Unequal Distribution of Fixed Negative Charges in Isolated Cell Walls of Various Tissues in Primary Leaves of Phaseolus

Abstract

Summary Microelectrode technique was used to determine the activity of fixed charges of cell walls of various tissues of primary leaves of Phaseolus coccineus L. by measuring the electric potential differences generated by the fixed negative charges of the isolated but morphologically intact walls. An unequal distribution of the activity of fixed charges was found in the walls of parenchyma and collenchyma cells of the petiole: at the layer facing the cell lumen the activity was low (approx. 220 meq 1 −1 in 100 mM KCl at pH 7) compared to that of the wall layer facing the intercellularies in parenchyma cells (approx. 550 meq 1 −1 in 100 mM KCI, pH 7) or of the thickened corners of the collenchyma cell walls (approx. 610 meq 1 -1 in 100 mM KCI, pH 7). In contrast, the activity of fixed charges determined in pulvinar motor cells at the wall layers facing the cell lumen was high. In both the primary and secondary pulvinus the walls of motor cells located above the stele exhibited a higher activity of fixed charges (approx. 640–680 meq 1 −1 in 100 mM KCI, pH 7) than the motor cells located below the stele (approx. 550–600 meq 1 −1 in 100 mM KCI, pH 7). The results indicate that wall layers rich in pectic substances exhibit high fixed charge activities. Consequently, the walls of most tissues should be characterized by a layering of fixed charge activities corresponding to the layering in the biochemical composition. The functional importance of the negative fixed charges of the cell wall and of their heterogenous distribution is discussed with respect to the activity of wall-bound enzymes, membrane transport processes and the role of walls as cation reservoirs.