Autoradiographic, plasmolysis, and (14)C-metabolite distribution studies indicate that the majority of exogenously supplied (14)C-sucrose enters the phloem directly from the apoplast in source leaf discs of Beta vulgaris. Phloem loading of sucrose is pH-dependent, being markedly inhibited at an apoplast pH of 8 compared to pH 5. Kinetic analyses indicate that the apparent K(m) of the loading process increases at the alkaline pH while the maximum velocity, V(max), is pH-independent. The pH dependence of sucrose loading into source leaf discs translates to phloem loading in and translocation of sucrose from intact source leaves. Studies using asymmetrically labeled sucrose (14)C-fructosyl-sucrose, show that sucrose is accumulated intact from the apoplast and not hydrolyzed to its hexose moieties by invertase prior to uptake. The results are discussed in terms of sucrose loading being coupled to the co-transport of protons (and membrane potential) in a manner consistent with the chemiosmotic hypothesis of nonelectrolyte transport.