Nitrate uptake and ATPase activity in oat seedlings in the presence of two humic fractions

Abstract

HEf is a water-soluble humic constituent of molecular size compatible with uptake by plant roots. In naturally-occurring humic material, HEf is usually encased in macrostructured, high-molecular-weight, humic constituents (HSp) without biological activity. One prerequisite for any direct biological action by humic substances is therefore dissociation of the HEf-HSp structure. In order to investigate the above interaction, the effects of HEf and HSp on nitrate uptake and microsomal (Mg 2+ K +) ATPase activity in oat roots were studied. HEf stimulated nitrate uptake with a peak value at 1.0 mg Cl −1, K +-stimulated ATPase activity of oat root microsomes, and H + extrusion from roots, but it decreased ATP concentrations and did not influence O 2 consumption. In contrast. HSp inhibited NO − 3 transport at concentrations higher than 0.5 mg Cl −1, and showed only negligible effects on the considered variables. These results suggest that HEf stimulates NO − 3 transport by activating K +-stimulated ATPase, whose activity lowered the ATP content of the cell. These effects are probably due both to the surfactant behaviour of humic substances (critical micelle concentration is 0.25 mg C ml −1 for HEf and 0.36 mg C ml −1 for HSp) and to their biological activity.