Respiration herbicide inhibition

The phenolic derivatives indicated in Figure 8.1 are also bound to the same binding niche on PS II as the triazines (Oettmeier, 1992). However, they have a somewhat different inhibition pattern than the classical family of PS II herbicides (e.g., triazines and ureas) and, therefore, were regarded as a separate family with a somewhat different mode of action (Van Rensen et al., 1978 Trebst and Draber, 1986). It is now clear that they just orient somewhat differently in the same binding niche, as discussed below. Although the phenolics are photosynthesis inhibitors, dinoseb and the halogenated benzonitriles also inhibit respiration. 

A large number of monoterpenes have been reported to be phytotoxic, and they have been proposed as potential starting structures for herbicides.44 Nevertheless, of the monoterpenes, we only know anything of significance about the mode of action of the cineoles. Early work showed that relataively high concentrations of 1,8-cineole (Fig. 10.1) inhibit mitochondrial respiration of isolated... 

Bipyridylium herbicides also produce changes in other biological processes of plants. It has been shown experimentally that they have an effect on the respiration of plants, photosynthetic oxygen production and the intake of carbon dioxide and that they inhibit transpiration (Mees, 1960 Funderburk and Lawrance, 1964 van Oorschot, 1966 Dodge, 1971). 

Synchronized cultures of Chlorella ellipsoidea were used as a model system to identify biochemical modes of action of two herbicides/ butamiphos (0-ethyl--0-(3-methyl-6-nitropheny1)-N-sec-buty1-phosphorothioamidate, formerly coded S-2846), and chlorpropham (isopropyl inch lorocarban il ate). Cell cycle studies showed that butamiphos inhibited cell division, whereas its effects on respiration and general biosynthesis were slight. Confirmatory experiments with onion root apices showed that mitosis was blocked at the metaphase and that the spindle apparatus was disrupted. 

If great persistence is required in a soil-acting herbicide, it should have, in addition to stability, a low water solubility and a low vapour pressure. The mode of herbicide action is in many cases unknown, but it is likely to involve the inhibition of metabolic processes such as photosynthesis, respiration, DNA or lipid synthesis. 

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). 

Although there are several mechanisms by which respiratory electron transport can be inhibited, the compounds in Figure 5.2 are classed as inhibitory uncouplers. " This activity is not restricted to mitochondrial electron transport as photosystem II (PSII) electron transport and phosphorylation are also affected by dinitrophenols and hydroxybenzonitriles (Refs. 3-5 and Chapter 1). In isolated mitochondrial preparations, lower concentrations of herbicides uncouple oxidative phosphorylation, that is, promote electron transport, without ATP synthesis, whereas higher concentrations inhibit electron transport. Fedtke calculated that uncoupling is 2-10 times more sensitive for a range of phenolic/acidic compounds." A similar difference has also been reported for dinoterb (Figure 5.2), which uncouples oxidative phosphorylation at 0.05-0.15 mM and inhibits respiration at 0.5-1.0mM. ... 

Nonspecific disruption of membrane permeability was proposed for difenopenten-ethyl ethyl 4-[(4-trifluoromethyl)phenoxyl-2-pentanoate, which uncoupled and inhibited respiration in corn and soybean mitochondria. This was considered to be a secondary effect of this aryloxy-phenoxypropionate analogue (see Chapter 3) due to the relatively high in vitro concentration. A similar conclusion was made for another aryl-oxyphenoxypropionate, diclofop, which uncoupled state 4 respiration, inhibited state 3 respiration, and caused swelling of isolated wheat and wild oat mitochondria. The inhibition of respiration was proposed to be due to an effect prior to the electron transport chain, that is, an effect on succinate or malate dehydrogenase. A potent inhibition of state 3 respiration was also reported for the herbicide UK J72J (2-ethylamino-4-amino-5-thiomethyl-6-chloropyrimidine), with succinate as substrate.Difenzoquat also inhibits state 3 respiration in isolated wheat and wild oat mitochondria however, this was not thought to be a significant contribution to the chlorotic contact symptoms observed with this translocated herbicide. Difenzoquat also possesses a weak bipyridinium-type PSI electron acceptor activity which causes chlorosis. ... 

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