The relationship of DNA content to nuclear and chromosome volumes and to radiosensitivity (LD50).

Factors which in the past have been invoked to explain the different responses of a wide range of organisms to ionizing radiation include the presence or absence of radioprotective chemicals,' different base ratios,2 variation in mitotic cycle times,3 or genetic complexity,46 to mention only a few. A different approach to the understanding of plant radiosensitivity was initiated by Sparrow and Miksche,7 who noted a high degree of correlation between the nuclear volume of a species and its radiosensitivity. They studied 23 species of herbaceous plants having a range of nuclear volumes and found that as nuclear volume increased, radiosensitivity increased correspondingly and in an almost direct manner. Based on this relationship, the authors concluded that the radiosensitivity of a plant could be predicted with a reasonable degree of accuracy by utilizing data concerning nuclear volume. In the same study it was noted that nuclear volume and DNA content were correlated, i.e., that DNA content increased as nuclear volume increased. Moreover, a comparison of DNA content per chromosome and radiosensitivity of five species of plants revealed an almost direct correlation between these two variables. In later studies," it was concluded that there was a better correlation between radiosensitivity and DNA content per chromosome than between radiosensitivity and DNA content per nucleus. A correlation between DNA content per cell and radiosensitivity has been reported for higher plants by Bowen9 and for a wide variety of organisms by Kaplan and Moses6 and by Sparrow, Underbrink, and Sparrow. '0 However, in Bowen's study, LD6o values were computed on the basis of the number of plants which reached maturity following irradiation of dry seeds, but DNA content was determined from leaf tissue. Further, the relationship he demonstrated was not as good as one might have expected. Thus, it seemed desirable to obtain additional and more comparable data which would establish more firmly the relation between nuclear volume and DNA content per cell, and DNA content per chromosome and radiosensitivity. For these reasons, the current investigation was initiated. Materials and Methods. -Plant material: The plants used in this study were obtained from sources too numerous to include here but are available from the authors on request. The chromosome numbers were obtained from Darlington and Wylie" or determined in our laboratory. Nuclear volume measurements: Nuclear volumes were determined basically in the same manner as described previously.7 The interphase chromosome volume (ICV) was obtained by dividing the nuclear volume by the somatic chromosome number characteristic of that species. Root tips used for nuclear volume measurements were collected at the same time that root tips were collected for DNA determinations. Apical meristems were removed from a sample of plants just prior to irradiation of the remaining plants. DNA measurements: For each determination of DNA, terminal 2-mm tips were removed from 40-100 roots and immediately transferred to ice-cold methanol. The material was subsequently ground in a tissue grinder in the presence of cold methanol and the DNA extracted according to the procedure of Schmidt-Tannhauser as modified by Davidson et al."2 The amount of DNA