Herbicide interference with photosynthesis

A number of other herbicides interfere with photosynthesis in specific ways. Amitrole inhibits biosynthesis of chlorophyll and carotenoids. The affected plants present a bleached appearance before they die because of the loss of their characteristic pigments. Another herbicide, atrazine, inhibits the oxidation of water to hydrogen ion and oxygen. Still other herbicides interfere with electron transfer in the two photosystems. In photosystem II, diuron inhibits electron transfer to plastoquinone, whereas bigyridylium herbicides accept electrons by competing with the electron acceptors in photosystem I. The inhibitors active in photosystem I include diquat and paraquat. The latter substance attained some notoriety when it was used to interfere with an... 

Uses Bromacil is a colorless crystalline solid. It is used for the control of annual and perennial grasses, broadleaf weeds, and woody plants.1213 Bromacil is a herbicide used for bush weed control on non-cropland areas. It is especially useful against perennial grasses. It is also used for selective weed control in pineapple and citrus crops. It interferes with photosynthesis of plants. It is available in granular, liquid, water-soluble liquid, and wettable powder formulations. 

Since the discovery of the herbicidal properties of the aryloxy-phenoxypropanoates, there have been many studies aimed at determining their mode of action. Fewer reports have been published regarding the mode of action of cyclohexanediones. Neither class of compounds interferes with photosynthesis, respiratory 02 uptake, protein biosynthesis, or nucleic acid biosynthesis (4-6). Several physiological processes are disrupted by both the cyclohexanediones and aryloxyphenoxypropanoates, namely growth and development, maintenance of membrane integrity, auxin induced growth, and lipid metabolism (4.5.7.8). In addition, the aryloxyphenoxypropanoates have been reported to depolarize membrane potentials (9.10). However, no specific target site had been identified for either class of compounds. 

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. 

Selective herbicides act like hormones, very selective biochemicals that control a particular chemical change in a particular type of organism at a particular stage in its development. Most selective herbicides in use today are growth hormones they cause cells to swell, so that leaves become too thick for chemicals to be transported through them and roots become too thick to absorb needed water and nutrients. Nonselective herbicides usually interfere with photosynthesis and thereby starve the plant to death. On application, the plant quickly loses its green color, withers, and dies. 

Given their relatively high costs relative to the herbicides discussed so far (this is due to the high cost of pyridine), the rapid expansion in their use is remarkable. Clearly it relates to their novel applications. Their mode of action involves reduction of the cation to a free radical during interference with photosynthesis. Re-oxidation back to the original cation by molecular oxygen then takes place, accompanied by production of hydrogen peroxide which is extremely toxic to plant cells. 

Chlamydomonas species have also been used in in vitro studies of the phenylurea herbicides. Loeppky and Tweedy (1969) observed that metobromuron was toxic to Chlamydomonas reinhardi and C. eugametos complete inhibition of growth occurring at 1.5 and 5.5 ppm, respectively. In heterothrophic growth studies, the herbicide was found to be as toxic to C. reinhardi as in autotrophic growth studies. This indicates that metobromuron, and possibly other phenylurea herbicides, inhibits algal growth not only by interference with photosynthesis but also by other routes. 

There are a number of other herbicides that affect photosynthesis indirectly. Pyrazole herbicides such as benzofenap, pyrazolynate and pyrazoxyfen interfere with chlorophyll biosynthesis and have found commercial application for the control of annual and perennial weeds in paddy rice and maize (Figure 2.4). 

Glufosinate is a postemergent, nonselective, partially systemic contact herbicide which acts on leaves as well as in the plant after uptake and transport. Its action is delayed at lower temperatures. It causes inhibition of photosynthesis and interferes with amino add metabolism of the plant, thereby causing accumulation of NH4. The most important uses are in the control of seed and root weeds in vineyards, and in fruit growing. A relatively quick degradation with a half-life between 30 and 40 days occurs in the soil. 

Several of the older and more common herbicides have been shown to interfere with one or more steps of photosynthesis. A number of these are powerful inhibitors at or near photosystem II (PS II) ( 1). Beginning in 1981, J. N. Phillips and J. L. Huppatz and their group at CSIRO (Australia) have shown that a series of cyanoacrylate (vinylogous carbamate) herbicides are potent inhibitors of PS II (2-6). 

Pyridazine herbicides reduce or inhibit photosynthesis and carotenoid biosynthesis, - and they interfere with the formation of polar... 

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. 

The first photosynthesis-inhibiting herbicides such as arylurea (e.g., diuron) and triazines derivatives (e.g., atrazine) were identified in 1956 even before the photosynthetic reactions and two photosystems were known and before plastoquinone had been discovered. Surprisingly, this group of herbicides still dominates the field. The second group, which includes phenolic compounds such as bromoxynil and ioxynil, were recognized later. Although phenolic herbicides inhibit the PS II reaction centre differently from triazine herbicides, they also interfere with the Qg function and bind the DI protein. 

Cell cultures are ideal axenic physiological systems to study herbicide action without problems of cuticular transfer or complications of translocation. Still, not all metabolic systems function in all cells at all times in the cell cycle. Inhibitors of photosynthesis are often inactive in non green cells, and root-active herbicides may be degraded in green cells. Nutritional components in the medium may interfere with herbicide action. 

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). ... 

Fluridone is a herbicide that is a gamma-pyridone derivative. It is a carotenoid inhibitor this is a well-known type of activity that interferes with the photosynthesis process in such a way as to lead to production of the plant-lethal singlet oxygen. 

---