Carbon import into barley roots: effects of sugars and relation to cell expansion

The import of photosynthate labelled with ¹¹C from a sourceleaf into the two halves of a split root system of an intactbarley plant was studied. When applied to one half of a splitroot system sugars that are absorbed and metabolized reducesubsequent import of ¹¹C into that root half. The non-metabolizedsugar analogue 3-O-methyl glucose has no effect on import, whilstmannose and 2–deoxyglucose inhibit both root elongationand import of ¹¹C. EDTA, PCMBS, and apoplastic pH in the range4–7, have little effect on partitioning. These resultsare interpreted in terms of a suggestion that phloem unloadsdirectly into expanding cells in the elongation zone of roottips. Key words: Carbon partitioning, roots, ¹¹C, sucrose, phloem, sugars, cell expansion