X-Irradiation of Root Meristems

Abstract

Zea mays seedlings were irradiated when their primary roots were 10-20 mm long at doses of 900 or 1800 rad of unfiltered x rays, given at 113 rad/min from a machine operating at 70 kv. For calculation of the rates of mitosis in the different regions of the meristem the method of metaphase accumulation was adapted. As a measure of the sensitivity of cells to x rays, the proportion of cells with micronuclei was used. After these heavy doses of x rays the average rate of mitosis at first falls in the normally meristematic cells and increases in the quiescent center. Proliferation in the quiescent center gives rise to a new meristem that continues the growth of the root and acquires a quiescent center of its own. Observations on micronuclei produced after irradiation suggest that the quiescent center is less sensitive to x rays than the rest of the meristem. This lower sensitivity, combined with the initial difference in time at which the radiation damage to cell viability occurs, presumably gives the quiescent center its role in the recovery of the meristem. Other evidence indicating this role of the quiescent center in the reconstruction of damaged roots came from autoradiographsmore » made from sections of apices fed with labeled adenine at various intervals after x irradiation. This also showed that in Zea the quiescent center often disappears after irradiation, its cells synthesize DNA, andd then divide. Many of the normally meristematic cells, on the other hand, stop synthesizing DNA andd stop dividing. It is concluded that the quiescent center is a reservoir of cells that will become meristematic after the normally meristematic cells have suffered genetic death. (BBB)« less