Sensitivity of aquatic plants to the herbicide metsulfuron-methyl

The sensitivity of 12 aquatic plant species to the herbicide metsulfuron-methyl was tested in microcosm experiments under two growth conditions. As reference species, barley (Hordeum vulgare) and oil-seed rape (Brassica napus) were grown with their roots submerged in the microcosms. Two response variables were chosen: relative growth rate and specific leaf area (SLA). SLA was the most sensitive response variable, with 11 of the 12 aquatic species responding to the herbicide. EC50 values varied 56-fold between species, with the commonly used aquatic test species Lemna minor being one of the most sensitive. Fast-growing species with a small exposed leaf area proved to be more sensitive to the herbicide than slow growing species with a large exposed leaf area, which was believed to be primarily due to variations in growth rates rather than to variations in exposed leaf area. The aquatic plants displayed high tolerance in growth to metsulfuron compared with the sensitive crop oil-seed rape. Hence, possible spray-drift events and leaching of the herbicide applied at agricultural rates are not considered to have a large impact on the growth of the aquatic flora tested.