Hexose accumulation and turgor-sensitive starch synthesis in discs derived from source versus sink potato tubers
1990
Wright, K.M. | Oparka, K.J.
The net total uptake (sum of soluble and insoluble components) of the hexoses, D-glucose and D-fructose, into sink potato (Solanum tuberosum L.) storage parenchyma was biphasic with respect to substrate concentration. Analysis of radioactive products revealed that the biphasic kinetics were composed of a linear, soluble component superimposed on saturating starch synthesis. In contrast, in source tuber tissue, there was negligible conversion of D-glucose to starch and the shape of the kinetic was the result of a biphasic soluble component. The uptake of D-fructose into source tissue was linear with respect to substrate concentration. Uptake of the non-metabolizable glucose analogue, 3-oxymethyl-D-glucose (3-OMG), into both sink and source tissue, demonstrated biphasic kinetics, indicating the presence of a carrier for glucose. The data demonstrate that in sink potato tubers, metabolism greatly influences apparent uptake kinetics, the kinetics of starch synthesis masking the kinetics of hexose transport at the plasmalemma. Uptake of L-glucose was linear with respect to substrate concentration, an observation consistent with this sugar not being recognized by a carrier. As in the case of sucrose, in sink tuber tissue the conversion of D-glucose and D-fructose to starch was sensitive to turgor, showing a marked optimum in external osmotica containing 300 mol m-3 mannitol. The mechanisms controlling turgor-sensitive starch synthesis in the potato tuber would, therefore, appear to be common to all three sugars.
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