Herbicides that inhibit photosynthesis

The inhibitors of photosynthesis are all nitrogen-containing substances with various structures. They may be derivatives of urea, s-triazines, anilides, 

Inhibition of akonitase Fluoroacetic acid (fluorocitrate) Most animals 

Inhibiting cytochrome oxidase Cyanide Phosphine All aerobic organisms 

Inhibitors of ATP synthase in Organonotin Fungi, mites, aquatic 

Superoxide generators Copper ions Most organisms 

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How would you show that a herbicide that inhibited photosynthesis required light to show its effect and that plant death was not due to starvation ... 

Figure 8.1 Examples of herbicides and herbicide classes containing some compounds that inhibit photosynthesis with a mode of action like that of the triazines. Structures within the boxes are essential elements common to all classical inhibitors of photosynthesis II at the QB site. Some phenolic compounds also inhibit in a similar way.

Insofar as they have been studied, all herbicides that inhibit the Hill reaction of isolated chloroplasts also inhibit photosynthesis of intact plants and photosynthetic microorganisms (2, 3). Phy to toxicity is produced only in the light, and severity of symptoms is proportional to light intensity. Studies with light quality have indicated that the chlorophylls are the principal absorbing pigments involved in the production of phytotoxicity. 

The development of herbicides that inhibit photosynthetic electron transport has proved to be outstandingly successful. Furthermore, because of the efficiency of these compounds and their use as tools to study photosynthesis, our knowledge of photosystem II in particular has been greatly enhanced by their use. Recent developments involving X-ray structural analysis of photosynthetic bacterial reaction centers as well as the ability to engineer herbicide resistance into crop plants have been outstanding scientific achievements. 

Members of an extensive group of sym-triazine herbicides, usually having one or two secondary amine substituents, block the Hill reaction and inhibit photosynthesis in a manner quite similar to that of the urea herbicides. The most widely used, 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), (10), is one of several hundred herbicidal analogs... 

Leaf discs have commonly been used for bioassays to determine if herbicides inhibit photosynthesis (Table 16.2). The simplest leaf-disc bioassay uses small discs cut from fully expanded cucumber or pumpkin cotyledons, floated in the light on a phosphate buffered medium containing suspected photosynthesis inhibitors.115 Qualitatively, if photosynthesis is inhibited, the leaf disc sinks. There are several variations of this method that can provide quantitative data. Evolution of O2 in the test solution can be measured with an oxygen electrode, CO2 induced pH changes colorimetrically determined with bromothymol-blue, or electrolyte leakage monitored with a conductivity meter. Leaf strips, algae, isolated chloroplasts, and duckweed (Lemna minor) have been used as test subjects. Although the bioassays presented in Table 16.2 are fairly easy to perform, few allelochemicals have been tested as possible inhibitors of photosynthesis. Many... 

Plastoquinone is one of the most important components of the photosynthetic electron transport chain. It shuttles both electrons and protons across the photosynthetic membrane system of the thylakoid. In photosynthetic electron flow, plastoquinone is reduced at the acceptor side of photosystem II and reoxidized by the cytochrome bg/f-complex. Herbicides that interfere with photosynthesis have been shown to specifically and effectively block plastoquinone reduction. However, the mechanisms of action of these herbicides, i. e., how inhibition of plastoquinone reduction is brought about, has not been established. Recent developments haVe brought a substantial increase to our knowledge in this field and one objective of this article will be to summarize the recent progress. 

At this point, evidence that similar molecules acted as effective antidotes by inducing needed metabolic pathways for herbicide detoxication was at most very speculative. However, another hypothesis emerged. Could early herbicide pretreatments increase crop tolerance to these herbicides by elevating the substrates and enzymes needed for detoxication While not a new concept in animal systems, such an idea has received little attention in plant systems and it certainly has not been exploited in any practical way. The whole idea has seemed much more credible with the study by Jacetta and Radosevich (19) of photosynthetic recovery in corn after treatment with atrazine. More specifically, they showed that inhibition of photosynthesis was reduced and the rate of recovery enhanced in corn plants treated for the second or third time with atrazine compared to "first exposed" plants (Figure 2). Furthermore, the faster recovery was related to enhanced rates of atrazine metabolism in the previously treated plants (Table III). 

For cyclic electron flow, an electron from the reduced form of ferredoxin moves back to the electron transfer chain between Photosystems I and II via the Cyt bCyclic electron flow does not involve Photosystem II, so it can be caused by far-red light absorbed only by Photosystem I — a fact that is often exploited in experimental studies. In particular, when far-red light absorbed by Photosystem I is used, cyclic electron flow can occur but noncyclic does not, so no NADPH is formed and no O2 is evolved (cyclic electron flow can lead to the formation of ATP, as is indicated in Chapter 6, Section 6.3D). When light absorbed by Photosystem II is added to cells exposed to far-red illumination, both CO2 fixation and O2 evolution can proceed, and photosynthetic enhancement is achieved. Treatment of chloroplasts or plant cells with the 02-evolution inhibitor DCMU [3-(3,4-dichlorophenyl)-l, 1-dimethyl urea], which displaces QB from its binding site for electron transfer, also leads to only cyclic electron flow DCMU therefore has many applications in the laboratory and is also an effective herbicide because it markedly inhibits photosynthesis. Cyclic electron flow may be more common in stromal lamellae because they have predominantly Photosystem I activity. 

Herbicides inhibit photosynthesis by interrupting electron flow on the reducing side of the reaction center of photosystem 11. As originally proposed by Wraight and by Velthuy , based on several lines of evidence, that inhibition by a herbicide occurs in the Qe-binding site in D1 protein and that the action arises from the ability of the herbicide molecule to compete with Qb forthebinding site, thus resulting in the disruption of electron transfer from to Qb-... 

Almost all of the carbamate herbicides inhibit photosynthesis, as has been shown by the investigations of Moreland and Hill (1959). Asulam and terbutol do not inhibit photosynthetic electron transport in vitro, while the other carbamates do only in high concentrations not occurring in vivo. The conclusion of Corbett (1974), that the inhibition of photosynthesis is only a side-effect of these compounds, therefore seems justified. 

Similarly, meturin can also be considered as a precursor (Baskakov, 1973 Stonov et al., 1974).Though the herbicide has a greater effect in light than in the dark, it has no effect on photoreactions I and II in isolated spinach and pea chloroplasts. It is assumed that meturin is metabolised within the plant to a compound or compounds inhibiting photosynthesis. 

Metflurazon inhibits photosynthesis and prevents the development of chloro-plastids in sensitive plants (Hilton et al., 1969). The authors also report on their investigation of the mode of action of 4 pyridazinone herbicides on barley. Metflurazon and its phenyl- and unsubstituted amino analogues, structurally similar to pyrazon, also inhibited the Hill reaction and photosynthesis, but showed two further biological features they resisted metabolic oxidation and inhibited chloroplast formation. The latter effect is similar to that of amitrol and dichlormate, but 100-1000 times stronger. 

Broad-leaved roadside weeds killed by herbicides that selectively inhibit photosynthesis. Note the unaffected corn in the background. 

For example, glyphosate inhibits the enzyme, EPSP (5-enolpyruvylshikimate 3-phosphate) synthase, that catalyzes a step in the synthesis of the aromatic amino acids. Similarly, both the imidazolinones and sulfonylureas inhibit acetolactate synthase (ALS), the enzyme that catalyzes the first step in the formation of branched-chain amino acids (11). Triazine herbicides act by binding to a specific protein in the thylakoid membranes of the chloroplasts, preventing the flow of electrons and inhibiting photosynthesis (12). 

Section 19.5.5 in the text describes inhibitors of the light reaction that are used as herbicides. Inhibiting photosynthesis is a good way to produce compounds that can kill plants while keeping toxicity toward animals to a minimum. Can you think of another way to produce herbicides that would be relatively safe for animals ... 

A method of detecting herbicides is proposed the photosynthetic herbicides act by binding to Photosystem II (PS II), a multiunit chlorophyll-protein complex which plays a vital role in photosynthesis. The inhibition of PS II causes a reduced photoinduced production of hydrogen peroxide, which can be measured by a chemiluminescence reaction with luminol and the enzyme horseradish peroxidase (HRP). The sensing device proposed combines the production and detection of hydrogen peroxide in a single flow assay by combining all the individual steps in a compact, portable device that utilises micro-fluidic components. 

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