Contribution of momilactone A and B to rice allelopathy.

Eight cultivars of rice (Oryza sativa L.) inhibited shoot and root growth of Echinochloa crus-galli when co-cultured with rice seedlings in a bioassay medium. Momilactone A and B were found in the bioassay medium of all rice cultivars, and concentrations of momilactone A and B in the medium were 0.21-1.5 and 0.66-3.8 micromol/L, respectively, indicating that all rice cultivars may secrete momilactone A and B into the medium. Exogenously applied momilactone A and B inhibited the growth of shoots and roots of E. crus-galli at concentrations greater than 30 and 1 micromol/L, respectively. The concentrations required for 50% growth inhibition of E. crus-galli shoots and roots, respectively, were 146 and 91 micromol/L for momilactone A and 6.5 and 6.9 micromol/L for momilactone B. Considering the growth inhibitory activity and concentrations found in the bioassay medium, momilactone A may have caused only 0.8-2.2% of the observed growth inhibition of E. crus-galli roots and shoots by rice. However, momilactone B in the medium was estimated to be able to cause 59-82% of the observed growth inhibition of E. crus-galli roots and shoots by the rice seedlings. In addition, the concentrations of momilactone B in the medium reflected the observed differences in the growth inhibition of E. crus-galli by the eight rice cultivars investigated. This suggests that the allelopathic activity of rice may depend primarily on the secretion level of momilactone B. Therefore, momilactone B may play a very important role in rice allelopathy.