Involvement of Protons as a Substrate for the Sucrose Carrier during Phloem Loading in Vicia faba Leaves

The effect of pH on uptake of exogenous sucrose by broadbean (Vicia faba L.) leaf discs without the lower epidermis has been investigated at various sucrose concentrations. The concentration dependence of sucrose uptake showed a biphasic saturation response. At high sucrose concentrations (>20 millimolar), sucrose uptake showed no pH dependence. At low sugar concentrations (<5 millimolar), plots of 1/V against 1/H(+) give straight lines which all intercept at the same point at the left of the ordinal axis. Calculations show that these data agree well with two-substrate kinetics for the carrier, the substrates being the protons and the sucrose molecules. Our results provide further evidence that protonation/deprotonation processes of the carrier are involved in phloem loading, especially for low sucrose concentrations of the apoplast.