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Environmental and Stress Physiology

Electrostatic Changes in Lycopersicon esculentum Root Plasma Membrane Resulting from Salt Stress

Department of Crop Science and Plant Ecology, University of Saskatchewan, Saskatoon, Saskatchewan S7N OWO Canada, Biology Department, St. Olaf College, Northfield, Minnesota 55057, Department of Agronomy, University of Illinois, Urbana, Illinois 61801, U.S. Department of Agriculture-Agricultural Research Service, U.S. Salinity Laboratory, Riverside, California 92501

Salinity-induced alterations in tomato (Lypersicon esculentum Mill. cv Heinz 1350) root plasma membrane properties were studied and characterized using a membrane vesicle system. Equivalent rates of MgATP-dependent H⁺-transport activity were measured by quinacrine fluorescence (pH) in plasma membrane vesicles isolated from control or salt-stressed (75 millimolar salt) tomato roots. However, when bis-[3-phenyl-5-oxoisoxazol-4-yl] pentamethine was used to measure MgATP-dependent membrane potential () formation, salt-stressed vesicles displayed a 50% greater initial quench rate and a 30% greater steady state quench than control vesicles. This differential probe response suggested a difference in surface properties between control and salt-stressed membranes. Fluorescence titration of vesicles with the surface potential probe, 8-anilino-1-napthalenesulphonic acid (ANS) provided dissociation constants (K_d) of 120 and 76 micromolar for dye binding to control and salt-stressed vesicles, respectively. Membrane surface potentials (_o) of–26.0 and –13.7 millivolts were calculated for control and salt-stressed membrane vesicles from the measured K_d values and the calculated intrinsic affinity constant, K_i. The concentration of cations and anions at the surface of control and salt-stressed membranes was estimated using _o values and the Boltzmann equation. The observed difference in membrane surface electrostatic properties was consistent with the measured differences in K⁺-stimulated kinetics of ATPase activity between control and salt-stressed vesicles and by the differential ability of Cl^– ions to stimulate H⁺-transport activity. Salinity-induced changes in plasma membrane electrostatic properties may influence ion transport across the plasma membrane.

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