Glycolate metabolism of three submersed aquatic angiosperms: Effect of heavy metals

Abstract

Glycolate metabolism and the effect of heavy metals e.g. mercury, lead, cadmium and copper, on glycolate metabolism of the three submersed aquatic macrophytes, i.e. Potamogeton pectinatus L., Vallisneria spiralis L. and Hydrilla verticillata (L.f.) Royle, were studied by estimating the endogenous levels of glycolate and hydrogen peroxide and assaying the activities of glycolate oxidase and catalase. The glycolate content was highest in Hydrilla, which also showed the lowest glycolate oxidase activity. Lowest glycolate content and highest glycolate oxidase activity was recorded in Potamogeton, while an intermediate state was observed in Vallisneria. The initial content of hydrogen peroxide was highest in Vallisneria and lowest in Hydrilla. The catalase activity was lowest in Vallisneria spiralis. Compared with the initial activity, the activity of the enzyme glycolate oxidase declined in Vallisneria and Hydrilla but rose in Potamogeton after three days of ageing of isolated leaves. Hydrogen peroxide content increased and catalase activity decreased in all three species during ageing. The highest content of hydrogen peroxide in Potamogeton was associated with maximum decline in catalase activity. The four heavy metals produced different effects on glycolate metabolism in the three species of aquatic macrophytes tested. All the heavy metals decreased glycolate content in Potamogeton, but in Vallisneria Hg and Cu caused it to increase while Pb and Cd gave a decrease. In Hydrilla, glycolate content decreased with Hg and Pb but increased with Cd and Cu. Glycolate oxidase activity was increased by Hg, Cd and Cu in Potamogeton, but decreased with Pb, whereas in Vallisneria this activity was decreased by Hg and Pb but increased by Cd and Cu. The activity of this enzyme was decreased by Hg, Cd and Cu but increased by Pb in Hydrilla. In Potamogeton and Vallisneria all the heavy metals except Pb increased H 2O 2 content, while in Hydrilla the effect was reversed. Catalase activity was decreased by all the heavy metals in Potamogeton and Vallisneria but increased in Hydrilla. A metal stimulatory to glycolate metabolism when treated alone became inhibitory in the presence of another, and the inhibitory metals became stimulatory to this metabolism in combination treatments. It was concluded from these results that the glycolate metabolism was stimulated in Potamogeton and inhibited in Hydrilla by most of the heavy metals used. Vallisneria was intermediately affected by these metals.