A series of 1,3,4(2H)-isoquinolinetriones have been found to be fastacting post-emergence herbicides, producing symptoms of desiccation. These redox-active compounds are very potent stimulators of the light-dependent consumption of oxygen at photosystem I in isolated chloroplasts. Pulse radiolysis studies on 2-ethyl-l,3,4(2H)-isoquinolinetrione have shown it to have free-radical properties which could enhance the generation of superoxide radicals in plants.Electrochemical studies further support a redox mediator mode of action for the series. The compounds were found to be unstable towards hydrolysis, and this was considered to be a major factor limiting the overall herbicidal effects. Other parameters, related to uptake and/or translocation, which may limit the full expression of the herbicidal activity of certain compounds, are discussed.which gives rise to the herbicidal activity of the isoquinolinetriones, and describe the results of a preliminary study designed to probe the structureactivity-property relationships within a series of compounds. EXPERIMENTAL METHODS Chemical synthesisThe three general methods used for the preparation of 1,3,4(2H)-isoquinolinetriones are outlined in Fig. 1.Method A involves the oxidation of a homophthalimide derivative to the corresponding isoquinolinetrione. This transformation has been conveniently carried out using a two-step one-pot procedure, in which the homophthalimide is first condensed with N,N-dimethyl-p-nitrosoaniline to give an intermediate imine which is then hydrolysed in situ by the addition of aqueous acid." Method B requires the 49 Pestic. Sci. 0031-613X/95/$09.00 0 1995 SCI. Printed in Great Britain 1,3,4(2H)-isoquinolinetrione herbicides~~~~~~ ~ ~ Average score across weed species, on a scale of 0-9 where 0 indicates no damage five days after treatment and 9 signifies Estimated from a half life of 3.8 min at 40°C. Estimated. Estimated from a measurement made at pH 6. Estimated from a measurement made at pH 5. complete kill (see Section 2.2).' Not measured; this compound could not be obtained >90% pure.

Fluazifop is a grass-selective herbicide that appears to act by inhibiting fatty acid synthesis de novo in sensitive species. Results from four different types of experiment show that this inhibition is due to an action of fluazifop on acetyl-CoA carboxylase and not on fatty acid synthetase. The acetyl-CoA carboxylase from sensitive barley (Hordeum vulgare), but not from resistant pea (Pisum sativum), is inhibited by the R stereoisomer, a finding that agrees with the herbicidal specificity of fluazifop.

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Soluble protein extracts and chloroplasts from a serial sequence of transverse sections of a 7-d-old wheat leaf (Triticum aestivum cv. Maris Huntsman) were used to study changes in the activity of glutamine synthetase (GS; EC 6.3.1.2) during cell and chloroplast development. Glutamine synthetase activity increased more than 50-fold per cell from the base to the tip of the wheat leaf. Two isoenzymes of GS were separated using fast protein liquid chromatography (FPLC). Glutamine synthetase localized in the cytoplasm (GS1) eluted at about 0.21 M NaCl, and the isoenzyme localized in the chloroplast (GS2) eluted at about 0.33 M NaCl. The increase in GS activity during leaf development was found to be caused primarily by an increase in the activity of the chloroplast GS2. The activity of the cytoplasmic GS1 remained constant as the cells were displaced from the base to the tip of the leaf, whereas GS2 activity increased within the chloroplast throughout development. At the base of the leaf, 26% of total GS activity was cytoplasmic; the remaining 74% was in the chloroplast. At 10 cm from the base, only 4% of the activity was cytoplasmic, and 96% was in the chloroplast. The results indicate that the chloroplast GS2 is probably responsible for most of the ammonia assimilation in the mature wheat leaf, whereas cytoplasmic GS1 may serve a role in immature developing leaf cells.

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