The impact of herbicides in aquatic vascular plants is often investigated in toxicity laboratory studies. Lemna minor is a fast growing aquatic vascular plant regularly used as a bioindicator in ecotoxicological dose-response studies. Chlorophylls and carotenoids are among the most commonly used biomarkers when monitoring the effects of toxicants. For this study we chose two herbicides, tritosulfuron and metribuzin, with different modes of action. Tritosulfuron is a sulfonylurea herbicide, an amino acid synthesis inhibitor. Metribuzin, on the other hand, is a triazinone herbicide, a photosystem II inhibitor.
In the study we investigated changes in content of photosynthetic pigments, in particular chlorophylls a, b and carotenoids, caused by tritosulfuron and metribuzin in Lemna minor. We also evaluated these changes compared to growth rate (based on frond number) endpoint on this species. The toxicity of the two herbicides was assessed by growth inhibition tests in Lemna minor based on standard OECD protocols (7 day test). Growth inhibition, caused by the two herbicides, was measured daily from day 0 until day 7. The four effect concentrations chosen (EC100, EC75, EC50, EC25), were evaluated from pre-tests. The amounts of chlorophyll a and b as well as carotenoids were measured pectrophotometrically on days 1, 3 and 5 after exposure.
Obtained results showed that adverse effects on Lemna minor, based on growth inhibition caused by the herbicides, were not clearly connected with a decrease in chlorophyll a, b and carotenoids content. After exposure to various concentrations of tritosulfuron, Lemna minor chlorophyll and carotenoid content showed a decrease which was more intense as the time of exposure increased. For metribuzin, although there was a decrease in pigment content in high concentration treatments on day 3, on day 5 there was no evident effect in pigment concentrations in any treatment. On the other hand, growth rate was reduced with the increase of the concentration of both herbicides on days 3 and 5; growth rate differed statistically between all concentration treatments, except the lowest concentration. We can conclude that for both herbicides growth rate is a more sensitive endpoint to measure the toxicity than pigments. Moreover, tritosulfuron (the amino acid biosynthesis inhibitor) affects pigment concentration more promptly and more intensely than metribuzin (the photosystem II inhibitor) in concentrations causing similar growth inhibition. Further research on the ways that chlorophylls and carotenoid are affected by these two herbicide categories is necessary.
