| Abstract: | The volume and composition of the endosperm apoplast of thedeveloping wheat grain, comprising endosperm cavity and intercellularfree-space, was examined in relation to kernel growth rate andsize. Samples of the cavity sap were collected by centrifugationof kernels during the linear phase of grain growth. The cavitysap contained 10–50 mM sucrose, a small amount of hexosesbut a high concentration of oligosaccharides (up to 9 timesthat of sucrose). In comparing cvs Yandilla King and Cleveland,high growth rate was associated with high cavity sap sucroseconcentration but with low K+ concentration. K+ concentrationin the endosperm cells (124 mM) was about 5 times higher thanin the cavity sap (10–40 mM). Cavity sap pH was 6.3–6.6.The uptake of sucrose by endosperm cells was partly inhibitedby PCMBS, an inhibitor of membrane-bound carriers. Several necessaryconditions for proton cotransport during sucrose uptake by endospermcells were met. The volume of the intercellular free-space, estimated by membranepermeating (14C-mannitol, 14C-sucrose) or non-permeating (3H-PEG900)markers averaged 2.2 µl or 5–7% of the water ingrains of cvs Yandilla King, Cleveland and SUN 9E. The cavityvolume was highly variable but tended to be larger in largergrains. Pulse labelling of 14CO2 to flag leaves showed that 14C-sucrosewas the principal 14C-assimilate in the cavity sap and was convertedto insoluble compounds in the endosperm while the cavity sapoligosaccharides acquired negligible label in 6 h. Key words: Wheat, Endosperm apoplast, Sugars |
