*Author for Correspondence

The naturally occuring auxin Indole-3-Acetic acid (IAA), is responsible for plant growth by cell elongation. Within plant cells, IAA concentration is controlled by the enzyme peroxidase. Peroxidases are also considered to be plant defence proteins due to their involvement in the lignification of cell walls, which restricts plant growth. The present study investigates whether exogenously supplied IAA can increase the production of peroxidase by in vitro plantlets of Armoracia rusticana or horseradish. Adding IAA to the growth medium was hypothesised to influence the activity of peroxidases in the horseradish plantlets, since IAA is a substrate of the enzyme peroxidase. In the experiments, horseradish plantlets were grown in medium supplemented with increasing concentrations of IAA. The IAA treated plantlets showed a significantly higher growth rate than the controls. The growth rate of the treated plantlets increases linearly (r = +0.969) with the concentration of IAA in the medium up to 1.5 mg L-1 (0.105 mg