The Allelopathy of the Invasive Plant Species <i>Ludwigia decurrens</i> against Rice and Paddy Weeds

<i>Ludwigia decurrens</i> Walter, belonging to the Onagraceae family, is native to southeastern and southcentral USA and has spread quickly, colonizing wetlands in South and Central America, East and South Asia and Africa. <i>Ludwigia decurrens</i> also infests rice paddy fields and causes serious damage to rice production. The phenomenon of allelopathy is often observed in invasive plant species and contributes to their invasive characteristics. However, no information has been made available on the allelopathy of <i>L. decurrens</i> against wetland species, including <i>Oryza sativa</i> (rice), or on its allelochemicals. Aqueous methanol extracts of whole <i>L. decurrens</i> plants showed allelopathic activity against <i>O. sativa</i> and paddy weeds, <i>Echinochloa crus-galli</i> and <i>Monochoria vaginalis</i> in an extract-concentration-dependent manner. Two allelochemicals, loliolide and dehydrololiolide, were isolated from the <i>L. decurrens</i> extracts through a bioassay-guided separation process using <i>O. sativa</i> as the test plant species. Loliolide and dehydrololiolide also suppressed the growth of <i>O. sativa</i> and <i>E. crus-galli</i> in a concentration-dependent manner. Thus, loliolide and dehydrololiolide may be involved in the allelopathy of <i>L. decurrens</i> and may provide a competitive advantage for <i>L. decurrens</i> due to their growth inhibitory activity. However, the inhibitory activity of loliolide was about 3-fold greater than that of dehydrololiolide. These investigations suggest the allelopathy of <i>L. decurrens</i> may play an important role in the invasion of the species into rice paddy fields and other wetlands. This is the first report on the allelopathy of <i>L. decurrens</i> against wetland species and the isolation and identification of the allelochemicals in <i>L. decurrens.</i>