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Articles

Compartmentation in Vicia faba Leaves

I. Kinetics of ¹⁴C in the Tissues following Pulse Labeling ^1,2

^a Department of Botany, University of Georgia, Athens, Georgia 30602

Leaflets of Vicia faba were pulse-labeled with ¹⁴CO₂ to follow the subsequent movement of photosynthate between leaf tissues. Samples were taken during a ¹²CO₂ chase, quick frozen, freeze-substituted, and embedded in methacrylate. Paradermal sections provided tissue samples consisting only of upper epidermis, palisade parenchyma, spongy parenchyma and veins, spongy parenchyma, or lower epidermis. Most CO₂ fixation occurred in the palisade parenchyma, but its ¹⁴C content declined rapidly after labeling. Concomitant with the decline of activity in the palisade parenchyma, there was an increase in activity in the spongy parenchyma and upper epidermis and a slow increase in the lower epidermis. Activity in the palisade parenchyma and spongy parenchyma eventually reached similar levels and remained constant. Tissue samples containing veins were consistently the most radioactive, and activity in those samples showed a decline. Very little change occurred in the insoluble fraction from any tissue. The results support previous assumptions regarding the pathway of assimilate transport to the veins, and demonstrate the rapidity of such transport. Sucrose is apparently the principal mobile compound.

⁴ Present address: Department of Biochemistry, Michigan State University, East Lansing, Mich. 48824.

¹ This work was supported by Grant GB-33905X1 from the National Science Foundation to D.B.F.

² This work was presented in part at the 1973 meeting of the American Society of Plant Physiologists.

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