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Articles

Alkali Cation/Sucrose Co-transport in the Root Sink of Sugar Beet ¹

Crops Research Laboratory, Agricultural Research, United States Department of Agriculture, Science and Education Administration, Logan, Utah 84322

The mechanism of sucrose transport into the vacuole of root parenchyma cells of sugar beet was investigated using discs of intact tissue. Active sucrose uptake was evident only at the tonoplast. Sucrose caused a transient 8.3 millivolts depolarization of the membrane potential, suggesting an ion co-transport mechanism. Sucrose also stimulated net proton efflux. Active (net) uptake of sucrose was strongly affected by factors that influence the alkali cation and proton gradients across biological membranes. Alkali cations (Na⁺ and K⁺) at 95 millimolar activity stimulated active uptake of sucrose 2.1- to 4-fold, whereas membrane-permeating anions inhibited active sucrose uptake. The pH optima for uptake was between 6.5 and 7.0, pH values slightly higher than those of the vacuole. The ionophores valinomycin, gramicidin D, and carbonyl cyanide m-chlorophenylhydrazone at 10 micromolar concentrations strongly inhibited active sucrose uptake. These data are consistent with the hypothesis that an alkali cation influx/proton efflux reaction is coupled to the active uptake of sucrose into the vacuole of parenchyma cells in the root sink of sugar beets.

³ To whom reprint requests should be addressed.

¹ Cooperative investigations of Agricultural Research (United States Department of Agriculture, Science and Education Administration), the Beet Sugar Development Foundation, and the Utah State Agriculture Experiment Station. Approved as Journal Paper No. 2466. This work was supported by the Science and Education Administration of the United States Department of Agriculture under Grant 5901-0410-8-0187-0 from the Competitive Research Grants Office.