Dark-grown sugarbeet(Beta vulgarisL.) leaves were used to investigate a possible role of apoplastic sucrose in the induction and development of the putative phloem-located sucrose carrier in relation to minor vein loading and export capacity. Unlabeled sucrose was introduced to the leaf apoplast after which vein accumulation of 14Csucrose was determined by autoradiogra-phy. Western blotting was used to detect the putative carrier. An affinity purified antibody against the sucrose binding protein of soybean did not cross-react with the protein in a plasmalemma-enriched fraction from sugarbeet leaves. Challenging the apoplast of leaf discs with buffer plus sucrose for 6 h (induction) resulted in decreased 14Csucrose uptake. When induction treatments were conducted with detached intact leaves in the dark, sucrose and glucose, but not buffer alone enhanced 14Csucrose uptake. Detached leaves induced under laboratory light conditions for 24 h showed enhanced 14Csucrose uptake even in the absence of any sugar introduced to the apoplast (buffer only). The data suggested that in the etiolated tissue, sucrose was not a direct and specific inducer of its putative carrier; instead sugars may have provided the energy for vein loading. Furthermore, the data suggested a role for light in the development of the putative sucrose carrier and vein accumulation of sucrose in transitional leaves of sugarbeet. The role of light may also be related to tissue energy level.
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