Activity of Fixed Negative Charges of Isolated Extensor Cell Walls of the Laminar Pulvinus of Primary Leaves of Phaseolus

Summary The activity of fixed charges of isolated, morphologically intact extensor cell walls of the laminar pulvinus of the primary leaves of Phaseolus coccineus L. was determined by measuring the electric potential generated by the fixed negative charges. The activity of fixed charges depends strongly on the pH and KCl concentration of the bathing solution. The maximum activity measured was approx. 570 meq 1 -1 at pH 5 in 100 mM KCl and 400 meq 1 -1 at pH 6 in 10 mM KCl, respectively. The apparent maximum activity of fixed charges in the walls of living extensor cells (250 meq 1 -1 in 10 mM KCl) is lower than in isolated cell walls. The apparent pKa value of 3.1 calculated from the titration curves of isolated extensor cell walls in both 100 mM and 10 mM KCl indicates that the carboxyl-groups of pectins are the source of the fixed charges. In isolated cell walls the mean concentration of fixed anions as calculated from the K + concentration in the walls is lower than that calculated from the wall potential. This indicates that a large fraction of the cell wall solution is unaffected by the fixed anions, i.e. may be divided into a water free space and Donnan free space. The volume of the Donnan free space is estimated to be 55 % of the total wall water or 2.9 ml g -1 dry weight. We suggest that, due to the high activity of fixed negative charges, the Donnan free space of the extensor cell wall serves as a K + reservoir which is used to change the cell volume via the pH of the apoplastic solution.