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

During sucrose uptake by the cotyledons of castor bean seedlings excised from their endosperms, small amounts of glucose and fructose accumulate in the medium. When these hexoses were supplied separately, it was found that the rate of fructose uptake exceeded that of glucose at concentrations up to 0.5 m. Sucrose uptake exceeded that of both hexoses combined, particularly at concentrations greater than 0.1 m. Only minute amounts of labeled hexoses were recovered from the cotyledons after exposure to glucose-U-(14)C or fructose-U-(14)C; sucrose contained virtually all of the (14)C in the sugar fraction. The fructose and glucose moieties of the extracted sucrose were equally labeled when glucose-U-(14)C was supplied, and the glucose moiety contained 41% of the (14)C when fructose-U-(14)C was the substrate.Several considerations led to the conclusion that by far the greater part of sucrose absorbed by the cotyledons was not hydrolyzed prior to or during uptake. These included the differential responses of hydrolysis and uptake to pH, and to Tris(hydroxymethyl) aminomethane, the absence of interference of the hexoses on sucrose absorption and particularly the demonstration that when the cotyledons were supplied with fructosyl-(14)C sucrose, 90% of the (14)C was retained in the fructose residue of sucrose in the cotyledons.