Axillary buds in normal and irradiated apple and pear

Abstract

Dormant buds of apple and pear are convenient material to irradiate for the production of mutations or chimeral rearrangements. The sites of action of ionizing radiation are the apical meristem and the secondary bud primordia of the dormant bud. Such buds and growing shoot tips of normal and irradiated McIntosh (2×) and Northern Spy (2×) apples, their cytochimeras Kimball McIntosh (2-4-4-4) and Loop Spy (2-2-4-4), and Bartlett pear (2×) were sectioned or dissected. Apple scions received 0 to 2850 rad of X-rays and pear scions were exposed to 0, 2000, 4000 or 6000 rad of γ-rays. Dormant pear buds had seven bud primordia which would normally continue growth while those of apple had only three. No bud primordium in a dormant bud had reached the mound stage. After irradiation of apple and pear radiation-damaged cells were found in the subsurface central region of both apical and axillary meristems. Greatest damage occurred in layers II and III. Cells adjacent to the damage, notably those surrounding the damaged area of the shoot between the youngest three leaves and those adaxial to the older bud primordia, became meristematic. One or more secondary buds developed from the axils of pre-irradiation leaves 1–5 in apple and 1–10 in pear. In older recovering shoots an adventitious bud might arise on the stem, as suggested by one cauline bud.