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

HEf is a water-soluble humic constitutent 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 (Mg2+K+) ATPase activity in oat roots were studied. HEf stimulated nitrate uptake with a peak value at 1.0 mg C l-1, K+-stimulated ATPase activity of oat root microsomes, and H+ extrusion from roots, but it decreased ATP concentrations and did not influence O2 consumption. In contrast, HSp inhibited NO3- transport at concentrations higher than 0.5 mg C l-1, and showed only negligible effects on the considered variables. These results suggest that HEf stimulates NO3- 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.