Abstract
A large field screening programs and laboratory experiments in many countries have proved that rice (Oryza sativa L.) is allelopathic and releases allelochemical(s) into the rice ecosystems. A great number of compounds, such as phenolic acids, fatty acids, phenylalkanoic acids, hydroxamic acids, terpenes and indoles, were identified in rice plants, root exudates and decomposing rice residues as potential allelochemicals. However, increasing evidence suggests that momilactone B among those compounds probably plays a critical role in rice allelopathy. Rice plants secrete momilactone B into the neighboring environments over their entire life cycle and momilactone B accounts for 59-82% of the observed rice allelopathy. In addition, genetic studies have shown that momilactone B deficient rice mutants significantly reduce allelopathy, demonstrating the importance of momilactone B in rice allelopathy. Several stress conditions increase momilactone B production and secretion in rice plants. It was also found that rice may response to the presence of neighboring barnyardgrass (Echinochloa crus-galli (L.) Beauv.) by sensing the components in barnyardgrass root exudates, and increase allelopathic activity by production of elevated concentration of momilactone B. Thus, rice allelopathy may be one of the inducible defense mechanisms by chemical-mediated plant interaction between rice and barnyardgrass, and the induced-allelopathy may provide a competitive advantage for rice through suppression of the growth of barnyardgrass. Momilactone B has so far been found only in rice and the moss Hypnum plumaeforme Wilson, although rice and the moss H. plumaeforme are taxonomically quite distinct.
