Justyna Szerement

The role of cell walls and pectins in cation exchange and surface area of plant roots

We aimed to assess role of cell walls in formation of cation exchange capacity, surface charge, surface acidity, specific surface, water adsorption energy and surface charge density of plant roots, and to find the input of the cell wall pectins to the above properties. Whole roots, isolated cell walls and the residue after the extraction of pectins from the cell walls of two Apiaceae L. species (celeriac and parsnip) were studied using potentiometric titration curves and water vapor adsorption - desorption isotherms. Total amount of surface charge, as well as the cation exchange capacity were markedly higher in roots than in their cell walls, suggesting large contribution of other cell organelles to the binding of cations by the whole root cells. Significantly lower charge of the residues after removal of pectins was noted indicating that pectins play the most important role in surface charge formation of cell walls. The specific surface was similar for all of the studied materials. For the separated cell walls it was around 10% smaller than of the whole roots, and it increased slightly after the removal of pectins. The surface charge density and water vapor adsorption energy were the highest for the whole roots and the lowest for the cell walls residues after removal of pectins. The results indicate that the cell walls and plasma membranes are jointly involved in root ion exchange and surface characteristics and their contribution depends upon the plant species.

Input of different kinds of soluble pectin to cation binding properties of roots cell walls

It is widely believed that pectin are responsible for the vast majority of cation binding positions in the root cell walls. To estimate the role of particular kinds of pectin, we studied the cell wall material isolated from the roots of monocots (wheat and rye) and dicots (clover and lupine) before and after removal of different fractions of soluble pectin. Simultaneously PME activity and degree of pectin methylation were determined. From potentiometric titration curves cation exchange capacity, total surface charge and acidic strength of surface functional groups responsible for surface charging were determined. Monocots had smaller cation exchange capacity and lower pectin content than dicots. Removal of pectin induced up to 50% reduction in the cell walls surface charge. Pectin seem to have more acidic character than the other roots components that is seen from an increase in very weakly acidic groups fraction and significant decrease in the average dissociation constant of the cell walls material after pectin removal. Water soluble pectin and non-pectic soluble compounds had the dominant role in surface charging, while chelator and diluted alkali soluble pectin contributed to surface charge only at high pHs.