The Use of Radiocobalt as a Source of Gamma Rays and Some Effects of Chronic Irradiation on Growing Plants

Abstract

Radiocobalt has been used as a source of gamma rays in a study of the effect of chronic irradiation on growing plants. The installation and method of operation of sources of approximately 16 and 145 curies is described. The Co60 was placed in the center of an experimental plot in such a fashion that it could be operated safely by remote control. By placing plants in concentric circles (or arcs) around the source at various distances it was possible to study the effects of a wide range of radiation intensities. Actual dosages used varied from less than 0.1 r per day up to dosages in excess of 2,000 r per day. The tolerance to ionizing radiation of different species of plants studied varied widely. Preliminary tolerances are given for nineteen different species of plants. Tradescantia paludosa showed severe effects at 30 r per day, whereas Gladiolus showed relatively little effect after 2,000 r per day for 42 days. Mutation rates of four endosperm characters of corn revealed the following: (1) There was no increase in mutation rate over the control when the radiation was less than 5 r/day. (2) Above 5 r/day there was a gradual increase up to 31 r/day with a more marked increase from 32 to 57 r/day. From 57 r/day to 127 r/day the increase in mutation rate was even more marked. The increase in mutation rate at the higher intensities of radiation was greater than expected if the increase is linearly proportional to dose. (3) The logarithm of the mutation rate seemed to show a linear relationship with dose received. (4) The gene with the greatest spontaneous mutation frequency (R) showed a smaller percentage increase in induced changes than the other three genes Su, Sh and Pr. Under chronic gamma radiation the percentage of abnormal pollen grains in corn increased with an increase in radiation. There was no marked increase over the control below 2.6 r/day and the maximum effect noted was 57 per cent. abnormal grains at 226 r/day. Chromosome fragments were scored in somatic and meiotic (first) anaphase of Tradescantia after 47 days exposure. Fragmentation in meiosis was more than tenfold greater than that observed in somatic cells. Micronuclei derived from chromosome breaks were scored in microspores of chronically exposed Tradescantia. A statistically significant increase in number of micronuclei was found at a dose rate as low as 0.41 r per day. The numbers of micronuclei did not increase significantly after 32 and 64 days as compared to that found after 16 days. Other effects observed included killing, various degrees of growth inhibition, inhibition of flowering, pollen abortion, reduced fertility (or complete sterility in some cases) abnormal growth pattern of both floral and vegetative parts as well as a strong tendency to proliferative growth in some cases. The effect of irradiation on production of crown gall in tomatoes is also briefly reported.