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 ⁴⁵Ca²⁺ 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 Ų 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.