Hill reaction in photosynthesis

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 diphenyl ether herbicides are active only in the presence of light and canse chlorosis of leaf tissue. They inhibit the Hill reaction in photosynthesis and photophosphorylation. However, the primary mode of action probably involves the photosynthetic rednction to form radicals, which initiate destructive reactions in lipid membranes leading to cell leakage. 

Triazine class of herbicides are used to control broadleaf weeds and annual grasses. Atrazine and other related symmetrical tri-azines are used to control broadleaf and grassy weeds in tomatoes, potatoes, com, asparagus and other vegetables. The mode of action of these compounds involve inhibition of Hill reaction in photosynthesis. All triazine herbicides are structurally related. They all contain a triazine ring in their structures. The general structure of this class of herbicides is discussed in Chapter 44. 

The triazines, such as simazine (4.62), are the most used of all soil-applied herbicides small changes in the substituents give excellent control over selectivity and persistence. These substances [and the phenylureas, such as diuron (4.63), which are also soil-types] act by interfering with the Hill reaction in photosynthesis (see Section 4.6). Both of these parent substances, simazine and diuron, have undergone extensive modification for particular purposes, but each is still extensively used. A newcomer, chlorsulfuron (6.73), combines molecular features from both parents, but is notable for the greater dilution at which it acts, and also for the speed (all cell division is halted within 1 hour). This substance, l-(2-chlorophenylsulfonyl)-3-(4-methoxy-6-methyl-l, 3,5-tri-azin-2-yl)urea, is taken up by wheat, oats, and barley plants which rapidly inactivate it. It is harmless to Man (Campion and Tichon, 1981). 

Distinct differences in cells with regard to the presence or absence of target structures or metabolic processes also offer opportunities for selectivity. Herbicides such as phenylureas, simazine, and so on, block the Hill reaction in chloroplasts, thereby killing plants without harm to animals. This is not always the case because paraquat, which blocks photosynthetic reactions in plants, is a pulmonary toxicant in mammals, due apparently to analogous free-radical reactions (see Figure 18.4) involving enzymes different from those involved in photosynthesis. 

The triazine herbicides are selective inhibitors of the Hill reaction in plant photosynthesis. In mammals, atrazine disrupts luteinizing hormone and prolactin secretion through direct action on the hypothalamus-pituitary axis. 

Diuron is a selective inhibitor of the Hill reaction in plant photosynthesis. 

Benthiocarb inhibits the Hill reaction in isolated chloroplasts but does not inhibit photosynthesis either in intact rice or in intact barnyard grass (Ishii, 1975). 

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. 

The identities of the reducing and oxidizing moieties of water are not known. It is possible that the chlorophyll molecule participates chemically in the Hill reaction in addition to absorbing the radiant energy on which all photosynthesis depends. 

Atrazine enters plants primarily by way of the roots and secondarily by way of the foliage, passively translocated in the xylem with the transpiration stream, and accumulates in the apical meristems and leaves (Hull 1967 Forney 1980 Reed 1982 Wolf and Jackson 1982). The main phytotoxic effect is the inhibition of photosynthesis by blocking the electron transport during Hill reaction of photosystem II. This blockage leads to inhibitory effects on the synthesis of carbohydrate, a reduction in the carbon pool, and a buildup of carbon dioxide within the leaf, which subsequently causes closure of the stomates, thus inhibiting transpiration (Stevenson et al. 1982 Jachetta et al. 1986 Shabana 1987). 

Ozone causes both quantitative and qualitative changes in carbon dioxide fixation patterns. Wilkinson and Bames, using carbon dioxide-found a reduction in radioactivity in soluble sugars and increases in free amino acids and sugar phosphates in white pine after a 10-min exposure to ozone at 0.10 ppm. Miller observed a decrease in carbon dioxide-fixation in ponderosa pines that correlated with loss of chlorophyll, after exposure to ozone at 0.30-0.35 ppm. The Hill reaction rates of chloroplasts isolated from healthy and ozone-injured ponderosa pine indicated that both light and dark reactions of the chloroplasts from ozone-injured plants were depressed. Barnes found depressed photosynthesis and stimulated respiration in seedlings of four pine species of the southeastern United States after exposure to ozone at 0.15 ppm. 

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... 

The idea that light drives the formation of oxidants and reductants was first advanced by C. B. van Niel in the 1920s. It was strengthened through experiments done by Robin Hill in 1939. Hill discovered that isolated chloro-plasts evolved 02 if illuminated in the presence of an added electron acceptor such as ferricyanide, Fe(CN)63-. The electron acceptor became reduced in the process. Because no fixation of C02 occurred under these conditions, this experiment demonstrated that the photochemical reactions of photosynthesis can be separated from the reactions that involve C02 fixation. 

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. 

In 1937 Robin Hill demonstrated that isolated chloroplasts, when placed in an aqueous solution in the presence of a suitable electron acceptor, can evolve oxygen in the light, a process that has become known as the Hill reaction. Oxygen evolution proceeds in the absence of CO2, suggesting that CO2 fixation and O2 evolution are separate processes, contrary to the then prevailing belief. Using lsO-labeled H20 and lsO-labeled CO2 in different experiments, Laurens Ruben and Martin Kamen showed in 1941 that the evolved O2 comes from water and not from CO2. Subsequent studies have elucidated the steps intervening between O2 evolution and CO2 fixation in photosynthesis. 

Uracils and substituted uracils owe their herbicidal activity to inhibition of photosynthesis by blocking the Hill reaction. This activity also present in the ureas and triazines. 

There are at least four known actions of pyridazines on plants involving a one or more of the following effects (a) inhibition of the Hill reaction (photolysis of water during the photosynthesis), (b) inhibition of pigment formation (chlorophylls, carotenoids), (c) change in the linolenic/linoleic acid ratio, and (d) influence on the chloroplast ribosomes. In addition, pyridazines may be used in plant protection as biocides. 

IP 50) of Photosynthesis in Thylakoid Membranes From Pea Via the Hill Reaction... 

Like the other urea herbicides, cycluron inhibits photosynthesis. In vitro, it inhibits the Hill reaction at a-concentration of 5.6 mole/dm (Moreland, 1969). 

Photoreaction II, the photolysis of water accompanied by Oj formation (Hill reaction), can be catalysed by isolated chloroplasts in the presence of synthetic electron acceptors, such as ferricyanides or reducible dyes. It can easily be seen if a compound has an inhibiting effect on this step of photosynthesis through measurement of the oxygen produced. 

Hilton et al. (1969) found that the 50% Hill reaction inhibiting concentration of metflurazon is 4 mole/dm, a low value, but according to the investigations of Bartels and Hyde (1970) the primary cause of herbicidal action is in fact not the inhibition of photosynthesis, but the inhibition of carotenoid synthesis or of carotenoid accumulation (Bartels and Hyde, 1970). Practically, metflurazon is not toxic, its acute oral lDj being 9100 mg/kg for rats. 

Isocil and bromacil are soil-applied herbicides. Both block the Hill reaction and interfere with a step in the photosynthetic pathway close to oxygen evolution. This blocking may cause the accumulation of a phytotoxic product, possibly a reactive free radical. Though this particular antiphotosynthetic action is not in itself sufficient to explain the total phytotoxic action (Hoffmann, 1972), it is certain that the herbicidal action of substituted uracils is based on the inhibition of photosynthesis. 

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