DORMANCY AND DEVELOPMENT OF APPLE AXILLARY BUDS INVESTIGATED IN VITRO

Single apple buds excised from dormant trees were cultured in vitro at 4°C for 3–30 weeks on MS medium either with or without added 10^-4M ABA (abscisic acid). After 18 weeks at 4°C the chilling requirements of the buds were satisfied and they showed their highest growth potential. Release from dormancy was accompanied by increase in water content, mobilization of storage nutrients and a temporary decrease in dry weight. When the bursting process was already advanced, the nutrients were intensively utilized and acid phosphatase activity and the fresh and dry weight of the buds increased rapidly. The cells of the axis and the mesophyll showed marked extension. Catabolic metabolism predominated after the transition from the dormant to the bursting stage. ABA inhibition was seen only at a late stage in the bursting process and was associated with reduced water uptake and a lack of utilization of reducing sugars and soluble proteins. Cell extension in the bud axis was completely inhibited by ABA, but growth of the cells in the leaf primordia was only partially reduced. It is suggested that ABA does not prolong bud dormancy, but suppresses the bursting process.