A Reanalysis of the Two-Component Phloem Loading System in Beta vulgaris

Kinetic analysis of ¹⁴C]sucrose loading into sugar beet leaf discs revealed the presence of two transport components. At low exogenous sucrose concentrations, a saturable component, which exhibited Michaelis-Menten characteristics, was the main mode of transport. At concentrations greater than 50 millimolar, phloem loading was dominated by a linear component which appeared to operate as a first order kinetic transport process. Over the exogenous sucrose concentrations employed, influx could be described by the equation v = V_maxS/(S + K_m) + kS. Influx via both processes was strongly pH-dependent. Evidence is presented that the linear component was not explicable in terms of simple diffusion, or exchange diffusion, into either mesophyll or minor vein phloem tissue. Extensive metabolic conversion of sucrose was not a factor contributing to influx at high external sucrose concentrations. At present, it is believed that both components operate in parallel at the membrane bounding the sieve element-companion cell complex. The saturable component is identified with sucrose-H⁺ cotransport. While the significance of the linear component has been established, its nature remains to be elucidated.