Sugar Uptake and Translocation in the Castor Bean Seedling II. Sugar Transformations During Uptake

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Abstract
During sucrose uptake by the cotyledons of castor bean seedlings excised from their endosperms, small amounts of glucose and fructose accumulate in the medium. The rate of fructose uptake exceeded that of glucose at concentrations up to 0.5[image] when these hexoses were supplied separately. Sucrose uptake exceeded that of both hexoses combined, particularly at concentrations greater than 0.1 [image]. Only minute amounts of labeled hexoses were recovered from the cotyledons after exposure to glucose-U-C14 or fructose-U-C14; sucrose contained virtually all of the C14 in the sugar fraction. The fructose and glucose moieties of the extracted sucrose were equally labeled when glucose-U-C14 was supplied, and the glucose moiety contained 41% of the C14 when fructose-U-C14 was the substrate. The greater part of sucrose absorbed by the cotyledons was not hydrolyzed prior to or during uptake. There were differential responses of hydrolysis and uptake to pH, and to Tris(hydroxymethyl)-aminomethane. No interference of the hexoses on sucrose absorption, occurred. When the cotyledons were supplied with fructosyl-C14 sucrose, 90% of the C14 was retained in the fructose residue of sucrose in the cotyledons.