Herbicides phytotoxicity

After use, herbicides decompose slowly, and so affect cultivated plants for many years. In 1990, investigations in many regions of the USSR detected herbicides phytotoxic effects, especially among the si/m-triazine class, on different cultivars in many varied situations [13]. These sym-triazine herbicides, such as protrazin, simazin, atrazine, metazin, and prometrin, were used in different oblasts of the Ukraine, Kirgizia, Kazakhstan, Russia and Moldavia in previous years, especially on corn. Residual herbicide aftereffects led to the suppression and death of crops such as winter wheat, oats, barley, rye, potatoes, beets and sugar beets, linen, onions, watermelons and other melons, and sunflowers. 

Eullaffroy, P. and Vemet, G. 2003. The F684/F735 chlorophyll fluorescence ratio A potential tool for rapid detection and determination of herbicide phytotoxicity in algae. Water Research, 37 1983-90. 

Figure 6. Mathematical models for the effects of herbicide phytotoxicity on crop growth and yield. A represents an estimation of final yield reduction [99].

Correlation studies of soil properties and herbicide phytotoxicity showed that while soil organic matter was highly related to the activity of many herbicides, it was poorly related to the activity of CDEC and CDAA (350). This was probably because these compounds were not adsorbed as strongly and were volatilized from the soils in different amounts depending upon the temperature and soil moisture contents. Other herbicides used in the study were not as affected by temperature and moisture as were the volatile compounds. 

Estimation of Phvtotoxicity. Several investigators have used cell cultures to study herbicide phytotoxicity (3. 5. 18-20). Callus and cell suspensions have potential in the estimation of phytotoxicity, especially in detecting the inherent phytotoxicity of a molecule that fails to penetrate into or translocate in a whole plant. Alteration of the molecule to enhance penetrability may provide a usable herbicide. Structure activity relationships have also been examined to determine the comparative potency of different plant growth regulators in cell cultures in comparison to whole plants (21). 

Urea and uracil herbicides tend to be persistent in soils and may carry over from one season to the next (299). However, there is significant variation between compounds. Bromacil is debrominated under anaerobic conditions but does not undergo further transformation (423), linuron is degraded in a field soil and does not accumulate or cause carryover problems (424), and terbacd [5902-51-2] is slowly degraded in a Russian soil by microbial means (425). The half-hves for this breakdown range from 76 to 2,475 days and are affected by several factors including moisture and temperature. Finally, tebuthiuron apphed to rangeland has been shown to be phytotoxic after 615 days, and the estimated time for total dissipation of the herbicide is from 2.9 to 7.2 years (426). 

Agricultural Products. Pesticides are frequendy appHed as emulsiftable concentrates. The active insecticide or herbicide is dissolved in a hydrocarbon solvent which also contains an emulsifier. Hydrocarbon solvent selection is critical for this appHcation. It can seriously impact the efficacy of the formulation. The solvent should have adequate solvency for the pesticide, promote good dispersion when diluted with water, and have a dash point high enough to minimise dammabiUty ha2ards. When used in herbicide formulas, low solvent phytotoxicity is important to avoid crop damage. Hydrocarbon solvents used in post-harvest appHcation require special testing to ensure that polycycHc aromatics are absent. 

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... 

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. 

Nitrophenols are phytotoxic, and dinoseb (6-iec-butyl-2,4-dinitrophenol) has been used as a herbicide, while nitrophenols have been detected in rainwater and plausible mechanisms for their abiotic formation have been proposed (Kohler and Heeb 2003 Vione et al. 2005). The pathway for the degradation of phenols with a single nitro group depends on the position of the substituents, while... 

Many herbicides and other chemicals have been reported to influence levels of various phenolic compounds in higher plants by unknown mechanisms. It is unlikely that more than a few of these compounds have a primary influence on secondary phenolic compound synthesis. For instance, in our survey of the effects of 17 herbicides on anthocyanin accumulation, only glyphosate appeared to directly influence accumulation (31). The effects of several compounds on secondary phenolic compound production for which the mechanism of influence is unknown are summarized in Table II. A much longer list could be derived from the literature. Unfortunately, many of these compounds are phytotoxic or are known to have effects other than on secondary aromatic compound production. In most cases the effects on these compounds correlate well with extractable PAL activity (31, 71, 72, 73, 74) (Figure 5), even though they do not directly affect the enzyme. 

Rationale. Microorganisms were isolated from soil and screened for toxin production according to the scheme in Figure 1. Some of the organisms causing strong inhibition on solid medium were tested for toxin production in liquid medium. Liquid culture will be required to obtain large amounts of material for commercial production of herbicides, however, the ability to produce toxins on solid medium does not necessarily imply toxin production in broth (34). Cyclo-heximide, a phytotoxic but relatively nonspecific antibiotic with little value as a herbicide, is produced by many actinomycetes. Liquid cultures were tested for cycloheximide to determine whether it caused the observed toxicity. 

Thus it is significant that for such si/m-triazine herbicides as simazine, atrazine, etc., the health and hygiene MPC (toxicity to warm-blooded animals, including humans) and phytotoxic MPC (toxicity to plants) differ by more than an order of magnitude 0.2-0.5 mg/kg for warm-blooded animals, and 0.01 mg/ kg for plants [89]. Warm-blooded animals and arthropods have a difference in sensitivity to many pyretroids that can reach tens of thousands of times [90]. 

The presence of free anilines or other metabolites in soils and plants has been reported [123, 139-143]. Some work has suggested that they are very strongly bound to soil components and the findings of Caverly and Denney [138] are in agreement with these conclusions. The presence in soils of metabolites of Linuron that possess the urea structure have been reported [123, 142] these are produced mainly by microbiological degradation. The dimethyl derivative is considered to be inactive whereas the monomethyl metabolite has phytotoxicity approaching that of the parent herbicide [142], It is probable that the procedure reported by... 

Isoxaflutole is an isoxazole herbicide that, in the presence of hypochlorite, hydrolyzed to a stable and phytotoxic metabolite, diketonitrile. This intermediate further degraded to yield benzoic acid as the major end product, which is nonphytotoxic [112]. 

GraysonBT, Webb JD, Factors offecting the performance and crop phytotoxicity of a new rice herbicide, cinmethylin. 1. Effects of water depth and soil type on the distribution and uptake of anmethylin by transplanted and direct-seeded rice,... 

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