Seed shattering habit of weedy rice [Oryza sativa] in Nagano prefecture, Japan, and germination ability of shattered seeds

A weedy rice with a red pericarp, known as 'Toukon', has become a serious problem for rice producers in Nagano prefecture. We studied the seed threshability and shattering habit of weedy rice accessions collected from paddy fields in Nagano to identify the appropriate timing for hand weeding of the weedy rice to prevent them from shattering their seeds and forming a seed bank in the fields. We also studied the germination ability of the weedy rice seed threshed after heading at intervals. We assessed the threshability of the weedy rice accessions grown under a condition undisturbed by wind by stroking their panicles slightly by hand at different times after heading and counted the number of threshed seeds. Some seeds were threshed about 2 weeks after heading, and almost all within 1 month. The number of threshed seeds per day was the highest about 3 to 4 weeks after heading. On the other hand, most accessions grown under field condition started to shatter their seed between 2 weeks and 1 month after heading. Early-heading accessions tended to start shattering sooner after heading than late-heading accessions. Although most threshed seeds in the early stage had green hulls and looked immature, more than 80% of them germinated. Additionally, about 0.1% to 0.2% of the seeds collected from each accession showed dormancy. These results suggest that weedy rice should be cleared from the field within 2 weeks after heading by hand or other weeding methods to prevent them from shattering their seeds and forming a seed bank in the soil.