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Translocation Patterns in Soybeans Exposed to ¹⁴CO₂ at Four Different Time Periods of the Day¹

Field grown soybeans [Glydne max (L.) Merr.] were exposed at the mid.bloom stage to ¹⁴CO² for 1 hour beginning at either 9 AM, 11 AM, 1 PM, or 3 PM CDT to evaluate the effects of time of day when exposed to ¹⁴CO² on the translocation and distribution of labeled assimilates. Plants were harvested 0, 1, 3, 6, and 24 hours after labeling. By the end of the labeling period, 16.9% of the total recovered ¹⁴C had been exported from the leaves and 36.9% had been exported by the end of 24 hours (averaged over the four exposure times).

Leaves labeled during the time periods associated with the highest net C fixation rates (11 to 12 AM and 3 to 4 PM) assimilated more ¹⁴C, translocated more to the stems and roots, and exhibited a higher rate of export from the leaves. Percent ¹⁴C recovered in each component (except nodules) was significantly different between the 9 to 10 AM and 3 to 4 PM exposure periods. Differences between the two midday exposures were relatively small, averaged over all harvest times and increased only when compared immediately and 24 hours after labeling.

Carbon-14 labeling and translocation experiments should be conducted during relatively constant time periods to allow nonconfounded intercomparisons between data when experimental technique dictates multiple times of exposure to ¹⁴C, especially between the early morning and late afternoon exposure periods.

Key Words: Giycine max (L.) Merr. • ¹⁴CO₂ labeling

² Instructor (now post-doctoral research associate) and assitant professor of agronomy, respectively, University of Missouri-Columbia.

Received for publication May 28, 1974.