SOME FACTORS AFFECTING THE RESPONSES OF PLANTS TO ACUTE AND CHRONIC RADIATION EXPOSURES

Abstract The radiobiological responses of a large number of species of higher plants have been studied after acute or chronic irradiation of growing plants mostly with γ radiation from Co60. Some acute treatments were made with X-rays. The radiation responses studied were of four main types: chromosome breakage, somatic mutation, growth inhibition and lethality. Somatic mutations in petals of several species of plants heterozygous for flower colour show responses most readily explicable on the basis that they result from deletion of the dominant locus, i.e., in most respects they behave as chromosome breakage events and not as point mutations. The yield of somatic mutations is reduced by dose fractionation of acute exposures or, within limits, by reduced dose rates with chronic exposures. The frequency per r of somatic mutations and of chromosome deletions produced under chronic irradiation is higher during periods of slow growth than during periods of fast growth. Definite evidence of long-term cumulative dosage effects was described for Pinus and Taxus. Over a long period (several years) at dose rates as low as 5 r/day, cumulative effects are expressed as severe morphological deformity, severe growth inhibition, or even death. A correlation was shown between the acute and daily chronic doses necessary to produce severe growth inhibition in young plants of several different species. To produce equivalent effects the acute dose required averages about thirteen times the daily chronic dose. In diploid species a clear relationship was shown between the average nuclear volume of apical meristem cells and tolerance to chronic gamma radiation. The larger the nuclear volume the greater the sensitivity of the nucleus and ultimately of the whole plant. The frequency of somatic mutation per r and nuclear size were shown to be directly related. Preliminary data also indicate a similar relationship between nuclear volume and frequency of chromosome aberration and for three species between average amount of DNA per nucleus and radiosensitivity (the more DNA the greater the sensitivity). Two polyploid series (Chrysanthemum and Sedum) showed increasing resistance as the degree of polyploidy increased. Preliminary data suggest that increasing chromosome number without known polyploidy and without a changing nuclear volume has a protective effect. A theoretical model was presented describing a possible explanation for the protective effect. The protection offered by doubling the chromosome number without polyploidy may be greater than that afforded by polyploidy at a doubling level. Some theoretical aspects relating to the interrelationship of certain major factors determining radiosensitivity are presented. Analysis of the relationship between sensitivity of different species and nuclear volume indicates that a constant (or nearly constant) number of ionizations per nucleus is required to produce growth inhibition or death. This relationship supports the concept that a cluster effect such as chromosome breakage (deletion) may be the critical event or events. Differential sensitivity results in part from the different doses required to produce a unit number of breaks in nuclei of different average volumes. The protective effect associated with increasing chromosome number or polyploidy is assumed to result from the reduced seriousness of an average break or deletion as the chromosome number increases. Clarification of the manner in which radiosensitivity is determined in different species should make predictions of radiosensitivities possible in the very near future. This should hold for animal species as well as for plants. Extrapolation of certain kinds of radiobiological data from one species to another can now be made on a reasonably sound basis.