Survival curves were determined for seven species of woody angiosperms and six gymnosperms given chronic60 Co γ-exposures for up to eight years. Young plants were transplanted to the gamma field in the spring and scored annually beginning the following year (first-year scoring). Since the rate of change of survival (based on R/day) had decreased by the third year of irradiation, third-year survival data were used in most cases. Values of LD10, LD50, and <tex-math>${\rm LD}_{90}$</tex-math> were determined from survival curves fitted by probit approximation; those of <tex-math>${\rm LD}_{100}$</tex-math> were observed (the lowest exposure used which killed all the plants). These end points were determined for both R/day and accumulated kR. On the basis of R/day data, LD50 s decreased gradually (after the second year) to 54% of the first-year value by the eighth year with an average change of only 3.3% per year from the second to the eighth year, but on the basis of accumulated kR they increased to 4.3 times the first-year value by the eighth year. This suggests that a highly effective repair system had developed after an extended exposure in the LD50 range. Third-year LD50 s in the gymnosperms ranged from 7.2 to 29.4 R/day (average, 12.5 R/day) and in the angiosperms from 32.5 to 132 R/day (average, 96 R/day). Data for one species (Pinus strobus) showed that greater damage was produced by a 5-month chronic exposure during a period of active growth than by a comparable exposure during dormancy. The relationship previously found between interphase chromosome volume (ICV) and radiosensitivity was confirmed for these end points (the larger the ICV, the more sensitive the species). The regressions obtained for values of LD10, LD50, <tex-math>${\rm LD}_{90}$</tex-math>, and <tex-math>${\rm LD}_{100}$</tex-math> (R/day and accumulated kR) versus ICV had slopes not significantly different (5% level) from -1. Since predicted sensitivity to chronic γ-irradiation of species not yet tested can be obtained from these regressions if the ICV is known, the distributions of ICV and predicted sensitivity ( LD50 in R/day) are given for 215 woody species. Whereas for 94% of the gymnosperms the predicted LD50 is 40 R/day or below (75% between 11 and 40 R/day), for 92% of the angiosperms it is above 40 R/day (63% between 101 and 570 R/day). Sensitivity of gymnosperms to chronic irradiation appears to be in the same general range as that of mammals.

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