The role of pH for ionic solute uptake by the non-aerial hypocotyl of mung bean plants

The non-aerial hypocotyl is an exceptional fast pathway for the uptake of soil-applied pre-emergent herbicides or systemic actives with soil mobility after release from coated seeds. The presence of charged groups at the interface of the non-aerial hypocotyl to the environment and its role for the penetration of likewise charged agrochemicals have not yet been investigated as a basis for the development of enhanced delivery systems. Recent work suggests pore-like structures having pore size diameters < 1.5 nm in mung bean hypocotyls, which is not associated with the existence of a real channel system having a continuous aqueous phase. The current work presents a detailed study on the role of pH for the sorption of ionic compounds by the non-aerial hypocotyl of mung bean plants. The presence of fixed dissociable charges on the surface of the non-aerial hypocotyl was characterized utilizing different methods, including local application of charged radiolabelled agrochemicals, incubation in osmotic solutions and differential dye staining experiments. The formation of a pH-dependent and continuous Donnan phase determined the adsorption or repulsion of compounds with opposite and equal ionic charge, which was shown for various ionic dyes and agrochemicals. Uptake via the non-aerial hypocotyl varied with the pH value of the test solution, indicating the presence of fixed charges in the membrane. This information is considered as essential for a better understanding of the transport properties through the non-aerial hypocotyl membrane.