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Articles

Compartmentation in Vicia faba Leaves

II. Kinetics of ¹⁴C-Sucrose Redistribution among Individual Tissues following Pulse Labeling ^1,2

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

Leaflets of Vicia faba L. were pulse labeled with ¹⁴CO₂ and the kinetics of ¹⁴C-sucrose redistribution among individual tissues was followed. Sucrose specific activity in the whole leaf peaked about 15 minutes after labeling and declined with a half-time of about 80 minutes. In one experiment, leaflet discs taken at various times during the ¹²CO₂ chase were quick frozen, freeze-substituted, and embedded in plastic. The tissue was sectioned paradermally and sections of palisade parenchyma, of spongy parenchyma, and of spongy parenchyma that contained veins were collected. Water extracts from these sections were assayed for sucrose specific activity. Sucrose specific activity in the palisade parenchyma was higher than that of the spongy parenchyma and reached a maximum in both tissues 9 to 15 minutes after labeling. Sucrose specific activity initially declined rapidly in the palisade parenchyma followed by a period during which little or no loss occurred. Sucrose specific activity in sections containing veins peaked at 15 minutes with a maximum value substantially higher than either mesophyll tissue, indicating that recently synthesized sucrose was preferentially exported from the mesophyll. Decline of activity in these sections containing veins continued for the remainder of the experiment. Sucrose specific activity in lower epidermal peels peaked several minutes after that of the whole leaflet and remained lower. Sucrose specific activity in upper epidermal peels was variable (probably due to contamination), but the limited data suggest that the sucrose specific activity there reached somewhat higher values than those of the lower epidermis. The experiments indicate that each leaf tissue contains a kinetically identifiable sucrose pool (which we refer to as "histological compartmentation"), and that further compartmentation may occur at the intracellular level. A simulation of leaf sucrose compartmentation is presented.

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

⁵ Present Address: Agricultural Research Department, Monsanto Company, St. Louis, Mo. 63166.

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 meetings of the American Society of Plant Physiologists.