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Kinetics of C-14 Translocation in Soybean

II. Kinetics in the Leaf ¹

^a Department of Biochemistry and Biophysics, Iowa State University, Ames, Iowa

The kinetics of ¹⁴C-assimilates in the soybean leaf were studied in pulse labeling and steady state labeling experiments. ¹⁴C-Sucrose apparently served as the ultimate source, at least, of translocated ¹⁴C-sucrose. However, since the specific activity of leaf sucrose reached a maximum within 5 minutes after pulse labeling, whereas that of exported sucrose did not reach a maximum until at least 20 minutes, it appeared that there were two sucrose compartments in the leaf. A possible physical basis for the two compartments may be the mesophyll (a photosynthetic compartment) and a specialized "paraveinal mesophyll" (a nonphotosynthetic compartment), through which photosynthate must pass on its way to the veins.

The ¹⁴C kinetics of sterol glucoside, and probably esterified sterol glucoside, were similar to those for ¹⁴C-sucrose export. Sterol glucoside was labeled only in its glucose moiety and was the only stem lipid which became strongly labeled during ¹⁴C-sucrose translocation. These sterol derivatives may act as membrane carriers of sucrose between the translocation stream and surrounding cells.

The kinetics of ¹⁴C-sucrose and its movement to the veins are discussed with reference to compartmentation within the leaf and metabolic exchange with other compounds, particularly with starch. Although a simple compartmental model gave a fairly accurate description of ¹⁴C-sucrose kinetics, an entirely accurate model could not be provided, primarily because of loss of ¹⁴C from sucrose, at an unknown rate, to starch.

¹ This work was supported in part by United States Public Health Service Fellowship 1-F1-GM-22,031, to the author, and in part by National Science Foundation Grant GB-114, to Dr. Sam Aronoff.