Oxidoreductase enzymes liberated by plant roots and their effects on soil humic material

Enzymes exuded by roots of 12 terrestrial plant species into sterile and unsterile soils were identified by spectrophotometry and gel electrophoretic methods. Aqueous extracts of planted unsterile soils contained up to 10 % of the high activities of peroxidases (EC 1.11.1.7), laccases (EC 1.10.3.2), monophenol monooxygenases (EC 1.14.18.1), and fluorescein diacetate hydrolyzing proteinase, esterase, and lipase enzymes which were found in planted sterile soils. Enzyme excretion was increased by minerally fertilized, dying, and water-stressed plants. The plant peroxidase/H2O2 system decreased the absorbance of humic macromolecules, and possibly, too, the reactivity of the humus phenolic groups to Fast Blue B salt. It is concluded that roots of certain members of Fabaceae, Gramineae, and Solanaceae release enough oxidoreductases to take part in the oxidative degradation of certain soil constituents.