Mapping membrane potential differences and dye-coupling in internodal tissues of tomato (Solanum lycopersicum L.)

Symplastic continuity in internodal tissues of tomato was investigated by electrophysiological and micro-injection techniques. Recordings of potential differences (pd) were combined with iontophoretic injection of 5,6-carboxyfluorescein (CF) and Lucifer Yellow CH (LYCH). Mapping of pds and dye-coupling respectively labeled the potential sites of major solute uptake and the spatial differentiation in symplast permeability. The cells of the central pith had low negative pds (-23 mV) and limited dye-coupling. In the pith periphery, pds (-43 mV) and dye-coupling were more pronounced. The pith was symplastically isolated from the secondary xylem. Axial contact cells (pd -54 to -109 mV) showed a strong dye-coupling within the vessel-enclosing sheath. No dye transfer was observed from the vessel sheaths to other cells. Fibre-tracheids and libriform wood fibres were dye-coupled. Ray cells (pd -39 to -94 mV) transported dye to adjacent fibres (pd -18 to -41 mV). The reverse was not observed. Except for a few cases of limited radial cell-cell transport, no dye movement through rays was observed. Cambium cells were connected mainly axially (ray cell initials) or both axially and tangentially (fusiform initials). Radial dye-coupling of cambium cells was rare. The limitations of the present approach and the significance of the differential symplast permeability for xylem-to-phloem transfer are discussed.