Patch Clamp Studies on Root Cell Vacuoles of a Salt-Tolerant and a Salt-Sensitive Plantago Species

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Abstract
Plantago media L. and Plantago maritima L. differ in their strategy toward salt stress, a major difference being the uptake and distribution of ions. Patch clamp techniques were applied to root cell vacuoles to study the tonoplast channel characteristics. In both species the major channel found was a 60 to 70 picosiemens channel with a low ion selectivity. The conductance of this channel for Na+ was the same as for K+, PK+/PNa+ = 1, whereas the cation/anion selectivity (PK+/Pc1-) was about 5. Gating characteristics were voltage and calcium dependent. An additional smaller channel of 25 picosiemens was present in P. maritima. In the whole vacuole configuration, the summation of the single channel currents resulted in slowly activated inward currents (t1/2 = 1.2 second). Inwardly directed, ATP-dependent currents could be measured against a .DELTA.pH gradient of 1.5 units over the tonoplast. This observation strongly indicated the physiological intactness of the used vacuoles. The open probability of the tonoplast channels dramatically decreased when plants were grown on NaCl, although single channel conductance and selectivity were not altered.