Diffusion of an organic cation into root cell walls

Uptake of a cationic dye (methylene blue) by isolated root cell walls, roots of whole transpiring seedlings, and excised roots was investigated using 7-day-old seedlings of cucumber, maize, and wheat. The number of ionogenic groups per 1 g dry and wet weight of the root cell walls, their swelling capacity (K-cw), time-dependence of methylene blue (M-cw) ion exchange capacity, and diffusion coefficients of the cation diffusion in the polymer matrix of the cell walls (D-cw) were determined. The Mcw value depended on pH (or carboxyl group dissociation); it changed in accordance with the number of carboxyl groups per I g cell wall dry weight. This parameter decreased in the order: cucumber > wheat > maize. For description of experimental kinetic curves and calculation of cation diffusion coefficients, the equation for ion diffusion into a cylinder of infinite length was used. The chosen model adequately described cation diffusion in cell walls and roots. Diffusion coefficient values for cucumber, wheat, and maize were 3.1.10(-8), 1.3-10(-8), and 8.4.10(-8) cm(2)/sec, respectively. There was a statistically significant linear dependence between K-cw and D-cw values, which characterize the same property of the polymer matrix, rigidity of its polymer structure or the degree of cross-linkage or permeability. This also confirms the right choice of the model selected for calculation of methylene blue diffusion coefficients, because K-cw and D-cw values were obtained in independent experiments. The coefficients determined for methylene blue diffusion in transpiring seedling roots (D-ts) and excised roots (D-er) depended on the plant species. The rate of methylene blue diffusion into the excised roots was either 1.5-fold lower (cucumber) or 3-4-times lower (maize, wheat) than in cell walls. The values of diffusion coefficients in roots of whole seedlings were comparable which those for the cell walls. On the basis of the experimental data and results of calculations, it is concluded that the mechanism of methylene blue uptake by plant roots involves ion exchange reactions between the organic cation and cell wall carboxyl groups, and the uptake rate is determined by the cation diffusion in the polymer matrix of the cell walls.