SUCROSE UPTAKE BY THE PHLOEM PARENCHYMA OF CARROT STORAGE ROOT

The characteristics of sucrose uptake into the symplast of phloem tissue discs harvested from fresh, actively-growing carrot storage roots are described. Sucrose uptake exhibited a curvilinear response with increasing sucrose concentration. The inhibitors p-chloromercuribenzenesulphonic acid (PCMBS) and carbonyl cyanide m-chlorophenylhydrazone (CCCP) decreased uptake and resulted in sorely linear relationships between uptake and sucrose concentration. These results suggest that active carrier-mediated transport occurs at the plasmalemma in addition to a diffusive mechanism. The former saturates at a lower concentration (approximately 20 mM) than the latter which does not saturate below 100 mM. Though similar in their effect on the ethanol-soluble fraction, CCCP and PCMBS had different effects on the conversion of sucrose to ethanol-insoluble material. Varying the osmotic environment with different mannitol concentrations did not affect uptake between 0 and 400 mM mannitol, but did cause an increase at 600 mM mannitol: an effect which may have been an artefact of plasmolysis. Metabolic conversion to ethanol-insoluble forms remained unchanged from 0 to 250 mM mannitol and declined above this. Thus metabolism, but not uptake may be responsive to changes in turgor.