Herbicide photosynthetic inhibitor

Sugarcane Sower initiation is dependent on day length, temperature, age, moisture, and variety, and can be prevented by chemical applications at, or very close to, the date of floral initiation. The effective chemicals have been of two types (a) photosynthetic inhibitors, such as 3-(p-chlorophenyl)-1,1-dimethylurea (monuron), or (b) leaf-burning, contact chemicals. The very effective bipyridylium herbicides 6,7-dihydrodipyrido[l,2-a 2, r-c]pyrazidinium dibromide (diquat) and 1, l -dimethyl-4,4 -bipyridinium bis (methyl sulfate) (paraquat) combine the two properties, although the... 

Many commercially available herbicides have been demonstrated to interfere with one or more steps of photosynthesis, by reacting near the photosystem II (PS II) center [for a recent review see ( 1), among others]. DCMU ( ) and other chemical families of photosynthetic inhibitors ( 3, ) were shown to shift the potential of the PS II secondary electron acceptor B, a specialized plastoquinone molecule, bound to a protein ( 5). ... 

The Role of Light and Oxygen in the Action of Photosynthetic Inhibitor Herbicides... 

James Franck wrote in 1949 U) ... it is one of the miracles of photosynthesis that the plant can use a dye able to fluoresce in the presence of oxygen, predominantly for the purpose of reduction, and is able to hold the process of photooxidation in check so that damage is prevented or minimized even under severe conditions . It is evident that the chloroplast is endowed with protective devices that are able to limit damage except under extreme conditions. Such situations are promoted by the presence of photosynthetic inhibitor herbicides. 

Promotion of the toxic action of photosynthetic inhibitor herbicides by light may be initially ascribed to type 1 reactions. Triplet chlorophyll, without the involvement of oxygen, may directly initiate electron or hydrogen abstraction from particularly susceptible molecules (e.g., unsaturated fatty acids (LH)) to yield lipid free radicals. 

Many experiments into the secondary effects of photosynthetic inhibitor herbicides have been performed with isolated chloroplasts, Isolated chloroplasts provide a convenient system for studying the generation and quenching of singlet oxygen. 

Figure 13. Summary scheme of the primary effects caused by A, photosynthetic inhibitor herbicides and B, photosynthetic deviator herbicides. 0% is triplet or ground state oxygen PQ represents paraquat.

Methabenzthiazuron is a preemergence herbicide, urea derivative and photosynthetic inhibitor that inhibits photosynthetic electron transport at the diuron site . After treatment with this herbicide, the wheat crop showed physiological effects , such as a delayed senescence of leaves, a greening effect and a decrease in carbohydrate content. 

Still working with Chlorella NMI and the herbicides atrazine, mon-uron, diuron, and neburon, Tchan et al. (1975) devised an assay procedure for the measurement of herbicides in soil and water. This technique was based on the eflEect that the photosynthetic inhibitors have on oxygen evolution during photosynthesis. Evolved oxygen was estimated by its influence on light generation by the bioluminescence system of a photobacterium. 

Sorgoleone was initially found to inhibit mitochondrial respiration, but it was later found to be a more potent inhibitor of photosyndietic electron transport of photosystem II (PSII) IS, 16). Sorgoleone is structurally similar to plastoquinone (PQ), a benzoquinone involved in photosyndietic electron transport. Sorgoleone competes for the PQ binding site of die D-1 protein in a manner similar to most commercial photosynthetic inhibitors IT). The in vitro PSII inhibiting activity of sorgoleone is similar to some of the commercial herbicides targeting this site e.g., atrazine and diuron). 

Research had confirmed that no parent simazine residues were found in treated com plants, and additional data on the dissipation pathway of simazine needed to be developed. Research also indicated that triazines interfered with the photosynthetic process on susceptible growing weeds, as evidenced by the appearance of chlorotic leaves. Steps were undertaken to elucidate simazine s dissipation pathway and herbicidal mode of action. In Basel, Dr. Gast (1958) showed that the accumulation of starch by common coleus (Coleus blumei Benth.) plants was inhibited from treatment with 2-chloro-4,6-bis-(alkyl-amino)-triazines due to the inhibition of sugar synthesis. At the same time, Moreland et al. (1958) found weed control activity could be reduced by supplying carbohydrates to the plants through their leaves and that simazine was a strong inhibitor of the Hill reaction in photosynthesis. Exer (1958) found that triazines inhibited the Hill reaction as strongly as urea of the CMU (monuron) type. 

The triazine herbicides currently used are mostly 4,6-alkylarmno-v-triazine compounds with either a 2-chloro, 2-methylthio, or 2-methoxy substituent (Table 23.1). The /V-alkyl groups may be methyl, ethyl, 1-methylethyl (isopropyl), 1,1-dimethylethyl (tertiary-butyl), 1,2-dimethylpropyl, or 2-methylpropanenitrile. Absorbed by roots or leaves of plants, these herbicides are applied either preemergence or postemergence to control annual broadleaf weeds and annual grasses in a wide variety of crops. The triazine herbicides listed in Table 23.1 have the same mechanism of action in plants, as all are photosynthetic electron transport inhibitors. 

An elegant example of this is the monitoring of herbicide residues via the photosynthetic electron transport (PET) pathway by utilising cyanobacteria or thylakoid membranes (5). For many herbicides the mode of action is as inhibitors of PET, often acting between the 2 photosystems as indicated in figure 3, and the result is a decrease in the photocurrent. 

Inhibitors of carotenoid synthesis also lead to chlorophyll destruction by destabilizing the photosynthetic apparatus. Total carotenoid content decreased with increased (-)-usnic concentration (Fig. 1.4). Carotenoid biosynthesis can be interrupted by inhibiting the enzyme phytoene desaturase that converts phytoene to carotenes or by inhibiting the enzyme HPPD responsible for plastoquinone (required for phytoene desaturase activity) synthesis.14 Usnic acid possesses some of the structural features of the triketone HPPD inhibitors, such as sulcotrione (Fig. 1.1C).8 (-)-Usnic acid had a strong inhibitory activity on HPPD, with an apparent IC50 of 70 nM, surpassing the activity obtained with the commercial herbicide sulcotrione (Fig. 1.5). 

These suggest that the proliferation of the derived cell line of photoautotrophic pak-bung hairy roots is dependent entirely on photosynthesis for acquiring carbon and energy sources because DCMU blocks the reaction in photosynthetic process as shown in Fig. 6. In addition,based on the sensitive response to DCMU, a kind of photosynthesis inhibitor, the photoautotrophic hairy roots maybe used to detect some herbicides existing in the surroundings. 

Tischer and Strotmann ( 7), the binding constant corresponds to the inhibition constant, i. e. the I,. value (the concentration necessary for 50% inhibition of photosynthetic electron transport), provided the I. value is extrapolated to zero chlorophyll concentration. The value of 527 molecules of chlorophyll per molecule of bound inhibitor indicates that roughly one molecule of herbicide binds per electron transport chain, because about 400-600 molecules of chlorophyll are considered to be associated with each electron transport chain. 

Thus, the concentration necessary for 50% displacement roughly corresponds to the PIjq value. It is possible, therefore, to assay the pl value of a new compound just by examination of its displacement behaviour. It is no longer necessary to determine the pi value by testing the inhibition of a light-driven photoreduction. Another very potent inhibitor of photosynthetic electron transport, DBMIB (2,5-dibromo-3-methyl-6-isopropyl-l,4-benzoquinone) (13), almost completely fails to displace metribuzin from the membrane (Figure 3). This is due to the fact that DBMIB has a completely different site of action as compared to the photosystem II herbicides, i. e. it inhibits plastohydroquinone oxidation by acting at the cytochrome b /f-complex (13). 6... 

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 have several different mechanisms of action. Carotenoid biosynthesis inhibitors (Norflurazon, Fluridone, Flurochoridone, Diflufenican) block the formation of antioxidants which protect the photosynthetic apparatus in plants [3]. Plants treated with this class of substance are bleached and become unable to photo-synthesize [4]. 

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