Mechanism of sucrose retrieval along the phloem path - a kinetic approach

In contrast to hexoses (fructose, glucose, 3-oxymethylglucose) sucrose is preferentially taken up by isolated conducting tissue ofCyclamen persicum Mill. Sucrose absorption follows a biphasic kinetic, with a saturable component (Km=5.2 mM; Vmax = 4.2 μmol · g FW(-1) · h(-1)) in the lower range (<20 mM) and a linear component in the upper range (20-400 mM). The saturable, carrier-mediated uptake operates against the concentration gradient without attaining any equilibrium over 6 h. It is characterized by a pH optimum at 4.5 and sensitivity to the protonophore CCCP (carbonylcyanidem-chlorophenylhydrazone). The activity of the saturable uptake increases with decreasing turgor and it is independent of the presence of hexoses. There is no effect of pH or CCCP on linear sucrose uptake at high external concentrations. Hexose uptake between 0.25 and 20 mM shows a entirely linear kinetic. Leakage after preincubation with 1 mM sucrose exhibits a minimum at pH 5.0 and enters a steady state within 100 min. Furthermore, it was found to be independent of the osmolarity of the external solution. Results are discussed in terms of apoplastic leakage and reabsorption along the phloem path: efflux of sucrose is compensated by a specific, carrier-mediated influx, which is qualitatively similar to apoplastic phloem loading accomplished by proton cotransport. In according with the balanced efflux and influx it should be considered rather as a retrieval than a loading mechanism.