GA) applications to clusters in Muscat Bailey A grape and also to explain the mechanism of the effect by substantial evidences.
1. When the GA treatment was applied to clusters of Muscat Bailey A vines receiving cane pruning, higher percentages of seedless berries were obtained in the clusters of strong shoots sprouted from the distal nodes of each cane than in those of weak shoots from the basal nodes. Further, this trend was consistent through 3 years′ experiments in various conditions such as 2 timings of prebloom GA application, 2 concentrations of GA and 2 levels of vine vigour. However, such a difference in the percentage of seedless berries between the clusters of strong and weak shoots was not detected in the spur-pruned vines through 2 years′ experiments with 2 timings of prebloom GA application. This is presumably due to a narrower margin of vigour between the strong and weak shoots in the vines of spur pruning than those of cane pruning.
2. It has been known that a high proportion of florets are usually composed of more than 3 carpels (loculi) in the clusters of Muscat Bailey A and we found that the percentage of seedless berries increased with the increment in the number of carpets per berry in the GA-treated clusters of the cultivar.
3. The number of seeds per berry in the non GA-treated clusters of strong shoots was smaller than that of weak shoots in cane-pruned vines of both Muscat Bailey A and Delaware. On the contrary, the number of carpels i.e. the number of ovules per berry in the clusters of strong shoots was larger than that of weak shoots in Muscat Bailey A; the berries of 3 or more carpels were extremely dominant in the former whereas those of 2 carpels in the latter. On the other hand, the number of seeds per carpel decreased in inverse proportion to the increment of the number of carpels per berry in the clusters of both strong and weak shoots in Muscat Bailey A. Further, when the berries having each equal number of carpels were compared between the clusters of strong and weak shoots, fewer seeds per berry were found in the former than in the latter in each class of carpel numbers.
4. From these results, it can be said that the ovules contained in the florets of strong shoots are larger in the number per ovary but lower in fertilization potential than those of weak shoots in Muscat Bailey A. Moreover, it seems most likely that the less potent ovules may be responsible for the higher percentages of seedless berries which were consistently obtained when the GA treatment was applied to the flower clusters of strong shoots.