Electrophysiological membrane characteristics of the salt tolerant Plantago maritima and the salt sensitive Plantago media

Membrane potential, electrical conductance and plasmamembrane permeability for Na+, K+ and Cl+t- of root cells of the salt tolerant Plantago maritima and the salt sensitive P. media were compared. Plants were grown with (25 mol m-3) or without NaCl. No differences were found in the membrane potentials between the species when measured in growth medium, either plus or minus NaCl. When plants were grown in 25 mol m-3 NaCl membrane potentials in both species were slightly more positive.

Na and K ions both caused depolarization of the membrane potential. These were not equal for the two species, depolarization caused by K+ ions was about 3.5 times greater than that caused by Na+ ions. Membrane permeability for both ions waemed less in the salt tolerant species. Passive Na+ permeability, i.e. in the presence of the uncoupler CCCP, was about 25% of the K+ permeability in P. media, and 60% in P. maritima. From comparison of permeabilities under ‘active’ and ‘passive’ conditions and the related ion concentrations in the root cells it was concluded that in both species active, i.e. against the electrochemical gradient, Na+ efflux pumping must exist at the cortex medium boundary.