PARENT SESSION
Poster Session 31: Rare, Threatened, and Endangered Species
Thursday, August 11, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Impact assessment of herbicide runoff from rice fields on threatened aquatic plants in Japan.

Ikeda, Hiroaki^*,1, Aida, Miki², Luo, Xiao-Yong¹, ¹ National Institute for Agro-Environmental Sciences, Tsukuba, Japan² University of Tsukuba, Tsukuba, Japan

ABSTRACT- We investigated the effects of rice herbicides on five threatened aquatic plants (one dicot, Penthorum chinense Pursh, and four ferns, Azolla japonica Franch. et Savat., Isoetes japonica A. Braun, Marsilea quadrifolia L. and Salvinia natans All.), distributed in rice-cropping areas in Japan. For P. chinense, a hydroponic culture exposed to each of four rice herbicides (bensulfuron-methyl, mefenacet, simetryn and thiobencarb) for 10 days was conducted using 6-well polystyrene plates under laboratory conditions. All herbicides reduced seedling growth at the 3rd leaf stage, but the growth recovered after treatment except for simetryn. Thus, we estimated the effective concentration of herbicides resulting in 50% inhibition (EC₅₀) for growth recovery after treatment. EC₅₀ values of bensulfuron-methyl, mefenacet, simetryn and thiobencarb were 3.5, 2600, 180 and 4500 nM respectively, indicating the greatest inhibitory effect of bensulfuron-methyl on the growth recovery of P. chinense. For the aquatic ferns, a 20-day treatment exposed to only bensulfuron-methyl was conducted using pots with a paddy soil under the ambient condition in May 2002. Although the herbicide concentration dissipated exponentially, the 20-day treatment reduced plant growth in all fern species. Thus, we estimated the effective concentration detected 1 day after application resulting in 50% inhibition (1-DAA EC₅₀) for dry matter production during 20-day treatment. The 1-DAA EC₅₀ values for A. japonica, I. japonica, M. quadrifolia and S. natans were 3.5, 40, 2.0 and 1.8 nM, respectively. The maximum concentrations of bensulfuron-methyl, mefenacet, simetryn and thiobencarb in river water in Japan which had been reported in recent years were 0.24-5.6, 23-88, 27-108 and 7.0-24 nM, respectively. As a result, it is suggested that bensulfuron-methyl runoff from paddy fields in rice-cropping areas in Japan may cause adverse effects on the four threatened plants, A. japonica, M. quadrifolia, P. chinense and S. natans.

Key words: bensulfuron-methyl, ecological risk, vascular plants, 50% effect concentration