Variation in Apple Cytochimeras

Abstract

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.

Red colour distribution in the skin of Gala apple and some of its sports

“…(4) Mutation of the LUI layer followed by layer replacement of LII by LIII Another possible cause of the sports is layer replacement of LU by LIII as described by Datyon (1959). The tendency for layer replacement occurring under normal conditions is greater in some cultivars than others (Pratt et al 1967). No estimate of the frequency with which layer replacement occurs in Gala can be made from our study.…”

“…The epidermis is derived from the LI layer (Dermen 1948;Pratt et al 1967). Because deterioration of the protoplasm occurs in some cultivars at maturity, a mutation for increased red colour in the LI layer may not always be apparent.…”

“…(2) Mutation of the LII layer The hypodermal layer of the fruit skin is derived from the LII layer (Dermen 1948(Dermen , 1960Pratt et al 1967). A mutation of the LII layer for increased red colour would produce fruit with increased numbers of red cells in the hypodermal layer.…”

“…Mutations occurring in the hypodermis LII layer could be heritable as the gametes are derived from this layer (Pratt et al 1967). Therefore the 4 selections with greater than 34% hypodermal colour could be expected to produce progeny with more highly coloured fruit than that of Gala.…”

Genetic Heterogeneity in the Histogenic Layers of Apple1

“…Wounds, heavy overall pruning, and particularly, heavy cuts on large limbs may stimulate such buds to develop. A tendency has been observed for sport limbs to develop on 'Delicious' fol lowing heavy pruning, and particularly near heavy pru- ning cuts.2 Pratt, et al (7) working with diploid-tetraploid chimeras, demonstrated that sprouts from heav ily pruned trees were more variable, in terms of the amount of internal tetraploid tissue, than were shoots from unpruned branches. The observed changes were principally in the direction of L II tissues replacing those derived from L III.…”