Proper control of barnyardgrass for direct sowing in flooded paddy rice cultivation in Japan’s cold regions requires knowledge of the relation between barnyardgrass seed emergence characteristics and paddy field water conditions after rice seeding. Barnyardgrass seed emergence characteristics were investigated under submergence conditions. Seeds were collected from paddy rice fields of 13 habitats in Akita prefecture. This study examined two water conditions in plastic containers: submergence and non-submergence. Their respective water depths were 2 cm and 0 cm from the soil surface. Seeds were sown into soil in May and June. The percentage of final emergence was significantly lower for submerged seeds than for non-submerged seeds of four habitats of the May sowing and of one habitat of the June sowing. The average length of days for emergence (ADE) was greater for submerged than for non-submerged seeds from all habitats, irrespective of the sowing time. The value of emergence uniformity (VEU) tended to be greater for submerged than for non-submerged seeds from all habitats, irrespective of the sowing time. For submerged May sown seeds, ADE was delayed 6.2–12.7 days, and VEU was 1.9–3.7 times greater than that of June-sown seeds. Additionally seeds collected from paddy rice fields in three habitats were sown into soil in the pot. Thereafter, the soil was submerged. In the plots, after submergence was ended at 20 and 30 days after submergence, the seed emergence percentage increased, irrespective of the habitat. Results of this study of barnyardgrass demonstrated that submergence causes a loss of emergence uniformity during the emergence period, especially at low temperatures, although emergence can occur under submergence. Furthermore, the cessation of submergence facilitates plant seed emergence.
This study aimed to investigate the geographical distribution patterns of the grass species Echinochloa oryzicola in Japan. Panicles of this species were collected from 636 sites, except Okinawa, and their spikelet forms (C or F) were identified (first study). Subsequently, the spikelet forms of 87 specimens of E. oryzicola that were deposited in four herbariums were identified (second study). In the first study, C- and F-form spikelets were collected from 396 and 377 sites, respectively. The degree of overlap indicated independent distribution patterns of both forms across the various sites. The Moran’s I value of both the C- and F-forms indicated positive spatial autocorrelation. The results of these two spatial statistical analyses strongly suggested that the C- and F-forms have geographical trends in Japan. In the second study, a relatively higher number of specimens was collected from the Hokkaido and Kinki regions, and the geographical distribution patterns of the C- and F-forms agreed well with those of the first study. These two studies indicated that the C-form is densely distributed in Tokyo, Kanagawa, Yamanashi, Shizuoka, Aichi, Kinki region, Okayama, Tottori, Hiroshima, Yamaguchi, Fukuoka, and Oita regions, whereas the F-form is densely distributed in Hokkaido, Tohoku, Hokuriku, Chiba, and Tottori regions. Thus, our results partially contradicted previous research suggesting that the C- and F-forms of E. oryzicola are predominantly distributed in areas on the Pacific Ocean side and the Japan Sea side of Japan, respectively (Yabuno 2001).