Journal of Weed Science and Technology

To clarify the characteristics of the floristic composition, three different weed management techniques, namely (1) the herbicide application at paddy fields and their levees, (2) hand weeding with rice bran application at paddy and (3) mowing by machine at levees, were investigated. The vegetational survey was carried out on the paddy fields and their levees in a vally in hill land at Sousa, Chiba prefecture. According to TWINSPAN analysis, the vegetation in surveyed area on the whole can be divided into paddy field and levee, and the former was subdivided corresponding the seasonal variation. The mean number of species at quadrates in herbicide non-application paddy was significantly bigger than that in the herbicide application paddy. Not only perennial species i.e. Equisetum arvense, Artemisia princeps, but annual species such as Alopecurus aequalis, Digitaria ciliaris, Cyperus microiria were conspicuous in the levees in herbicide application sites. While in the levees in herbicide non-application, the representative species of traditional levee flora like Imperata cylindrica, Rumex acetosa, and Ixeris debilis were patchily distributed, and threatened plant of Ricciocarpus natans was recognized in paddy in herbicide non-application. These results indicate that the introduction of paddy management by hand weeding or mowing machine, may contribute to the vegetation restoration like traditional paddy landscape.

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The effects of forty herbicides on cell morphologies of the freshwater microalgae such as Pseudokirchneriella subcapitata (green alga), Achnanthidium minutissimum (diatom) and Merismopedia tenuissima (blue green alga) were investigated by microscopic observation, measuring side scatter (SSC) intensity and autonomous fluorescence (AF₆₁₀) of chlorophyll a using a flow cytometry. Morphological changes and differences in chlorophyll a amount of these microalgae were induced by the exposures to high concentration (10 mg l^-1 or water solubility) of herbicides. Especially, the responses on cell shape of P. subcapitata were greater than those of two other species and these reactions observed in P. subcapitata were classified into four types.
The cell volumes were greatly increased at the highest concentrations (6 to 12 times density of 72h-EC₅₀) of the five herbicides (bensulfuronmethyl, esprocarb, mefenacet, pretilachlor, and thiobencarb) and the blanching cells were observed in incubations with three herbicides (esprocarb, quinoclamine and thiobencarb). The recoverability from morphological changes of P. subcapitata cells was confirmed in exposure experiments with seven kinds of herbicides (bensulfuronmethyl, esprocarb, mefenacet, pretilachlor, quinoclamine, simetryn and thiobencarb). It is clear that every treated P. subcapitata population did not extinct in a short time (72h) exposures of high concentration of seven herbicides, and that the recoverability of P. subcapitata population is apt to decline with increasing the amount of blanching cells.

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