RELATIONSHIP BETWEEN INTENSITY OF BUD DORMANCY AND LEVEL OF ABA IN JAPANESE PEAR NIJISSEIKI

Two experiments were conducted to investigate the chilling requirement s of vegetative and flower buds of the Japanese pear, 'Nijisseiki' and their ABA contents during the rest period. In Experiment 1, excised s hoots of 'Nijisseiki' pear were kept at 5, 10, 15 and 20-degrees-C for 0 to 1,500 hr, and the depth of rest of leaf bud was examined. Shoots kept at 5-degrees and 15-degrees-C were examined for the depth of res t of flower buds. The ABA levels in the leaf buds and floral primordia were determined after exposures to 5-degrees and 15-degrees-C for 0 t o 1, 500 hr. 1. On shoots that were exposed at 5-degrees-C, the rest o f the leaf bud was broken after 1,400 hr ; that of flower bud was sati sfied after 1, 200 hr. After 1, 500 hr, percentages of bud-break of le af buds were : 64 at 0-degrees-C ; 94 at 5-degrees-C ; 46 at 10-degree s-C ; 30 at 15-degrees-C, and 22 at 20-degrees-C. 2. ABA concentration s in the leaf buds and floral primordia kept at 5-degrees-C decreased rapidly during deepest rest phase ; a decrease of ABA in the leaf buds and floral primordia kept at 15-degrees-C was detectable only after 1 , 100 hr of exposure. In Experiment 2, excised shoots of 'Nijisseiki' pear, exposed to 5-degrees-C for 0, 300, 600 and 1,000 hr, were inject ed with 100 ppm ABA and kept at 5-degrees-C for 200 more hr. The depth of rest of the flower buds and their ABA levels were determined as st ated above. 1. Budbreak of flower buds on shoots exposed 200 and 500 h r were inhibited by injection of 100 ppm ABA ; budbreak on shoots expo sed for 800 and 1,200 hr, there were no significant difference between those injected with ABA or water. 2. Flower primordia of shoots admin istered 100 ppm of ABA contained 2 to 3 times more ABA than control sh oots or those injected with water.