FACTORS THAT AFFECT LEAF REGENERATION EFFICIENCY IN APPLE, AND EFFECTOF ANTIBIOTICS IN MORPHOGENESIS

Several factors that affect the frequency of organogenesis in apple le af explants were examined for the scion cultivars 'Empire', 'Freedom', 'Golden Delicious', 'Liberty', 'McIntosh', and 'Mutsu' and for the ro otstocks Malling 7A and Malling 26. The main factors affecting morphog enesis were BA concentration, basal medium, leaf explant origin and ma turity, explant orientation, and photosynthetic photon flux. Depending on the genotype, optimal regeneration was obtained using either 22.2 or 31.1 mu M BA and the N6 basal medium, with the exception of 'Golden Delicious' which regenerated better on MS medium. After 6 weeks, the average number of shoots per segment varied from 5 to 16, and the perc entage of regeneration between 70 and 100%, depending on the genotype tested and the maturity of the explant. Regeneration capacity increase d dramatically from the tip towards the base of the leaf, and was high er from the middle to the proximal end. Cefotaxime and carbenicillin, two antibiotics commonly used during transformation studies to elimina te Agrobacterium tumefaciens from plant tissue, were tested to determi ne their effect on morphogenesis. Cefotaxime at a dose of 250 mg 1(-1) enhanced regeneration and shoot development, whereas carbenicillin at a dose of 500 mg 1(-1) induced abundant callus formation and inhibite d regeneration. Kanamycin, a widely used selection agent for plant tra nsformation, strongly inhibited regeneration even at very low doses. S chemes for selection and recovery of transgenic apple plants when kana mycin is used as the selection agent are discussed.