Gymnosperm seedling sensitivity to gamma radiation: Its relation to seed radiosensitivity and nuclear variables

Abstract

Newly germinated seedlings of Larix laricina, Picea glauca, Picea mariana, Pinus banksiana, Pinus contorta, Pinus resinosa, and Thuja occidentalis were gamma irradiated at exposures of 150, 300, 450, 600, 900, 1200, 1800, 2400 and 3600 R. Number of leaves produced and shoot dry weight were determined 50 days after irradiation and survival and shoot dry weight after 130 days. Of the 4 endpoints scored, shoot dry weight at 50 days showed greatest radiosensitivity with a D50 ranging from 195 R for Pinus resinosa to 380 R for Picea mariana. Except for shoot dry weight at 50 days, all endpoints showed a sharp response over a relatively narrow range of exposures, generally between 600 and 900 R. The 7 species differed in seedling radiosensitivity by a factor of less than 2 for all endpoints. Comparison with seed radiosensitivity of the same populations showed that seedlings are from 7 to 15 times more sensitive than seeds. Correlation analyses showed that the only relationship among the endpoints studied was between D50 for number of leaves at 50 days and shoot dry weight at 130 days: the seedlings with the greatest number of leaves at 50 days had the greatest dry weight at 130 days. No relation was found between the D50 following seedling irradiation and the D50 for irradiated seed previously reported for the same populations of the 7 species. The observed seedling radiosensitivity was not correlated with predicted mature plant sensitivity, thus adding further evidence on the variation in relative radiosensitivity patterns between stages of the life cycle among gymnosperm species suggested previously. Seedling radiosensitivity as determined from 3 of the 4 endpoints studied was related to DNA content per cell determined cytophotometrically in Feulgen absorption units, but unrelated to DNA content determined chemically or to nuclear volume.