Photosynthesis herbicides

The advances from the physiology to the biochemistry, biophysics, molecular biology, and molecular genetics kept this field of photosynthesis herbicides always at the top of scientific progress, and it remains a prime example of the complete and comprehensive clarification of the mode of action of a pesticide. 

Photosynthesis in all photosynthetic organisms is blocked by triazines, as well as by other PS II herbicides, when isolated thylakoid systems are tested. However, in intact plants, they express either different inhibitory potency or no inhibition. This shows that the specificity of these photosynthesis herbicides to certain weeds is not related to a difference in the chemistry of their primary target, but rather is attributed to degradative mechanisms, translocation, and translocation mechanisms. 

CgH,3BrN202. A soil-acting herbicide. White crystalline solid, m.p. 158-159" C. It is a non-selective inhibitor of photosynthesis used for weed control In citrus and cane fruit plantations. It is relatively non-toxic to animal life. 

Bleaching Herbicides. Membrane-based modes of herbicidal action relevant to photosynthesis (37) include those of inhibitors of carotenoid biosynthesis, eg, norflura2on, diftmon, y -phenoxyben2amines inhibitors of chlorophyll biosynthesis, eg, oxadia2on, DTP or... 

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. 

Mechanism of action can be an important factor determining selectivity. In the extreme case, one group of organisms has a site of action that is not present in another group. Thus, most of the insecticides that are neurotoxic have very little phytotoxicity indeed, some of them (e.g., the OPs dimethoate, disyston, and demeton-5 -methyl) are good systemic insecticides. Most herbicides that act upon photosynthesis (e.g., triaz-ines and substituted ureas) have very low toxicity to animals (Table 2.7). The resistance of certain strains of insects to insecticides is due to their possessing a mutant form of the site of action, which is insensitive to the pesticide. Examples include certain strains of housefly with knockdown resistance (mutant form of Na+ channel that is insensitive to DDT and pyrethroids) and strains of several species of insects that are resistant to OPs because they have mutant forms of acetylcholinesterase. These... 

Ureides (e.g., diuron, linuron) and triazines (e.g., atrazine, simazine, ametryne) all act as inhibitors of photosynthesis and are applied to soil (see Figure 14.1 for structures). They are toxic to seedling weeds, which they can absorb from the soil. Some of them (e.g., simazine) have very low water solubility and, consequently, are persistent and relatively immobile in soil (see Chapter 4, Section 4.3, which also mentions the question of depth selection when these soil-acting herbicides are used for selective weed control). 

These high levels were sporadic and transitory. However, some of them were high enough to have caused phytotoxicity, and more work needs to be done to establish whether herbicides are having adverse effects upon populations of aquatic plants in areas highlighted in this study. It should also be borne in mind that there may have been additive or synergistic effects caused by the combinations of herbicides found in these samples. For example, urea herbicides such as diuron and chlortoluron act upon photosynthesis by a common mechanism, so it seems likely that any effects upon aquatic plants will be additive. Similarly, simazine and atrazine share a common mechanism of action. 

El-Sheekh, M.M., H.M. Kotkat, and O.H.E. Hammouda. 1994. Effect of atrazine herbicide on growth, photosynthesis, protein synthesis, and fatty acid composition in the unicellular green alga Chlorella kessleri. Ecotoxicol. Environ. Safety 29 349-358. 

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). 

Animals acquire many of their nutrients in a ready made form from the food that they eat. Plants, however, have to biosynthesise everything that they need for efficient growth. The first section of this chapter discussed photosynthesis, a fundamental biosynthetic process, but plants also synthesise other components that animals do not. These biosynthetic processes are good examples of potentially plant selective herbicidal targets. Amino acids are the building blocks of proteins and as such their biosynthesis is one such process. 

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 ... 

The pyridazin-3-ones are interesting because they include herbicides having two different modes of action, distinguished only by small changes in substitution pattern. Thus pyrazon (8) (61GEP1105232) is a photosynthesis inhibitor, while other discussed later are carotenoid biosynthesis inhibitors. The pyridazin-3-one ring is constructed by condensation of phenyl-hydrazine with 3,4-dichloro-2,5-dihydro-5-hydroxyfuran-2-one (9), in turn produced by chlorination of furan-2-carbaldehyde. Amination of (10) then occurs exclusively at the 5-position to give pyrazon (Scheme 4). 

The most obvious effect of 3-amino-s-triazole (amitrole) (5) in plants is the lack of chloroplasts, resulting in a bleached appearance and diminished photosynthesis, although an effect on the synthesis of protein is probably involved as a more fundamental mechanism of herbicidal activity. 

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... 

Plant Agriculture transfer of stress-, herbicide-, or pest-resistance traits to crop species, development of plants with the increased abilities of photosynthesis or nitrogen fixation, development of biological insecticides and nonice nucleating bacterium. 

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. 

Yanase, D., and A. Andoh. 1992. Translocation of Photosynthesis-Inhibiting Herbicides in Wheat Leaves Measured by Phytofluorography, the Chlorophyll Fluorescence Imaging. Pestic. Biochem. Physiol. 44, 60-67. 

---