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Articles

Effects of Chronic Internal -Radiation from Photoassimilated ¹⁴CO₂ on the Retention and Distribution of ¹⁴C in Young White Pine Plants ¹

^a Department of Biological Sciences, Brock University, St. Catharines, Ontario, Canada

Eastern white pines (Pinus strobus L.) in their 3rd year of growth photoassimilated 35, 110, 220, or 400 microcuries of ¹⁴CO₂ on a single occasion in the spring when needle expansion was occurring. Once assimilated into organic products and translocated to various sinks, the retained ¹⁴C represented an internal chronic source of ionizing radiation to the plant. About 2.5 months later, the pines were harvested, and the distribution of ¹⁴C activity was determined. In addition, new needle lengths, fresh weights, and rates of processes involving CO₂ exchange were measured.

All parameters measured were affected in those pines which initially assimilated 220 or 400 microcuries of ¹⁴CO₂, whereas no significant differences were observed between control plants and those initially assimilating 35 microcuries. Plants incorporating 110 microcuries were intermediate in their responses. The percentage of assimilated ¹⁴C retained by the plants decreased as the initial activity of assimilated ¹⁴C increased, with the losses of ¹⁴C occurring through both respiration and the shedding of needles. The pines which assimilated 35 microcuries of ¹⁴CO₂ retained about 70% of the ¹⁴C; those which assimilated 400 microcuries retained only about 28%. The distribution of the ¹⁴C activity recovered from all the plants was essentially the same; about 65% of the total ¹⁴C recovered was in the new needles and 13% in the roots.