Crop injury by Pyributicarb was evaluated in direct-seeded rice plants, depending on the seeding depth and application time. On the other hand, crop injury in combinations with dymron or dimepiperate was also evaluated in rice plants at 1cm seeding depth. Pyributicarb caused some crop injury in rice plants seeded on the soil surface on different application times (Fig. 1-a, b). However, pyributicarb did not cause a significant crop injury in direct-seeded rice plants at 1cm depth under the soil surface by the application of the standard dose (630g a. i./ha), more than 6 days (2 leaf stage) after seeding (Fig. 1-d, 2-c, d, 3-c, d). It is considered that pyributicarb does not injure rice plants seeded under the soil surface, because their rooting parts are not exposed to the chemical. In the case of combinations, dymron was effective in decreasing the crop injury of pyributicarb in rice plants seeded under the soil surface (Fig. 2-a, b), while dimepiperate did not significantly decrease the crop injury by pyributicarb under such conditions (Fig. 3-a, b). In conclusion, pyributicarb could be used as a paddy herbicide for direct-seeded rice cultivation.

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Peroxidizing herbicides have a common mode of action, i. e. inhibition of chlorophyll biosynthesis. A molecular target of these herbicides is protoporphyrinogen-IX oxidase (Protox). Corn is widely used for Protox inhibitory assays by peroxidizing herbicides because the plant enzyme has high and stable activity. The Protox isolated from one corn variety, Anjou, has been used for the assay in our laboratory and several others. This variety can no longer be used however, because of its reduced tolerance against corn diseases. Therefore, we collected twelve corn varieties and compared their Protox activities and sensitivities against peroxidizing herbicides. Among the tested varieties, DK212MF exhibited the highest activity. This variety has approximately two times higher activity than Anjou. Tolerance of Anjou and DK212MF to peroxidizing herbicides was evaluated by a pot test, because the varieties have quite different Protox activities. DK212MF was less sensitive to the peroxidizing herbicides oxyfluorfen and chlorophthalim than Anjou. Furthermore, DK212MF accumulated less Proto-IX than Anjou, which was believed to be one of the causes of its lower sensitivity in vivo to the peroxidizing herbicides.

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Response to sulfonylurea herbicides was investigated in three Lindernia weeds {L. procumbens (Krock.) Borbás, L. dubia var. major Pennell and L. dubia Pennell var. dubia} using plants from different sites in the Tohoku region of Japan. Plants of L. dubia var. dubia from a paddy field of Yuza Town, Yamagata Prefecture, were resistant to bensulfuron-methyl with a I₅₀ value (concentration of bensulfuron-methyl for inhibiting dry matter production of weeds by 50%), 5, 110 times higher than that of the plants from Shizukuishi Town, Iwate Prefecture, in the Wagner pot experiment (Fig. 1, Table 1). In the deep petri dish experiment, plants of L. procumbens from Yuza Town, L. dubia var. major from Tajiri Town, Miyagi Prefecture, and L. dubia var. dubia from Yuza Town, were also resistant to bensulfuron-methyl with a I₅₀ value, 60.7, 10, 200, and 14, 100 times higher than that of the susceptible plants of each species from Omagari City, Akita Prefecture or Shizukuishi Town (Fig. 2, Table 1).
The plants of L. dubia var. dubia from Yuza Town showed a cross-resistance to the other sulfonylurea rice herbicides, such as ethoxysulfuron, cyclosulfamuron, azimsulfuron, imazosulfuron and pyrazosulfuron-ethyl in the treatment of the recommended dosage in this region (Table 2, Table 4). Preemergence application of pretilachlor, bifenox and cafenstrole controlled the resistant plants effectively in the Wagner pot experiment (Table 3). Naproanilide and simetryn/thiobencarb/MCPB controlled the resistant plants effectively when applied at the 1st leaf stage of the weeds in the Wagner pot experiment (Table 3). Application of herbicides including pretilachlor or naproanilide, simetryn/thiobencarb/MCPB and bentazone controlled the resistant plants effectively in the field experiment (Table 4). Application of herbicides containing these ingredients, therefore, could be effective for the control of resistant Lindernia in paddy fields.

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Responses of barnyardgrass (Echinochloa crus-galli var. crus-galli) to water drained, saturated, or flooded soils were assessed in relation to the aerenchyma formation in crown roots. Plant height, fresh weight of shoots, the number of crown roots, and the fresh weight of roots of 4-week-old barnyardgrass increased in the following order: saturated>drained>flooded. By contrast, aerenchyma was radially well developed in the cortex of crown roots when flooded, but no extensive aerenchyma formation was observed when the soil was drained. Both root porosity and the total area of aerenchyma increased in the following order: flooded>saturated>drained; however, these values varied with the portion and/or ages of crown roots tested. In relation to the diameter of the crown roots, barnyardgrass showed different responses from those of root aerenchyma, i. e., the root diameter increased significantly when the soil was saturated but decreased as the root aged. These results suggest that the response to flooding in barnyardgrass is related to the formation of root aerenchyma which developed in a polar direction from the basal parts to the apical parts as the roots aged.

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Protoporphyrinogen oxidase-inhibiting herbicides cause massive accumulation of protoporphyrin IX (Proto IX) in treated plant cells. Reactive oxygen species (ROS) induced by the accumulated Proto IX is thought to cause peroxidative damage, however, the process of peroxidation is still unclear. To investigate the mechanism of the peroxidation of lipids by Proto IX-induced ROS, we used an oxygen electrode. By measuring dissolved oxygen consumption in the reaction mixture, the rate of peroxidation was evaluated in vitro. Production of lipid hydroperoxide was confirmed by measuring thiobarbituric acid reacting substances (TBARS). The linear decrease of dissolved oxygen and linear increase of lipid hydroperoxides occurred immediately after light irradiation, suggesting that the method used in this study is applicable as a model system to investigate lipid peroxidation by Proto IX-induced ROS. Determination of the antioxidants effect on the reduction of dissolved oxygen indicated that both superoxide and hydrogen peroxide scavenging enzymes as well as singlet oxygen scavengers remarkably suppressed the peroxidative reaction. These results suggest that in addition to singlet oxygen, other ROS are involved in the Proto IX-induced peroxidation processes in plant cells. Furthermore, comparison of porphyrin intermediates derived from chlorophyll synthesis showed that the peroxidizing ability of Proto IX was not higher than other porphyries. This suggests that greater accumulation of Proto IX is necessary to initiate lipid peroxidation in plant cells.

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