Abstract
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.
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