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Allocation and Turnover of Photosynthetically Assimilated ¹⁴CO₂ in Leaves of Glycine max L. Clark ¹

Biochemistry Department, University of Missouri-Columbia, Columbia, Missouri 65211, Division of Molecular Plant Biology, University of California, Berkeley, California 94720

The allocation and turnover of photosynthetically assimilated ¹⁴CO₂ in lipid and protein fractions of soybean (Glycine max L. Clark) leaves and stem materials was measured. In whole plant labeling experiments, allocation of photosynthate from a pulse of ¹⁴CO₂ into polymeric compounds was: 25% to proteins in 4 days, 20% to metabolically inert cell wall products in 1 to 2 days, 10% to lipids in 4 days, and 4% to starch in 1 day. The amount of ¹⁴C labeled photosynthate that an actively growing leaf (leaf 4) used for its own lipid synthesis immediately following pulse labeling was about 25%. The ¹⁴C of labeled proteins turned over with half-lives of 3.8, 3.3, and 4.1 days in leaves 1, 2, and 3, respectively; and turnover of ¹⁴C in total shoot protein proceeded with a half-life of 5.2 days. Three kinetic ¹⁴C turnover patterns were observed in lipids: a rapid turnover fraction (within a day), an intermediate fraction (half-life about 5 days), and a slow turnover fraction. These results are discussed in terms of previously published accounts of translocation, carbon budgets, carbon use, and turnover in starch, lipid, protein, and cell wall materials of various plants including soybeans.