Abstract
About 200 1-year old Pinus sylvestris seedlings were placed at the Welcome Residue Site, a radioactive waste disposal area located near Port Hope, Ontario, on 21 April 1981, and thereafter continuously exposed to an average gamma dose rate of 10.15 mR/hr. At the same time another 200 seedlings were placed at a nearby control site which provided a background dose rate of 0.03 mR/hr. Seedlings from both locations were sampled at 2-week intervals for an analysis of stem growth and cambial activity. Our results show that, while such continuous irradiation treatment had suppressed the normal stem elongation by as much as 21% by 2 June, it had at the same time stimulated the mitotic activity of cambium cells and thus caused a significant increase in the number of xylem and phloem cells per radial file over that of control. It is suggested that the suppression of stem height of continuously irradiated seedlings was caused by the reduction in the level of endogenous auxin, IAA, and that the stimulation of cambial activity was possibly caused by two factors: an increased supply of nutrients which were not utilized for stem elongation, and an alteration in the normal proportions of IAA and other endogenous growth regulating substances such as gibberellins and/or cytokinins.
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