Abstract
Humic acid fractions of different molecular weights inhibit IAA-oxidase activity to varying extents. The higher the molecular weight, the lower the IAA-oxidase inhibition; the parent material, however, having the greatest effect. A direct relationship was found between the degree of enzyme inhibition and the number of phenolic groups, C/H ratio and the carbon and nitrogen contents of the humic acid fractions, whereas the relationship was negative for oxygen and hydrogen percentages. The ash content of the humic acid preparations was not related to their effects on the enzyme. These results seem to suggest that phenolic groups, not carboxyl groups, are responsible for the influence of humic acids on enzyme activity.
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