In the absence of H2O2, isoforms of vacuolar phenol-dependent peroxidase (PO) in beet (Beta vulgaris) roots oxidized phenolic compounds like tyrosinases. Tyrosinase activity of PO manifested a clearly expressed pH-dependence with the optimum at pH of 8.0–9.0; peroxidase activity was the highest at pH 5.0–7.0. The inhibitory analysis confirmed a specificity of observed reactions. Along with tyrosinase activity, PO manifested SOD-like activity, which was also expressed in the absence of H2O2 and depended on some factors, for example, on the way of analyzed sample preparation. This activity appeared at a long-term dialysis and low temperature. SOD-like PO activity was observed in the presence of such substrates as 3,3′-diaminobenzidine and IAA. The results obtained allow a conclusion that vacuolar PO, as well as PO of other localization and origin is a polyfunctional enzyme, which, under definite conditions, can catalyze reactions of oxidase type.

Influence of herbicides on the hydrogen peroxide generation in vacuolar extracts of red beet root (Beta vulgaris L.) was investigated. Belonging to different chemical classes of herbicide compounds have been used. Herbicides differ from each other in the mechanism of effects on plants. Clopyralid (aromatic acid herbicide, derivative of picolinic acid) and 2.4-D (phenoxyacetic herbicide), characterized by hormone-like effects, contributed to the formation of H2O2 in vacuolar extracts. Fluorodifen (nitrophenyl ether herbicide) and diuron (urea herbicide) also have increased contents H2O2. These compounds inhibit the electron transport, photosynthesis, and photorespiration in sensitive plants. Herbicidal effect of glyphosate (organophosphorus herbicide) is due to the inhibition of amino acid synthesis in plant cells. Glyphosate did not affect the content of H2O2 in vacuolar extracts. Herbicide dependent H2O2-generation did not occur with oxidoreductase inhibitors, potassium cyanide and sodium azide. The results suggest that the formation of ROS in the vacuoles due to activity of oxidoreductases, which could interact with herbicides.