Uniform rice and barnyard grass seedlings (fifty of each) were then transferred onto a holed plate of polystyrene foam that was floated on the medium of 1 L Hoagland nutrient solution in plastic container (15 cm × 10 cm × 10 (height) cm; Kim et al., 2005; Song et al., 2008). Only roots of both plants were immersed in the medium through the hole of the plate and all manipulations were carried out under sterile conditions. The pH value of the medium was maintained at 6.0 throughout the experiments by using 0.5 mol/L NaOH or H2SO4 (Kim et al., 2005; Song et al., 2008). After 10 days incubation at 25 ◦C with a 12-h photoperiod, rice seedlings were harvested for bioassay and momilactone B determination.