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

The import of photosynthate labelled with C-11 from a source leaf into the two halves of a split root system of an intact barley plant was studied. When applied to one half of a split root system sugars that are absorbed and metabolized reduce subsequent import of C-11 into that root half. The non-metabolized sugar analogue 3-O-methyl glucose has no effect on import, whilst mannose and 2-deoxyglucose inhibit both root elongation and import of C-11. EDTA, PCMBS, and apoplastic pH in the range 4-7, have little effect on partitioning. These results are interpreted in terms of a suggestion that phloem unloads directly into expanding cells in the elongation zone of root tips.