Growth inhibition, mitotic cycle time and cell number in chronically irradiated root meristems of Pisum

Abstract

Seedlings of Pisum sativum were chronically irradiated with gamma rays from a Co 60 source at daily exposure rates sufficient to produce growth inhibition. Exposure rates of 500 and 1000 r/day reduced root growth by the fourth day. In order to determine the factor(s) that might be responsible for the reduced growth rate, the following measurements were made: (1) the number of cells comprising the root meristem was determined daily for seven days, (2) the percentage of aberrant cells at anaphase was scored after three days exposure, and (3) the minimum mitotic cycle time for meristematic cells was established, also after three days of exposure. Cell counts showed an approximate reduction of 5, 25, and 45 per cent compared to control on the third day of exposure at rates of 250, 500 and 1000 r per day. The minimum mitotic cycle time was measured by producing a tetraploid cell population with colchicine and noting the span of time between the production of this population and its initial appearance in the subsequent mitosis. These measurements indicated no change in the minimum cycle time of the cells in the irradiated meristems as compared to the unirradiated controls. Analysis of cells at anaphase showed an increase in the number of cells with broken chromosomes with increased exposure rate. Therefore, the reduction in growth observed in irradiated seedlings was attributed to the loss of reproductive integrity not due to a general slowing down of the cells' division rate but rather to genetic loss and/or unbalance and mitotic arrest of meristematic cells.