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Accumulation and conversion of sugars by developing wheat grains. 3. Non‐diffusional uptake of sucrose, the substrate preferred by endosperm slices

1983; Wiley; Volume: 6; Issue: 5 Linguagem: Inglês

10.1111/j.1365-3040.1983.tb01275.x

1365-3040

A. H. G. C. RIJVEN, Roger M. Gifford,

Polysaccharides and Plant Cell Walls

Abstract. Starch synthesis by developing wheat endosperm slices incubated in liquid media was more rapid, at optimum concentration, from sucrose as external substrate than from glucose and/or fructose. Fructose inhibited conversion of sucrose or glucose. The results are consistent with the hypothesis that sucrose is not hydrolysed in the apoplast before uptake. Besides a diffusional influx and efflux of labelled sucrose there was a non‐diffusional influx; it was inhibited by dinitrophenol, potassium arsenate, potassium iodide, and parachloromercuribenzene sulphonate (PCMBS). PCMBS inhibited both uptake and conversion of label from 150 molm −3 14 C‐sucrose by 75%. Uptake and conversion of sucrose were stimulated by lowering pH and by fusicoccin, a promoter of proton extrusion. Extracellular solutes like raffinosc and polyethylene glycol stimulated net uptake of label from 14 C‐sucrose — the larger molecule being more effective — this being due to a non‐specific inhibition of diffusional efflux. At too high an osmotic concentration such solutes reduced net uptake; the larger the molecule the lower this transitional concentration. In conclusion, wheat endosperm is better equipped to convert apoplastic sucrose rather than the hydrolysis products to starch; active loading of sucrose possibly involves proton co‐transport; and large molecules in the extracellular solution reduce the diffusional elllux of loaded substrate.