Effects of gamma rays on apple and peach leaf buds at different stages of development—II Injury to apical and axillary meristems and regeneration of shoot apices

Abstract

Injury to shoot apices and subsequent regeneration of injured tissues were studied in leaf buds of apple and peach that were exposed to gamma rays at different stages of bud development, in the interval from developing buds at shoot tips to fully developed dormant buds. Radiation injury to apical and axillary meristems was manifested as deceleration or cessation of mitosis, untimely maturation and vacuolation of cells, pycnosis of nuclei, loss of cell contents, disintegration and collapse of cells, and distortion of the apex. Within the shoot apex, sensitivity to radiation injury decreased from promeristem, through peripheral meristem, to pith meristem. The most actively dividing cells were not necessarily the most sensitive cells to radiation, as the highest frequencies of cell divisions were recorded in the peripheral meristem. Within the promeristem of both the main apex and the axillary meristems, the second cell layer was the most sensitive to radiation injury and the first layer the most resistant. As no periclinal divisions were observed in the cells of the first layer, this layer played no part in the regeneration of the severely injured deeper layers of apple and peach bud meristems. In slightly injured apices, local or direct healing began by activation of individual cells. If injury were moderate, restoration of the central apical region began from the periphery—from flank and pith meristems. In severely injured apices with the central zone of apex killed, regeneration began from the flanks, in the area of the youngest leaf primordia or above this area. The center of regeneration (and mutations) was the main apex of the primary bud rather than axillary or accessory meristems.