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Acetyl-coenzyme A (CoA) carboxylase from maize (Zea mays L.) is inhibited by nanomolar concentradons of both haloxyfop, an aryloxyphenoxypropionate, ad tralkoxydim, a cyclohexanedione herbicide.These results suggest that acetyl-CoA carboxylase, which catalyzes the first committed step in fatty acid biosynthesis, may be the target of these herbicides, contrary to an earlier report su ng that aryloxyphenoxypropionate herbicides do not inhibit acetyl-CoA carboxylase.Aryloxyphenoxypropionates and cyclohexanediones are two classes of herbicides that control many monocotyledoneous species. Although these herbicides are structurally very different, there has been some conjecture based upon similarities in selectivity and symptomology that these herbicides have a similar mode of action. There have been independent reports that these herbicides disrupt lipid metabolism (2, 6-1 1).Research by Hoppe (7-9) and Hoppe and Zacher (10, 11) has indicated that the aryloxyphenoxypropionate diclofop-methyl (2- [

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The onset of senescence, defined as the time that photosynthesis began to decline irreversibly, was compared with ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) (RuBPCase) activity, chlorophyll content, protein content, and leaf diffusive resistance in outdoor-grown soybean (Glycine max (L.) Merr. cv. Amsoy 71) leaves that had emerged and senesced during different stages of plant ontogeny. The purpose was to compare metabolic events in soybean leaves associated with progressive and monocarpic senescence. Soluble protein, chlorophyll, and RuBPCase activity declined coincidently with photosynthesis in all leaves. In leaves that emerged before flowering, the decline in RuBPCase activity could be accounted for by protein degradation, whereas in a leaf that emerged after flowering had begun, the decline also was attributable to, but to a much lesser extent, a change in specific activity. All parameters declined more rapidly in leaves at nodes higher on the plant. There was an indication that developing pods temporarily retarded the rate of decline in photosynthesis. Regression of photosynthesis on chlorophyll, protein, and RuBPCase activity yielded coefficients that were different in a leaf that developed during the vegetative period from those of leaves that developed later, whereas no differences were detected among the later leaves. We conclude from our results that the factors associated with senescence are similar among leaves that senesce during different stages of plant development. But there is evidence that the rate of senescence may be differentially regulated.

Delaying the onset or reducing the rate of leaf senescence may be one means of increasing crop productivity. But before senescence can be controlled, it must first be characterized. We compared the onset of leaf senescence, defined as the time at which apparent photosynthesis began to decline irreversibly, with ribulose‐l,5‐bisphosphate carboxylase (RuBPCase) activity, chlorophyll content, total protein content, leaf diffusive resistance, and dark respiration in outdoorgrown soybean [Glycine max (L.) Merr. ‘Amsoy 71’] leaves that had emerged during plant reproduction. We sought to determine which parameter(s) was associated with the onset of senescence. Apparent photosynthesis, as estimated by infrared gas analysis, began to decline about 85 days after planting in leaves at both nodes 12 and 15. There was no change in diffusive resistance, protein content, or dark respiration until about 3 weeks later. On the other hand, RuBPCase activity and chlorophyll content declined concomitantly with photosynthesis. This pattern was similar in both leaves. Thus, we conclude that RuBPCase activity and chlorophyll were best associated with the onset of senescence and that a similar mechanism seems to be operating in both leaves.