Screening of allelopathic activity from rice cultivars by bioassay and field test.

Preliminary results are presented of experiments conducted to determine the allelopathic activity of 500 rice genotypes against weeds in rice fields, without or with minimum herbicide applications, by bioassay and field tests. The plant box method was used for assaying the activity of diffusible phytotoxic metabolites from rice Nippon (Its in agar in the laboratory using lettuce (cv. Great Lakes 366) as the receiver plant. Roots of several rice lines, including Awa-Akamai, an ot red rice cultivar reserved in the old shrine in Shikoku, were highly inhibitory to the elongation of lettuce roots. Some genotypes showed no activity, some showed strong activity, and in some cases, the top of the lettuce radicle became brown to black. The strongest allelopathic activity was observed in Awa-Akamai. In a 5-year field trial conducted in Tsukuba, Japan, apparent allelopathic activity was observed in the second year. Awa-Akamai reduced the total weed biomass by approximately 80–90% compared with the commercial Japanese cultivar Nippon. Bare suppression of barnyard grass (Echinochloa crus-galli) and broad leaf rice weeds was observed in the field with other red rice cultivars. The mechanisms responsible for the weed suppression in Awa-Akamai may be valuable for improving weed suppression in commercial rice cultivars.