Effect of Herbicides in Paddy Runoff on Seed Germination of Vallisneria asiatica and Ammannia multiflora

Although rice production provides stable environments for aquatic plants, the wide use of herbicides is a concern for wild plants inhabiting the surroundings of rice paddies. Because commercial herbicides are typically a cocktail of chemicals, they may pose a threat to wild plants even when the constituent chemicals do not individually have detrimental effects. We sampled water from a rice paddy and a river receiving the paddy drainage immediately after the transplanting period to identify and compare the concentrations of herbicides. We also examined the effect of the sampled water on the germination of two plants: Ammannia multiflora (hygrophyte) and Vallisneria asiatica (submerged). We found that the concentrations of glufosinate in the paddy and river waters were 0.0015 and 0.0013 mg L-1, respectively, and those of pyraclonil were 0.0010 and 0.0009 mg L-1 in the same waters, indicating that these chemicals persist outside the rice paddy. The germination rate of A. multiflora was significantly diminished with exposure to river and paddy water under fluctuating temperature conditions, whereas no difference was observed for V. asiatica. For a comprehensive understanding of the influences of residual herbicides on wetland biodiversity, it is necessary to analyse the effects of herbicides on a wide range of aquatic plants and at various stages of growth.