Radiation Botany最新文献

Experiments were performed to find out whether the level of ploidy bestowed a species with any additional resistance against the lethal and mutagenic damage of alkylating agents. Seeds of 11 species of Solanum of the non-tuberous group formed the test system. Of these 2 were diploids (2n = 24); 4 tetraploids (2n = 48) and 5 hexaploids (2n = 72). Seeds of all the species were exposed to aqueous solutions of ethyl mathanesulphonate; ethylenimine ; methyl nitrosoguanidine and 1,3-propanesultone and effect of these mutagens studied through M₁ and M₂ generations.

A considerable difference in sensitivity was found even between different species of the same ploidy level and no positive relationship could be established between the level of ploidy and its sensitivity to alkylating mutagens. However, in general it has been observed that as the level of ploidy increased from diploid to hexaploid the tolerance to alkylating damage increased but not proportional to the level of ploidy. The increase in resistance varied from marginal to a highly significant level.

The effects of 2,4-D (10, 50 and 100 ppm) and gamma rays (10, 20 and 30 kR), both individually and in combination, were studied on the vegetative shoot apex of safflower. While 2,4-D had no effect on the shoot apex organization, gamma rays caused much injury. The extent of injury was, however, much less in the shoot tips treated with 10 ppm 2,4-D plus gamma ray exposures than in the apices treated with gamma rays alone. Occurrence of multinucleate cells and abnormally positioned tracheidal cells was observed in the apices treated with gamma rays and 2,4-D plus gamma ray combinations.

Germinating seeds of barley (Hordeum vulgare ‘Himalaya’) were irradiated with ¹³⁷Cs 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.

Freshly isolated haploid and diploid tobacco mesophyll protoplasts were plated in agar medium and their survival, after irradiation with acute doses of X-rays, applied on the day of isolation or on any of three consecutive days was evaluated by scoring their ability to form cell colonies. Survival of diploid protoplasts irradiated on the day of isolation was reduced to about one half by 750 R. This dose-effect is similar to that of plant meristem cells and cultured mammalian cells and rather lower than that reported for plant calli and cell suspensions. The cells showed a relatively low radiosensitivity on the day of isolation which increased drastically in the two subsequent days. On the third day, when most of the cells completed their first karyokinesis, their sensitivity to irradiation was again reduced.