The effect of a low molecular weight (< 5kDa, LMW) and a high molecular weight (> 5kDa, HMW) humic fractions on surface redox activities of oat roots was studied. Oxidation of the electron donor NADH and reduction of the artificial electron acceptor ferricyanide [K3Fe(CN)6] exogenously supplied to the roots both alone of in combination, was measured in the presence or absence of soil humic substances. HMW humic fraction inhibited NADH oxidation either in the presence or absence of ferricyanide, while LMW humic fraction inhibited NADH:ferricyanide oxidoreductase activity due to the contemporary addition of the two redox compounds to the solution bathing the roots. NADH:ferricyanide oxido-reduction was partially due to the release of substances from the roots. However, the presence of soil humic fractions (LMW or HMW) did not significantly modify this behaviour. Rather, the inhibitory effect of soil humic substances was even more evident when the oxidoreduction solely due to the root activity was considered. The results confirm the presence of multiple types of oxidoreductase activities at the surface of oat roots and show that the two humic fractions HMW and LMW may differently affect these activities. Partial inhibition of NADH oxidase activity is interpreted as a possible way of interference of humic substances with metabolic processes involved in cell wall formation. The effects on surface redox activities are discussed in terms of the role of soil humic substances in promotion of plant growth.
