Accumulation and Conversion of Sugars by Developing Wheat Grains

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Abstract
The volume and composition of the endosperm apoplast of the developing wheat grain, comprising endosperm cavity and intercellular free-space, was examined in relation to kernel growth rate and size. Samples of the cavity sap were collected by centrifugation of kernels during the linear phase of grain growth. The cavity sap contained 10–50 mM sucrose, a small amount of hexoses but a high concentration of oligosaccharides (up to 9 times that of sucrose). In comparing cvs Yandilla King and Cleveland, high growth rate was associated with high cavity sap sucrose concentration but with low K+ concentration. K+ concentration in the endosperm cells (124 mM) was about 5 times higher than in the cavity sap (10–40 mM). Cavity sap pH was 6.3–6.6. The uptake of sucrose by endosperm cells was partly inhibited by PCMBS, an inhibitor of membrane-bound carriers. Several necessary conditions for proton cotransport during sucrose uptake by endosperm cells were met. The volume of the intercellular free-space, estimated by membrane permeating (14C-mannitol, 14C-sucrose) or non-permeating (3H-PEG900) markers averaged 2.2 μl or 5–7% of the water in grains of cvs Yandilla King, Cleveland and SUN 9E. The cavity volume was highly variable but tended to be larger in larger grains. Pulse labelling of 14CO2 to flag leaves showed that 14C-sucrose was the principal 14C-assimilate in the cavity sap and was converted to insoluble compounds in the endosperm while the cavity sap oligosaccharides acquired negligible label in 6 h.

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