A new approach to the investigation on the tonogenic groups of root cell walls

Abstract

Acid-base properties of wheat, lupin, pea root cell walls were investigated. The roots of etiolated and green plants of different age were analysed by the potentiometric method. The ion exchange capacity of root cell walls (Si) was estimated at various pH values (pHi 2 to pHi 12) and constant ion strength of the solution (10 mM). To analyse polysigmoid curves pHi =f (Si), Gregor’s equation was used. It was shown that Gregor’s model fits fairly well the experimental data. The total quantities of cation-exchange (St cat) and anion-exchange (St an) groups were determined in the root cell walls. It was shown that the quantity of anion exchange groups is varied through a small range (60–185 µmol/g dry wt.) in plant species tested, and that the St cat differs widely from 550 to 1300 µmol/g dry wt. For leguininous plants the quantity of acidic groups (fixed anions) is nearly twice as large as that for cereals. It was found that in seedlings as well as in plants, there are 3 cation-exchange groups and one anion-exchange group in root cell walls. The quantity of functional groups of each type (Sj) was estimated, and the corresponding values of nj and pKa j were calculated. It can be assumed that the groups with the pKa 1 ≈ 3.2 are amine groups, the ones with PKa 2 ≈ 5 are groups of galacturonic acid, the ones with pKa ≈ 7.5 are the carboxyl groups of the second species, and the ones with pKa 4 ≈ 40 are the phenolic groups. The values of dissociation constants (pKa j) and Sj indicate that the root cell walls of wheat, lupin and pea are identical in qualitative structure of ionogenic groups but vary in the quantity of each ionogenic group. It was demonstrated that the summarized quantity of carboxyl groups (S2 + S3) should be connected directly with the pH gradient in the extracellar space at the membrane surface. The gradient arises from ion-exchange reactions between cations of an outer medium and protons of the ionized carboxyl groups of the cell walls. The results suggest that, St cat and St an allow the quantitative estimation of ion exchange properties of the cell walls. The resulting parameters (Sj, pKa j and nj) allow prediction of changes in an ionic composition of a medium that bathes the cell membrane, during the first step of mineral nutrition uptake.