Paddy herbicides have the potential to cause adverse effects on non-target plants. Susceptibilities of some aquatic ferns ( Azolla japonica Franch. et Savat., Isoetes japonica A. Braun, Marsilea quadrifolia L. and Salvinia natans All.) and duckweeds ( Lemna minor L. and Spirodela polyrhiza Schleid.) to paddy herbicide bensulfuron methyl (BSM) were evaluated with a 20 day exposure experiment using 200 cm 2 pots. The BSM concentrations in the surface water of monitoring pots with no plants dissipated exponentially with half lives of 3.5 and 3.9 days at application rates of 15 and 150 g ha -1 , respectively. The BSM concentrations in the surface water 1 day after application in the culture pots were comparable among plant species, and were lower than those in the monitoring pots. Bensulfuron methyl reduced the plant growth in all species. I . japonica showed the lowest intrinsic relative growth rate (RGR) and the lowest susceptibility with an effective dose resulting in 50% growth inhibition (ED 50 ) of 21 g ha -1 . Except for I . japonica , the RGR of the duckweeds was similar to the ferns, and ED 50 for the duckweeds was higher than the ferns. ED 50 for Sa . natans , A . japonica and M . quadrifolia were 1.1, 1.8 and 1.2 g ha -1 , respectively, which were smaller than 1/20 of the recommended field dose (51-75 g ha -1 ) and ranged from 1/2 to 1/6 of ED 50 for L . minor and Sp . polyrhiza (6.5 and 3.2 g ha -1 , respectively). These results suggest that BSM application in paddy fields and its runoff in some localities is expected to have adverse effects on the growth of Sa . natans , A . japonica and M . quadrifolia .
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