Peroxidases in grass dew derived from guttation: possible role in polymerization of soil organic matter

Peroxidases are enzymes that catalyze the oxidative cross-linking and polymerization of certain organic compounds by hydrogen peroxide and other organic peroxides. This study demonstrates that peroxidases are present in dew (droplets formed as the result of guttation) collected from Bermuda grass hybrids 419 and Tifway 2 [Cynodon dactylon (L.) x Cynodon transvaalensis Davy], which are warm-season C(4) grasses, and Kentucky bluegrass (Poa pratensis L.), which is a cool-season C(3) grass. Peroxidase activity [quantified with horseradish peroxidase (HRP) (activity 152 purpurogallin units/mg) as standard] in guttational fluids collected from grasses during early morning was in the 80 to 120 mu g/L range. Isoelectric focusing was used to determine isoelectric points (pI) of the isozymes present in the Bermuda grass dew following dialysis and lyophilization of the collected dew. The pi values ranged from 4.3 to 8.3 with 14 isozymes being detected using guaiacol and hydrogen peroxide as substrates. Peroxidases also were extracted from soil supporting the growth of Bermuda grass. Peroxidases in these soils were most abundant in the top 5 cm layer (activity was in the 6.8 to 16 purpurogallin units/g range). Stability and activity of these peroxidases in the presence of fulvic and humic acids were evaluated. Compared to controls with no added humic substances, peroxidase activity was inhibited by a soil fulvic acid and prolonged by a humic acid. Field measurements indicated that peroxidase activity did not meetly decrease during the winter when the grass was dormant, indicating that the peroxidases released into the soil remain active for a considerable time. Based on results in these studies and previously determined dry and wet deposition of atmospheric peroxides, we estimate that peroxidase-catalyzed reactions in areas planted in these grasses may convert about 8 g C m(-2) yr(-1) of labile soil organic compounds to more persistent oligomers and polymers.