Searching for a reliable measure in inducing parthenocarpy of Muscat Bailey A grapes，the effects of the timing, concentration and frequency of GA(gibberellic acid) application on the seedlessness ratio and bunch quality were studied from 1969 to 1971 at the vineyard of tbe experimental farm, Shimane University.
Dipping of clusters into 100 ppm solution of GA about 15 days before and 10 days after full bloom has been the established technique in Japan for the production of seedless Delaware grapes, by which almost all berries become seedless if the timing of the prebloom application is adequate.
This technique was not fit to Muscat Bailey A since the seedlessness ratio fractuated year by year from 94.5% to 78.5% despite the applications being conducted at the appointed date. When the prebloom treatment was applied earlier or later than the appointed date, the seedlessness ratio declined. The higher GA concentrations (200 and 300ppm) in the prebloom treatment were effective for raising the seedlessness ratio.
The most promissing method found in this investigation was the repeated prebloom applications (15 days at 100 ppm and again 8 days at 200 ppm before full bloom) with an ordinal after bloom application, by which the seedlessness ratio reached to 97.0% and the superior bunches in quality were produced. The only fault of the method was that it loosened too much the bunch resulting from the promoted elongation of peduncle and rachis.
On the other hand, it was observed in a portion of the seedless berries induced by any types of GA application that an abnormal tissue like a pit developed at the core, which was too hard to be eaten together with freshy pericarp. The appearance of the tissue may be connected with the seed abortion caused by the GA application though it has never been reported by others not only in the variety but also in Delaware grapes. The origin and histological characteristics of the abnormal tissue should be studied further.