VARIATION IN APPLE CYTOCHIMERAS

Diploid-tetraploid cytochimeras of apple were investigated by cytological examination of buds from branches selected by the characteristics of the fruits, flowers or pollen. Eight types of cytochimeras were identified on the basis of relative size of cells, nuclei and metaphase mitoses in the apical meristem (protomeristem) and mitoses in meristematic primary tissues. There appeared to be five apical layers which contributed to the stem and four which contributed to the leaves and flower parts. Buds of various cytochimeral patterns (designated by a formula giving the ploidy of the apical layers) were found on some bearing trees propagated from known cytochimeral sources. The most frequently associated types were (a) 2–4–2–2–2 and 2–4–4–4–4 (and 2–4–2–4–4 in one cultivar), or (b) 2–2–2–4–4 and 2x. Some sports were uniformly 2–4–4–4–4. The stability of apple cytochimeras under normal conditions appeared greater in some eultivars than in others. Sprouts from severely pruned 2–2–4–4–4 trees were more variable than unpruned branches. Buds of shoots which grew from radiation-damaged buds were more variable than those from non-irradiated buds and included types not yet found by branch selection. Cytochimeral variation was interpreted to be due to layer replacement resulting from infrequent periclinal divisions in apical or axillary meristems, or from wounding of meristems by ionizing radiation.