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Distribution and Utilization of ¹⁴C-labelled Assimilates in Soybeans¹

Individual soybean [Glycine max (L.) Merr.] plants grown in the field during 2 years were allowed to assimilate ¹⁴CO2 either during vegetative growth, at first flowering, at early pod development, or during rapid seed development. Some of the labeled plants were harvested immediately after labelling, and others after 1 week, at incipient leaf drop, or at maturity. Radioactivity was measured in the component parts of each labelled plant.

Plant parts that were growing actively at the tune of labelling accumulated more UC than those parts that were not growing actively. When the plants were growing vegetatively, most of the ¹⁴C was incorporated into structural leaf and stem tissue. As labelling tunes became later during ontogeny, stems contained decreasing amounts of ¹⁴C, and more ¹⁴C accumulated in seeds by maturity. After rapid seed development began, roots and nodules accumulated little ¹⁴C.

If plants were labelled before vegetative growth was completed, 30 to 40% of the ¹⁴C present immediately after labelling was recovered in plants grown to maturity. Plants labelled during rapid seed development retained 65% of the original ¹⁴C at maturity. Calculated losses of ¹⁴C due to respiration averaged 20%. Most of the ¹⁴C lost by maturity was in abscised tissue.

The proportions of ¹⁴C assimilated at the four stages of development and recovered in seeds at maturity were quite similar to comparable proportions of assimilated ¹⁴C recovered in the seeds of maize (Zea mays L.) grown nearby.

Key Words: Translocation • Respiration • Glycine max (L.) Merr. • Zea mays L.

² Associate Professor and Research Scientist, respectively.

Received for publication February 5, 1973.