PREDICTION OF THE SENSITIVITY OF PLANTS TO CHRONIC GAMMA IRRADIATION

The principal site of damage to plants by ionizing radiation is the cell nucleus. The effect on plant growth, measured as size or weight, used as the criterion of sensiviti to chronic gamma irradiation, shows the variation in sensitivity between different species of higher plants to approach 500-fold. This variation is correlated with chromosomal and nuclear characteristics: that is, plants which have low chromosome numbers and large nuclear volumes are most sensitive while polyploids and plants with high chromosome numbers and small nuclear volumes are highly resistant. The rate of nuclear division is also important. A slow rate increases the exposure time of each interphase nucleus to chronic irradiation and thereby increases the nuclear damage. Any environmental factor which influences the rate of growth will by influencing the rate of cell division, affect sensitivity. Environmental factors influencing the degree of response caused by ionizing radiation fall into four general categories: (1) those modifying dosage, dose rate or dose fractionation; (2) those involving type of ionizing radiation; (3) those influencing growth rate or rate of cell division; and (4) those affecting recovery from radiation damage. Consideration of these nuclear and environmental factors permits the prediction of radiosensitivity of plant species not previously irradiated. At the plant population level, chronic irradiation can probably be expected to have its most severe effects on sexual reproduction because during and after meiosis: (1) nuclear volume is high; (2) chromosome number is reduced after meiosis; (3) the rate of nuclear division may be low, some species requiring 2 years between meiosis and full maturation of seeds; (4) meiotic pairing and reduction tend to enhance the damage wrought by aberrations which may survive in diploid somatic cells. The high sensitivity of sexual reproduction is probably further enhanced by seed dormancy, during which damage accumulates. The relative dosage levels necessary to produce specified responses in growth rate, reproductive capacity or in degree of mortality vary greatly within a species. Both vegetative growth and the integrity of the sexual reproductive process of pines appear to be highly susceptible to damage from ionizing radiation and the genus Pinus includes some of the most radiosensitive plants known. Chronic exposures averaging about 8 months per year for 9 years produced detectable effects on growth of P. rigida at average dose rates as low as 2 r/day. The lethal acute dose for P. strobus seedlings of 600 r also indicates a high radiosensitivity. The principal reasons for these high sensitivities are the large nuclear volume of pines and, for chronic exposures, the long period between production of the meiocytes and the maturation of seed. It is predicted that many other gymnosperms will have a radiosensitivity approaching that of pines. A comparison of the estimated maximum exposure of vegetation from fallout occurring in New York City in 1958 with the minimum chronic radiation levels known to influence growth and reproduction of Pinus rigida indicates that these levels of fallout are approximately 1/800th of the 2 r/day necessary to produce visible morphological effects within several years. A forest stand representative of the Long Island oak-pine forest is currently being irradiated with chronic gamma radiation from about 9500 c of cesium-137 in an effort to provide quantitative estimates of radiation effects at the population and community levels. Present information makes possible the prediction, on the basis of nuclear characteristics, that the first year's exposure will cause selective killing. All trees within 35 m of the source (85 r/day) will die leaving a shrub community dominated by Gaylussacia baccata, probably one of the most resistant of the species present. The pitch pine (Pinus rigida), with its large nuclear volume, is the most sensitive plant present and will probably be killed by the second year at distances up to 75 m (15 r/day). This radiation facility is available to research workers interested in the general problem of radiation effects on organisms.