23Na- and 7Li-NMR studies were conducted on isolated cell walls of Nitella to monitor the changes induced in the pectin matrix after the cation exchange capacity was reduced to about 60% of its initial value by a pretreatment with a 500 mM LiCl solution. Walls in Ca2+ form were exchanged with NaCl and LiCl solutions before and after this pretreatment. In the whole cell wall equilibrated with 10, 35 and 100 mM NaCl solutions or mixtures at the same total concentrations of Na+ and Li+ in the same proportions, the 23Na- and 7Li-NMR spectra were all characterized by only one signal. The Na+ and Li+ relaxation times were drastically shorter than those recorded in aqueous solution but the two Na+ relaxation rates increased linearly with the Na+ concentration in the external solutions. In contrast, in the cell wall where the CEC had been reduced, the Na+ relaxation times were longer and no correlation with external concentrations was obtained. The 7Li spectra in walls equilibrated with 500 mM LiCl displayed two distinct lines suggesting different Li+ bonding affinities for the wall polyuronides. These data are interpreted in the scheme proposed by Gillet et al. (1994) to explain the competitive effects of alkaline ions on the wall pectin disorganization via disruption of divalent cation crosslinks, but also of solvation-like bonds between cell wall polymers.
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Gillet et al. (1994) studied this question.