Biological Activities of Rice Allelochemicals Momilactone A and B
Momilactone A and B had been demonstrated to play critical roles in rice allelopathy by the findings of 
physiological and genetical approaches. Rice plants secrete momilactones into the rhizosphere over their entire life cycle at phytotoxic levels, and momilactones are able to account for the majority of the observed rice allelopathy. However, allelopathic activities of momilactones were determined on only a few test plant species. Therefore, this study was to determine the allelopathic activities of momilactones against nine test plant species including weed species, and four rice cultivars. 
Momilactone A and B inhibited Arabidopsis, alfalfa, lettuce, cress, timothy, barnyard grass, E. colonum, crabgrass and ryegrass at concentrations greater than 3 and 0.3 μM, respectively. The inhibition on those test plants was concentration dependent. On the other hand, effectiveness of momilactone A and B on rice cultivars, Koshihikari, Nipponbare, Norin 8 and Sasanishiki was very weak. Those rice cultivars were only inhibited by momilactone A and B at concentrations greater than 300 and 100 μM, respectively. Momilactone A and B may have potential as templates for the development of new plant control substances because of their selective inhibitory activities on weed plant 
species. More importantly, momilactone A and B as allelochemicals in rice may provide a molecular marker for 
breeding and/or engineering efforts directed at increasing allelopathic activity of this critical staple food crop.