A comparison of the effect of triclopyr triethylamine salt on two species of duckweed (lemna) examined for a 7- and 14-day test period

The objective of this study was to estimate the level of phytotoxicity (50% reduction in the number of plants or fronds as compared to controls) to five clones of Lemna over a 7 and 14-day period. The end-points examined were the number of fronds, the number of plants, biomass, chlorophyll a and chlorophyll b . All data gathered during this investigation confirm the fact that this herbicide is only slightly toxic to duckweed clones using the U.S. EPA classificatory scheme. There is no statistically significant difference among the five end-points for the 7-day test for any of the plant clones with the exception of chlorophyll b . The 14-day results are not nearly as consistent. L. gibba G-3 is more sensitive than the four clones of L. minor . The EC 50 for biomass is more tolerant in L. minor 6591, L. minor 7102 and L. minor 7101 than for L. gibba G-3. The difference of greatest interest is that of chlorophyll production between the two test periods. In all cases chlorophyll a is significantly reduced in the 14-day test over that of the 7-day test, while chlorophyll b is only significantly reduced in the 14-day test for L. minor 6591, L. minor 7102 and L. minor 7136. It is hypothesized that the reason for this is that the system runs out of nutrients. This alone militates against the use of the 14-day test and favors the use of the 7-day one.