Radiation damage and recovery in diploid and cytochimeral varieties of apples

Abstract

Radiation damage and recovery of diploid and cytochimeral varieties of apple and the chromosomal constitution of recovered shoots of irradiated cytochimeras were studied. Dormant scions were exposed to 3000 r of X-rays or 3 hr of thermal neutrons (average flux 6·57 × 10 8 neutrons/cm 2 per sec.) and grafted. As soon as the irradiated buds of Northern Spy (2x) and Loop Spy (2-2-4-4) began to grow, the central cells below the first layer of apical and axillary meristems were conspicuous by their failure to stain normally. Such radiation-damaged cells did not divide. The cells lateral and basal to them hypertrophied and divided. Shoot growth, when it occurred, was resumed in one or more of three ways: (1) the damaged cells of the apical meristem were crushed by the growth of adjacent cells and the healed meristem resumed growth; (2) the cells lateral to the damaged apical meristem formed a substitute meristem; (3) axillary meristems, many of them healed, developed at one or more nodes below the apical meristem. More shoots recovered in the diploid varieties, Northern Spy and McIntosh, than in their chimeral sports, Loop Spy and Kimball McIntosh. The chromosomal constitution of recovered shoots depended upon the ploidy of the cells taking part in recovery and their subsequent intrasomatic selection. Two types of changes in chromosomal constitution were found in cytochimeral shoots examined one to five years after irradiation. The determination of chromosomal type was based on the distribution of diploid and tetraploid tissue in growing buds, and on pollen size and fruit shape wherever possible. The types are described by a formula indicating the ploidy (2x or 4x) of the uppermost four layers of the meristem. One change was the increase in tetraploid (inner) tissue in Loop Spy and Welday Jonathan (2-2-4-4 to 2-4-4-4 or 4x). No such increase was observed in Kimball McIntosh (2-4-4-4) or Bates Lobo (2-2-2-4). Increase in diploid (outer) tissue was found more frequently than increase in tetraploid (inner) tissue. The latter occurred more frequently in long shoots than in short shoots from irradiated buds of Loop Spy. Increase in diploid tissue was found often in Loop Spy and Welday Jonathan (2-2-4-4 to 2-2-2-4, 2-2-2-2-4 or 2x) and rarely in Kimball McIntosh (2-4-4-4 to 2-2-4-4 and 2-2-2-4).