CRITICAL PERIOD OF FLOWER BUD INDUCTION IN RED-FUJI AND RALLS-JANET APPLE-TREES

The critical period of flower bud induction in 'Red Fuji' 'Ralls Janet ' apple trees were studied for two years. The onset and duration of fl ower bud induction were found to be dependent upon crop load of trees and type of fruiting-wood. Floral induction took place earlier and its duration was longer for buds on 'on-year' trees compared to those on 'off-year' trees. In general, most of the buds on light cropping or 'o ff-year' trees of 'Red Full and 'Ralls Janet' apples had been induced, for 10 to 20 days from 10 June to 30 June while a critical period of flower bud induction lasted about 40 days from late May to early July on heavy cropping or 'on-year' trees of 'Ralls Janet' apples. Spur bud s were the first to enter into their physiological conversion process of floral induction, followed by the terminal buds of medium new shoot s. The onset of floral induction was most delayed on the terminal buds of long new shoots. However, the induction process finished almost at the same time regardless of the types of buds, consequently the durat ion of the flower induction was longer on spur buds than that on the t erminal buds of new shoots. Moreover, by comparing with the classical methods, fruit and leaf removal, the possibility of using GA(3) sprays was confirmed as a suitable method for investigating the critical per iod of flower bud induction in apple trees in these studies. 45 to 200 mg I-1 were the suitable range of GA(3) rate for evaluating the criti cal period of flower bud induction. However, a rate of 100 mg I-1 was recommended.