The non-uniform radiosensitivity of the dormant root meristem of barley seed revealed by chromosome aberrations in cells at metaphase of the first division cycle

Abstract

It is generally assumed for radiation-induced chromosomal aberration studies, that the dormant root meristem of seeds has a uniform radiosensitivity with respect to time. Therefore, provided that only cells in the first division cycle are scored, the effects of differential delay and cell mixing, which are also produced by radiation, can be disregarded. This paper presents details of experiments designed to test the validity of this assumption for seeds of barley ( Hordeum vulgare). A single dose of 40 krad of 60Co γ radiation was given to dormant seeds (∼10 per cent moisture) which, together with unirradiated controls were soaked immediately in aerated distilled water, and germinated on filter paper in a constant environment at 21·5°C. Fixations were made every 3 hr from 15 to 48 hr from the start of soaking, some seeds receiving no colchicine prior to fixation, others a 3 hr colchicine treatment, and a third lot continuous colchicine, from 12 hr post-soaking until the conclusion of the experiment. Various parameters have been scored to examine the perturbations produced by radiation in the cell population, and at the same time, detailed scores of chromosome-type aberrations at metaphase in diploid cells have been made. The results show that at least for low LET radiations, the dormant meristem cannot be regarded as a homogeneous cell population, and the assumption of a uniform radiosensitivity with respect to time is invalid. It follows that no single post irradiation fixation time can give a reliable estimate of aberration yield, and that any treatment of the seed, either pre or post irradiation, which will perturb the orderly progression of meristem cells, will also change the observed pattern of response, rendering the assessment of comparative effects almost impossible.