Richard H. Shimabukuro

Physiological effects of methyl 2-[4(2,4-dichlorophenoxy)phenoxy] propanoate on oat, wild oat, and wheat

Abstract Methyl 2-[4-(2,4-dichlorophenoxy)phenoxy]propanoate (dichlofop-methyl) is a selective herbicide for wild oat (Avena fatua L.) control in wheat (Triticum aestivum L.). Dichlofop-methyl inhibited IAA-stimulated elongation of oat and wheat coleoptile segments by 51 and 13%, respectively, at 10 μM concentrations. Dichlofop-methyl alone had no auxin activity at concentrations of 0.1, 1.0, and 10 μM. The inhibitory effect of dichlofop-methyl was overcome partially by increasing the IAA concentration or by application of 3,6-dichloro-o-anisic acid (dicamba), a herbicide with weak auxin activity. The de-esterified free acid metabolite, 2-[4-(2,4-dichlorophenoxy)phenoxy]-propionic acid (dichlofop), at 10 μM inhibited auxin-stimulated oat coleoptile elongation by 23%, but it did not affect wheat coleoptile elongation at the same concentration. Both dichlofop-methyl and dichlofop inhibited root growth in excised shoots and seedlings of wild oat but had no effect on wheat. Dichlofop was a more effective inhibitor of root growth than dichlofop-methyl. The results suggest that dichlofop-methyl functions as a strong auxin antagonist, while the metabolite, dichlofop, inhibits root growth and development by another mechanism. The herbicidal effect of dichlofop-methyl may be the net effect of two biologically active forms of the compound each with a different mode of action acting at different sites within a susceptible plant.

GLUTATHIONE AND GLUCOSIDE CONJUGATION IN HERBICIDE SELECTIVITY

The conjugation of herbicides with glutathione or glucose are frequently species specific reactions that result in herbicide detoxification. Therefore, glutathione and glucoside conjugation play an important role in herbicide detoxification and selectivity. Phase 1 activation reactions are sometimes necessary before glutathione or glucoside conjugation can occur. In these cases, the Phase 1 reaction rather than conjugation may be responsible for herbicide selectivity. This appears to be particularly true in the metabolism of herbicides to 0-glucoside conjugates. The glutathione-S-transferases and glucosyltransferases that catalyze these conjugation reactions, the role that these reactions play in the selectivity of specific herbicides, factors that affect these reactions, and the secondary metabolism of glutathione and glucoside conjugates are reviewed.

Metabolism of diclofop-methyl in susceptible and resistant biotypes of Lolium rigidum

Abstract Differential metabolism and detoxication of the postemergence graminicide diclofop-methyl (DM) [methyl 2-(4-(2′,4′-dichlorophenoxy)phenoxy)propanoate] is not the basis for selectivity between the DM resistant (R) and susceptible (S) biotypes of Lolium rigidum. Metabolism of DM did not differ significantly between the two biotypes. DM metabolism in Lolium sp. resembled that of susceptible oat and wild oat (Avena sp.) and not resistant wheat (Triticum aestivum). Metabolism of DM was assayed in excised shoots, roots and leaves (leaf surface-applied DM) of R and S Lolium biotypes. Higher concentrations of phytotoxic diclofop (acid) remained in shoots and roots of R and S Lolium biotypes (42 to 47% of absorbed DM after 72 hr in roots), than was found in susceptible oat. Conjugates of both diclofop (ester conjugate) and aryl hydroxylated diclofop (phenolic conjugate) were formed in Lolium sp. Aryl hydroxylation may be slightly higher in roots than in shoots of both biotypes but hydroxylation was less than in resistan wheat. The mode of action of DM in the S biotype of Lolium is probably similar to that in susceptible Avena sp. since the phytotoxicity of DM in both species is reversed by 2,4-dichlorophenoxyacetic acid.

Metabolism of substituted diphenylether herbicides in plants. II. Identification of a new fluorodifen metabolite, S-(2-nitro-4-trifluoromethylphenyl)-glutathione in peanut

Abstract The herbicide, 2,4′-dinitro-4-trifluoromethyl diphenylether (fluorodifen), is eleaved in peanut to give the metabolite, S -(2-nitro-4-trifluoromethylphenyl)-glutathione. A comparison of the glutathione conjugate isolated from treated peanut leaves and from in vitro pea epicotyl glutathione S -transferase reaction showed that both metabolites were identical. Other polar metabolites were also isolated, but not identified. The structure of the glutathione conjugate was confirmed by amino acid analysis and by mass, NMR, and infrared spectroscopy. The p -nitrophenyl moiety is also conjugated to natural products and is released as the free p -nitrophenol upon acid hydrolysis.

Pesticide Metabolism in Plants Reactions and Mechanisms

The use of pesticides to protect crop plants from weeds, insects, and other pests has increased steadily. In 1975 the total production of organic pesticides in the United States was 1609 million pounds (USDA, 1977). A great variety of pesticides, including herbicides, insecticides, and fungicides is applied to crop plants. Pesticides are inherently toxic and may be degraded to either toxic or nontoxic forms. Therefore, it is important to know the intermediate degradation products and the ultimate fate of pesticides in plants.

Response of wheat and oat seedlings to root-applied diclofop-methyl and 2,4-dichlorophenoxyacetic acid

Abstract Roots of wheat and oat seedlings were treated with diclofop-methyl (methyl 2-[4-(2′,4′-dichlorophenoxy)phenoxy]propanoate) in a specially designed Plexiglas treatment apparatus. Diclofopmethyl severely inhibited the root growth of susceptible oat seedlings but roots of resistant wheat seedlings were unaffected. Diclofop-methyl at 0.3 μ M reduced the growth of oat roots to 50% of the control. Direct contact between diclofop-methyl and the inhibited root zone was necessary for growth inhibition since other parts of the seedling (roots and shoots) isolated from contact with diclofop-methyl solution by a physical barrier were unaffected. Diclofop (2-[4-(2′,4′-dichlorophenoxy)phenoxy]propionic acid), the free acid metabolite of diclofop-methyl, was somewhat more phytotoxic than the parent compound. The herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), which engenders auxin responses, slightly enhanced the inhibition of oat root growth by diclofop-methyl. The primary wheat metabolite, ring-hydroxylated diclofop, was nonphytotoxic to oat root growth, whereas the acetylated derivative of the primary water-soluble oat metabolite (neutral glucose ester of diclofop) inhibited oat root growth to the same extent as diclofop-methyl. These results support the hypothesis that the basis for selectivity between resistant wheat and susceptible oat is the metabolism of diclofop-methyl by aryl hydroxylation and conjugation but not glucose ester conjugation. Translocation is also not an important factor in the phytotoxic activity of diclofop-methyl.

GLUTATHIONE CONJUGATION: A MECHANISM FOR HERBICIDE DETOXICATION AND SELECTIVITY IN PLANTS

Glutathione conjugation of herbicides appears to be a common reaction in plants and mammals. However, the terminal product in plants is not the mercapturic acid formed in mammals. In plants, glutathione conjugation is a detoxication mechanism which effectively reduces the active internal concentration of the herbicide and prevents irreversible injury.

N-Dealkylation of atrazine and simazine in Senecio vulgaris biotypes: A major degradation pathway

Abstract Separate populations of Senecio vulgaris were found that evolved partial tolerance to s -triazine herbicides and others that were totally resistant (plastid resistance). In plants from the susceptible, tolerant, and resistant populations, about one half of applied [ 14 C]atrazine (2-chloro-4-ethylamino-6-isopropylamino- s -triazine) was rapidly N -dealkylated to the des-ethyl and des-isopropyl products. These products were relatively inactive in inhibiting photosystem II and did not compete with atrazine. After 6 days, less than 25% of the applied [ 14 C]atrazine was metabolized to water-soluble degradation products but they were not 2-hydroxy derivatives. Less than 2% of the applied atrazine was incorporated into methanol-insoluble residues. The results on metabolism do not explain the tolerance of Senecio to atrazine. However, our results show that N -dealkylation of the s -triazines is more active than previously reported.

The role of coleoptile on barban sensitivity between wild oat and wheat

Abstract Differential sensitivity of wild oat ( Avena fatua L.) and wheat ( Triticum aestivum L.) to barban (4-chloro-2-butynyl- m -chlorocarbanilate) was highest when barban was applied to the coleoptile. The coleoptile acts as a physical and physiological barrier to reduce the concentration of free barban in the stem section where the sensitive meristematic sites are located. Metabolism of barban was higher in the coleoptile of tolerant wheat than in that of susceptible wild oat. Free barban concentration was higher in the stem of wild oat than in the stem of wheat after 24 hr, but after 48 hr, concentrations were similar. The coleoptile appears to enhance the differential sensitivity to barban between wild oat and wheat.

The absorption, translocation, and metabolism of difenopenten-ethyl in soybean and wheat

Abstract The postemergence graminicide, difenopenten-ethyl [ethyl 4-(4-(4′-trifluoromethylphenoxy)-phenoxy)-2-pentenoate], was rapidly hydrolyzed to its acid, difenopenten, in both susceptible wheat and resistant soybean followed by hydroxylation of the pentenoic acid side chain to a 3-hydroxy metabolite and subsequent conjugation to water-soluble polar metabolite(s). Metabolism of the herbicide did not differ greatly between the two species. A minor decarboxylation product of the 3-hydroxy metabolite, isolated from soybean cell suspension culture, was identified as a substituted 2-butanone metabolite. Difenopenten-ethyl is rapidly absorbed by both species through the roots and leaf surfaces but apoplastic and symplastic translocation is very limited. Differential absorption, translocation, and metabolism may not be major factors in the selective action of difenopenten-ethyl.