The absence of phloem loading in willow leaves.

Willow (Salix babylonica L.) is representative of a large group of plants that have extensive plasmodesmatal connections between minor vein phloem and adjoining cells. Because plasmodesmata provide a diffusion pathway for small molecules, it is unclear how sucrose could be loaded from the mesophyll into the phloem against a concentration gradient. In the studies reported here, the minor vein phloem of willow leaves plasmolyzed in approximately the same concentration of osmoticum as the mesophyll. Sucrose concentrations in mesophyll cells were greater than those reported in the literature for aphid stylet exudate from willow stems. Calculated turgor pressures in the mesophyll and minor vein phloem were greater than turgor reported in the literature for sieve elements in the stems of willow. Images of minor veins were not obtained in autoradiographs when attached leaves, or leaf pieces, were provided with 14CO2 or [14C]sucrose. Therefore, no evidence could be found for accumulation of sucrose against a concentration gradient in the minor vein phloem of willow. In these leaves, the mesophyll apparently acts as the "source" for long distance transport of sugar. The mechanism of translocation in willow, and the evolution of phloem loading, are discussed.