Interrelation of exposure and exposure rate in germinating seeds of barley and its concurrence with dose-rate theory

Abstract

Germinating seeds of barley ( Hordeum vulgare ‘Himalaya’) were irradiated with 137 Cs gamma rays at various combinations of total exposure (400–3200 R) and exposure rate (30–24,000 R/hr). Seedling height was measured 5 days after the initiation of irradiation and the various levels of growth inhibition produced by each combination of treatments were determined. The results obtained ranged from no effect on growth to 100 per cent growth inhibition. Growth inhibition curves based on both total exposure and exposure rate were constructed. The exposures required to produce 20 and 35 per cent growth inhibition at each exposure rate were determined, 35 per cent growth inhibition being the highest level that could be determined over the entire range of rates used (20 per cent growth inhibition was used for comparative purposes). For both levels of growth inhibition, as exposure rate increased (or, concomitantly, as exposure time decreased), the total exposure required to produce the end point decreased (effectiveness increased) as a straight line relationship on a double logarithmic plot between 30 and 1500 R/hr (0·03 to 0·3 hr exposure time). Above 1500 R/hr, further increases in exposure rate (or decreases in exposure time) increased the total exposure required for a given effect, i.e., effectiveness decreased. Conversion of exposure rate to exposure time demonstrates this point of change in effectiveness to occur well within one mitotic cycle. These results are discussed with regard to current dose-rate theory and are at least partially consistent therewith. A straight-line dependency of the exposure rate producing maximum growth inhibition on total exposure is shown. The point at which the combinations of exposure and exposure rate for 35 per cent growth inhibition occurs is restricted to barley, and may differ for other species. This may depend on chromosome size or DNA content and/or the mitotic cycle time characteristic of a species.