Carbohydrate transport in discs of storage parenchyma of celery petioles

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Abstract
Petioles of celery leaves function successively as net importing (sink) and net exporting (source) organs during ontogeny. The parenchyma of these petioles is the main store for large amounts of D-glucose, D-fructose and mannitol. The mechanism of uptake of glucose and fructose into discs isolated from storage parenchyma of celery petioles was investigated. Uptake kinetics showed a biphasic response to increasing concentrations for both hexoses with a saturable component at low concentrations and a non-saturable, linear, diffusion-like component at higher concentrations (at least up to 16 mM). The apparent Kns -values were an order of magnitude higher for fructose (2-3 mM) than for glucose (034 RIM) uptake. The saturable components of uptake of the two hexoses were inhibited similarly by the SH-reagents PCMBS and NEM, the uncouplers CCCP and DNP, the ATPase inhibitor DES and the hydrophobic reagents phloretin and phlorizin. They were stimulated by the plasmalemma H+ -ATPase stimulator FC and showed a broad pH-optimum around pH 6. Competition studies revealed that glucose uptake was very specific whereas fructose uptake was inhibited by D-glucose, L-sorbose and D-tagatose. The saturable components of uptake of both hexoses were clearly turgor-dependent. Lowering of cell turgor resulted in a linear increase of Vmax and a constant Km . It is concluded that the saturable components of glucose and fructose uptake are of a similar, active, sugar-proton cotransport type with carriers containing SH-groups and interacting hydrophobically. It is suggested that two different hexose carriers might be operative, one for glucose only and one for both glucose and fructose.