HEXOSE-TRANSPORT IN ISOLATED IMMATURE BARLEY EMBRYOS

The pathways of monosaccharide assimilation in developing barley embryos have been investigated in an in vitro assay system, and the results are discussed in relation to the primary carbohydrate nutrients of the developing and germinating embryo. Glucose and fructose are both rapidly taken up by immature embryos, and at a substrate concentration of 100 mM their assimilation rate exceeds by threefold that of sucrose. The kinetics of hexose uptake are complex, reflecting a combination of passive diffusion and metabolically mediated uptake mechanisms. The hexose transport energetics were further studied with the glucose analogue 3-O-methyl-D-glucose, shown to be a non-metabolite in the barley embryo. The kinetics of 3-O-methyl-D-glucose influx and efflux indicated that passive diffusion accounts for a major part of hexose assimilation in barley embryos. However, an active transport component was revealed by the sensitivity of 3-O-methyl-D-glucose uptake to uncoupling agents and the demonstration of competitive inhibition of uptake between glucose analogues. These observations supported the view that part of the observed energy-dependence of hexose uptake might be asociated with a direct coupling of energy to transport. A high-affinity active group translocation mechanism for glucose assimilation could be important in embryo metabolism during the first hours of germination prior to hydrolysis of endosperm starch reserves. Analysis of the fate of assimilated hexoses showed that the rapid consumption of the hexose pool in the synthesis of sucrose must contribute to the influx of hexoses by the maintenance of a hexose concentration gradient between the embryo and its environment. © 1979, Annals of Botany Company.