QUANTITATIVE COMPETITION OF CALCIUM WITH SODIUM OR MAGNESIUM FOR SORPTION SITES ON PLASMA-MEMBRANE VESICLES OF MELON (CUCUMIS-MELO L) ROOT-CELLS

The presence of Ca2+ ions in solution is vital for root growth. The plasma membrane is one of the first sites where competition between Ca2+ and other ions occurs. We studied the competition between Ca2+ and Na+ or Mg2+ for sorption sites on the plasma membrane of melon root cells. Sorption of Ca-45(2+) to right-side-out PM vesicles of melon (Cucumis melo L.) roots (prepared by aqueous two-phase partitioning) was studied at various Ca2+ concentrations, in the presence of increasing concentrations of Na+ or Mg2+ chlorides. Experimentally determined amounts of Ca2+ sorbed to the plasma membrane vesicles agreed fairly well with those calculated from a competitive sorption model. The best fit of the model to the experimental data was obtained for an average surface area of 370 Angstrom(2) per charge, and binding coefficients for Na+, Mg2+ and Ca2+ of 0.8, 9 and 50 M(-1), respectively. Our results suggest that nonphospholipid components in the plasma membrane contribute significantly to Ca2+ binding. The high affinity of Ca2+ binding to the plasma membrane found in this study might explain the specific role of Ca2+ in relieving salt stress in plant roots.