Herbicidal Activity Assay

Herbicidal activity assessment is the first and most important step in developing novel herbicides. Test procedures and conditions must meet testing requirements of chemical screening. A series of test procedures has been developed for testing the biological activity of new compounds. Data from these tests will help us in assessing the herbicidal ability of new chemicals. 

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Further Herbicide Activity Assay of Some l-(Substituted Phenoxyacetoxy)Alkylphosphonates... 

In order to confirm the pre-emergence herbicidal activity of 1-(substituted phenoxyacetoxy)alkylphosphonates, seven compounds were chosen for further herbicide activity assay at a lower rate for pre-emergence herbicidal activity. As shown in Table 2.27, they displayed very poor herbicidal activity against tested plants for pre-emergence application except IA-4 and IC-2 with 48-53 % inhibitory potency against dicotyledonous plants at 450 g ai/ha. All tested compounds had no pre-emergence herbicidal activity against tested plants at 150 or 75 g ai/ha, irrespective of difference in structure. 

In 1989 Cutler et al.1 reported the isolation of 3,7-dimethyl-8-hydroxy-6-methoxyisochro-man (1) from Penicillium corylophilum and demonstrated that it inhibited etiolated wheat coleoptiles at 10-3 and 1(T4M, as did the acetoxy (2) and methoxy (3) derivatives (Figure 5.1).2 The Parent compound had originally been isolated from moldy millet hay implicated in the death of cattle,3 but the metabolite had not been tested in plants. Because of the encouraging results obtained in the wheat coleoptile bioassay, isochromans 1,2, and 3 were assayed on greenhouse-grown bean, com, and tobacco plants. The methyl ether exhibited the greatest herbicidal activity in all the plants treated, while the parent and its acetoxy derivative were active only on corn. 

Aminoacid-tRNA synthetases were isolated from spinach chloroplasts by methods developed at Jealott s Hill. Cytoplasmic aaTRS s from carrot were isolated by standard procedures. Compounds were assayed as inhibitors of the chloroplastic aaTRS s at a single rate, 10 pM. The reaction was followed by phosphate generation and the percentage inhibition determined using literature methods. The IC50 value for the spinach chloroplastic enzymes and inhibition data for carrot cytoplasmic aaTRS s were determined for compounds of interest. All compounds were tested for post-emergence herbicidal activity, at 0.125, or at 0.5, or at 2.0 k a in standard screens." ... 

Inhibits ALS activity by 50%. A good correlation exists between the herbicidal activity of sulfonylureas and their ability to inhibit acetolactate synthase (2). This in vitro assay using the target enzyme along with the three-dimensioFTI structure of the enzyme should aid in the generation of a substantial data base that can be used to design potent inhibitors. 

Herbicide Binding Assays. Control and trypsin-treated chloroplast thylakoids were suspended in PSNM buffer. Buffer (1 ml volume) containing 50 Chi was incubated 3 min with l C-atrazine (specific activity 27.2 pCi/mg). Chloroplasts were pelleted and an aliquot of the supernatant was removed for determination of the amount of unbound atrazine. Details of this procedure are described elsewhere (6, 9). Radiolabeled atrazine was a gift of Dr. H. LeBaron, CTBA-GEIGY, N. Carolina. 

The action mechanism of herbicidal activity of the alkylphosphonates lo was explored on the basis of biochemical experiments. Representative alkylphosphonates lo were selected to examine their inhibition against plant PDHc in vitro including enzyme activity assays, kinetic experiments, and enzyme selective inhibition. Correlation between the inhibitory potency against plant PDHc and the herbicidal activity of the alkylphosphonates lo was summarized on the basis of bioassay. 

As described in Chaps. 2-6, most of l-(substimted phenoxyacetoxy)alkyl-phosphonates with 2,4-Cl2 as on the phenoxy-benzene ring exhibited much better herbicidal activity against dicotyledons than monocotyledons. Therefore, some alkylphosphonates and the corresponding sodium salts with 2,4-Cl2 as were chosen to examine their inhibition against PDHc from both dicotyledons and monocotyledons plants in vitro. Mung bean (V. radiata) and pea (Pisum sativum) as representative dicotyledonous plants, rice Oryza sativa) as a representative monocotyledonous plant were used in enzyme assays. The IC50 values of the tested compounds against plant PDHc are listed in Table 7.3. 

Sevin. 1-Naphthalenol methylcarbanate [63-25-2] (Sevin) (44) was developed as an insecticide. However, the conception of the molecule, in the mid-1950s, was as a possible herbicide. The compound ultimately was useless as a herbicide, but in routine testing it was discovered to be an excellent insecticide. Sevin was active in the oat mesocotyl assay and demonstrated weak auxin-like activity. During the development of Sevin, it caused massive apple drop in the western United States in an orchard being treated for insects. It is used (ca 1993) as an abscising agent to thin apples. 

R.N. Schlop, M.H. Hardy, and M.T. Goldberg, Comparison of the activity of topically applied pesticides and the herbicide 2,4-D in two short-term in vivo assays of genotoxicity in the mouse. Fundam. Appl. Toxicol. 15 666-675, 1990. 

Duke, S. O. and Kenyon, W. H. 1993. Peroxidizing activity determined by cellular leakage. In Boger, P. and Sandmann, G. (Eds.), Target Assays for Modern Herbicides and Related Phytotoxic Compounds, Lewis Publishers, Boca Raton, FL, 61-66... 

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