Glass microelectrode measurements of sieve tube membrane potentials in internode discs and petiole strips of tomato (Solanum lycopersicum L.)

Summary

In tissue slices of tomato (Solanum lycopersicum L.) sieve tube membrane potentials (E_m) were measured by use of glass microelectrodes. In internode discs, the potential differences (pd) of phloem cells near the cut surface fell into two distinct categories with average values of −66 and −109 mV. More distant from the cut surface the values decreased to averages of −71 and −140 mV. These pds were associated with phloem parenchyma cells and sieve tube/companion cell complexes, respectively. In petiole strips, pds were recorded from cells which were identified by iontophoretic injection of fluorescent dye. Averages in two different bathing media, were −140/−146mV, −149/−152mV, and −70/−68mV for sieve tubes, companion cells, and phloem parenchyma cells, respectively. The membrane potentials recorded from sieve tubes were transiently reduced upon sucrose addition. Reduction by CCCP and KCN was more permanent. Sieve tube E_ms recovered more slowly from potassium than from sucrose-induced depolarizations. Light/ dark (L/D) responses were minute (±3 mV). The limitations of the present experimentation are evaluated with special reference to the question as whether the recorded E_ms represent sieve tube membrane potentials occurring in the intact plant.