The effects of external gamma radiation from radioactive fallout on plants with special reference to crop production

This paper describes the major problems involved in attempting to predict for economically useful plants the degree of radiation damage which would arise from exposure to high level radioactive fallout. Since almost no data exist on the deleterious effects inflicted on crops by actual fallout radiation, it is necessary to extrapolate from the existing radiobotanical data concerned with the effects of gamma radiation on survival and yield of plants. A number of factors can modify the effects of the radiation and, hence, influence the accuracy of predictions of postattack injury. The most important variables are (a) species differences in interphase chromosome volume (the larger this value the more sensitive the plant), (b) exposure rate (high rates are more effective than lower rates), (c) stage of development of the plant (a complex and difficult variable to assess), (d) postirradiation time (generally the longer the time the greater the degree of damage), and (e) numerous environmental factors such as moisture, temperature, light, competition, etc., which normally modify plant growth and yield. These factors acting singly or in various combinations can have a considerable effect on the radiation response and thereby make more difficult the prediction of postattack injury. Survival and yield data obtained from irradiation of growing plants are presented for many species. The most useful values in comparing sensitivities are LD 10 , LD 50 and LD 90 (exposures required to reduce survival by 10, 50 and 90 per cent), and the YD 10 , YD 50 and YD 90 (exposures required to reduce yield by 10, 50 and 90 per cent). A log—log regression of LD 10 vs. YD 50 for 36-hr FDS (fallout decay simulation) gamma exposures has a slope not significantly different from + 1, indicating that, in general, an exposure which produces an LD 10 will reduce yield by 50 per cent. A prediction of other LD 10 values may also be made from regressions of interphase chromosome volume on LD 10 . Predicted YD 50 values following FDS exposures are given for 89 crop plants and for 82 woody plants for a 16-hr constant rate exposure. Using these predictions and the available radiobiological data some conclusions may be drawn concerning the vulnerability of crop plants to fallout radiation. The cereals, i.e. wheat, barley, oats and maize, which are probably our most important group of crop plants, would be the most sensitive with YD 50 values ranging from about 1 to 4 kR (rice is much more resistant). The legumes, i.e. peas and beans, include both sensitive and resistant species with YD 50 s ranging from less than 1 to 12 kR. Root crops, i.e. onion, garlic, beet, potato and radish, have a wider range in sensitivity with YD 50 s between 1 and 16 kR. For pasture and forage crops the YD 50 varies from 2 to 20 kR. Of the herbaceous crop species 70 per cent fall in the predicted sensitivity range between 4 and 16 kR. Woody species have a range of predicted LD 50 s between about 0·4 and 8 kR with the gymnosperms predominating below 2 kR. These predictions are for average conditions only and we still lack a significant amount of radiobiological data to make confident predictions of the expected response of many species to high level fallout gamma exposure. Also, inadequate information about beta radiation injury and its possible interaction with gamma radiation make extrapolation to actual fallout conditions even more difficult (see also Summary pp. 112–114).