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Articles

Phloem Unloading in Developing Leaves of Sugar Beet ¹

I. Evidence for Pathway through the Symplast

Biology Department, University of Dayton, Dayton, Ohio 45469-0001

Physiological and transport data are presented in support of a symplastic pathway of phloem unloading in importing leaves of Beta vulgaris L. (`Klein E multigerm'). The sulfhydryl reagent p-chloromercuribenzene sulfonic acid (PCMBS) at concentration of 10 millimolar inhibited uptake of exogenous [¹⁴C]sucrose by sink leaf tissue over sucrose concentrations of 0.1 to 5.0 millimolar. Inhibited uptake was 24% of controls. The same PCMBS treatment did not affect import of ¹⁴C-label into sink leaves during steady state labeling of a source leaf with ¹⁴CO₂. Lack of inhibition of import implies that sucrose did not pass through the free space during unloading. A passively transported xenobiotic sugar, L-[¹⁴C]glucose, imported by a sink leaf through the phloem, was evenly distributed throughout the leaf as seen by whole-leaf autoradiography. In contrast, L-[¹⁴C]glucose supplied to the apoplast through the cut petiole or into a vein of a sink leaf collected mainly in the vicinity of the major veins with little entering the mesophyll. These patterns are best explained by transport through the symplast from phloem to mesophyll.

¹ Supported by National Science Foundation Grant DMB-8303957 and a grant from Monsanto Agricultural Products Co. (D. R. G.).

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