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 pl asma 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 cell s. Sorption of Ca-45(2+) to right-side-out PM vesicles of melon (Cucum is melo L.) roots (prepared by aqueous two-phase partitioning) was stu died at various Ca2+ concentrations, in the presence of increasing con centrations of Na+ or Mg2+ chlorides. Experimentally determined amount s of Ca2+ sorbed to the plasma membrane vesicles agreed fairly well wi th those calculated from a competitive sorption model. The best fit of the model to the experimental data was obtained for an average surfac e 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 sugg est that nonphospholipid components in the plasma membrane contribute significantly to Ca2+ binding. The high affinity of Ca2+ binding to th e plasma membrane found in this study might explain the specific role of Ca2+ in relieving salt stress in plant roots.