Metabolism of pyroxasulfone in a tolerant crop, wheat and a susceptible plant, rigid ryegrass, was studied using 14 C-pyroxasulfone. The main metabolites were a cysteine conjugate of the isoxazoline ring (M-26), deaminated M-26 (M-29) and a glucose conjugate of M-29, suggesting that the main metabolic route in both plants was the cleavage of the methylenesulfonyl linkage caused by glutathione conjugation. The difference in the metabolic activity was assumed to be one of the factors in determining the selectivity of pyroxasulfone between wheat and rigid ryegrass.

Rice production is one of the major non-point sources of pesticide pollution in Japan because more than half of all arable land is paddy fields, from which pesticides can easily be transported to rivers. Public concerns regarding the adverse effects of pesticides in runoff on aquatic ecosystems and drinking water are increasing. In 2003, the Ministry of the Environment of Japan imposed a new pesticide registration scheme concerning ecological risk in an aquatic environment based on concepts adopted in the EU and the USA. In this scheme, the acute effect concentration in the assessment species (i.e., fish, Daphnia, or algae) is compared with the predicted environmental concentration calculated using environmental models based on the standard scenario of the pesticide use. 1)Recently, mathematical models (e.g., PADDY 2,3) and PCPF-1 4-6) ) have been developed to simulate the transport and fate of paddy rice pesticides in Japan; however, these models do not consider metabolites derived from the parent compound by hydrolysis, photolysis, or microbial degradation. The RICEWQ model, developed in the EU and the USA, is the only one that considers metabolites under paddy conditions. 7)The metabolites of some pesticides, such as organophosphorus insecticides, may be more toxic to aquatic organisms than the parent compound. Therefore, for ecological risk assessment it is necessary to evaluate the exposure of aquatic organisms not only to the parent compound but also to its metabolites. PADDY 2,3) is used to predict pesticide behavior in paddies. The objective of this study was to develop and evaluate an improved PADDY-based model that includes photoisomerization and metabolic pathways for the herbicide pyriminobacmethyl [methyl 2-(4,6-dimethoxy-2-pyrimidinyloxy)-6-(E,Z)-(1-methoxyiminoethyl)benzoate], which is used for selective control of early water grass. To validate the model, values calculated by the improved model were compared with measured values from paddy lysimeter and field experiments. Theory, Materials, and Methods ChemicalsPyriminobac-methyl herbicide consists of a mixture of its (E)-isomer (I) and (Z)-isomer (II) as active ingredients, with an (E)/(Z) ratio of ca. 5/1 in a technical grade of the active ingredient. The photo-transformation of this compound is the Improved PADDY model including photoisomerization and metabolic pathways for predicting pesticide behavior in paddy fields:Application to the herbicide pyriminobac-methyl Kumiai Chemical Ind. Co., Ltd., Tamari, Kakegawa, Shizuoka 436-0011, Japan (Received May 16, 2009; Accepted September 17, 2009) An improved simulation model for predicting pesticide behavior in paddy fields based on PADDY was developed to include photoisomerization and metabolic pathways and applied to the herbicide pyriminobac-methyl. Isomerization of pyriminobac-methyl in paddy water by sunlight was modeled as a reversible first-order reaction between the (E)-and (Z)-isomers in response to UV-B irradiation. The formation and degradation of its main metabolites in flooded so...

Fenquinotrione is a novel herbicide that can control a wide range of broadleaf and sedge weeds with excellent rice selectivity. We revealed that fenquinotrione potently inhibited the 4-hydroxyphenylpyruvate dioxygenase (HPPD) activity in Arabidopsis thaliana with an IC 50 of 44.7 nM. The docking study suggested that the 1,3-diketone moiety of fenquinotrione formed a bidentate interaction with Fe(II) at the active site. Furthermore, π-π stacking interactions occurred between the oxoquinoxaline ring and the conserved Phe409 and Phe452 rings, indicating that fenquinotrione competes with the substrate, similar to existing HPPD inhibitors. A more than 16-fold difference in the herbicidal activity of fenquinotrione in rice and the sedge, Schoenoplectus juncoides, was observed. However, fenquinotrione showed high inhibitory activity against rice HPPD. Comparative metabolism study suggested that the potent demethylating metabolism followed by glucose conjugation in rice was responsible for the selectivity of fenquinotrione.

The action mechanism of thiobencarb was studied by examining the inhibitory effects of this herbicide on the biosynthesis of very-long-chain fatty acids (VLCFAs). Thiobencarb treatment decreased VLCFAs, such as C20:0, C20:1, C22:0, C24:0, C24:1 and C26:0 fatty acids, and increased long-chain-fatty acids and medium-chain-fatty acids, such as C14:0, C15:0, C18:0 and C18:1 fatty acids, which are precursors of VLCFAs, in barnyard millet cultured cells. Thiobencarb sulfoxide and sulfone potently inhibited VLCFA elongase (VLCFAE) activity in the microsomal fraction of etiolated barnyard millet seedlings, although thiobencarb itself slightly inhibited it. These results suggested that thiobencarb is a VLCFAE-inhibiting herbicide whose active forms are its oxidized metabolites, such as sulfoxide and sulfone. Thiobencarb sulfoxide inhibited the VLCFAE activity of the microsomal fraction of etiolated barnyard millet seedlings in a time-independent manner. This time-independent inhibition proposed a reversible inhibition mechanism of the VLCFAE by thiobencarb sulfoxide, likely with isoxazoline-type herbicides, such as pyroxasulfone, which have been classified into group K3 of the Herbicide Resistance Action Committee (HRAC) in the U.S. It is assumed that the time-independent reversible inhibition of VLCFAE is applicable to other thiocarbamate herbicides presently classified into group N of the HRAC. © Pesticide Science Society of Japan Keywords: thiobencarb, thiobencarb sulfoxide, very-long-chain fatty acid (VLCFA), very-long-chain fatty acid elongase (VLCFAE), time-independent manner, N group. IntroductionThiobencarb, S-(4-chlorobenzyl)-N,N-diethylthiocarbamate, is a herbicide developed by Kumiai Chemical Industry for crops such as rice, wheat, corn and soybean (Fig. 1A).1) This herbicide at the application rate of 1.5-7.5 kg a.i./ha provides good efficacy on both grass and broad leaf weeds, especially needle spikerush and barnyard grass.2) Barnyard grass from pre-emergence up to the three-leaf stage is effectively controlled by this herbicide. This herbicide shows excellent selectivity for rice plants.Differences in the physiological activity between rice and barnyard grass for absorbing and metabolizing this herbicide have been reported. 3-5)Thiobencarb does not inhibit the germination of seeds but potently inhibits the shoot elongation of germinated seeds. This herbicide belongs to the thiocarbamate herbicides and is classified into group N of the Herbicide Resistance Action Committee (HRAC) in the U.S.6) According to the HRAC, the target site of the group N herbicides is described as the lipid synthesis except for acetyl-CoA carboxylase (ACCase), indicating that their mode of action has not been completely elucidated. 7,8) However, it has been reported that sulfoxide of pebulate, which is also a thiocarbamate herbicide, inhibits the incorporation of 14 Clabeled acetic acid into very-long-chain fatty acids (VLCFAs) in some plants. 9,10) Therefore, the thiocarbamate herbicides were presumed to inhibit the biosynthesis of...

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