Plants phytotoxicity

Possible effects on soil hfe, plants (phytotoxicity) and on ground water are of concern in all types of ecosystems. Food quality criteria are, however, of relevance for arable land only, whereas possible secondary poisoning effects on domestic animals or terrestrial fauna are relevant in grassland and non-agricultural land. A final critical limit can be based on the most sensitive receptor. Even though effects vary for each metal, soil microbes and soil fauna are generally most sensitive. 

The US EPA characterizes As, Be, Sb, Cd, Cr, Cu, Pb, Hg, Ni, Se, Ag, Tl, and Zn as priority metals because of their potential hazardousness to human health. However, the environmental fate and effect of only a few metals (As, B, Cd, Cr, Cu, Mo, Ni, Pb, and Zn) have been studied extensively (Rechcigl 1995). For a given metal the potential to cause harm depends on the identifiable risk pathway, which is different for different metals. One pathway usually provides the highest probability of adverse affects to some receptor and is, therefore, the limiting pathway (Ryan Bryndzia 1997). The most toxic elements to humans are Hg, Pb, Cd, Ni, and Co. Some of the principal limiting pathways for various metals are the direct ingestion of Pb-contaminated soil by children plant phytotoxicity from Cu, Zn, Ni food-chain concentration and transfer of Cd and Hg to humans and food-chain transfer of Se and Mo to livestock (Ryan Bryndzia 1997). 

In continously wet, hot weather elemental sulfur may cause injury, particularly to certain sulfur-sensitive plants. Phytotoxicity is manifested by a small or large reduction in photosynthesis and respiration, in the scorching of leaves and, in severe cases, in retarded foliage growth (Hoffman, 1933, 1934, 1936). Turell (1950) attributes phytotoxicity to a decrease in critical temperature and to the absorption of sun rays (lens effect). In lemon cultures, damage due to sulfur which would otherwise occur only at higher temperatures has been observed at lower temperatures. However, the true reason for its phytotoxicity is most probably the fact that sulfur penetrates the plant tissues and, as a hydrogen acceptor, detrimentally influences metabolic processes. 

Although traditional investigations into the toxicity of chemicals in the natural environment have focused on animals, the toxicity of agrichemical and environmental pollutants to plants (phytotoxicity) has gained interest. Despite being well documented in literature, phytotoxicity is measured in various ways by agronomists and plant scientists a standard quantification unit is therefore difficult to devise, see also Venom. 

M.p. I08-5 C. Ordinary DDT contains about 15% of the 2,4 -isomer, and is prepared from chloral, chlorobenzene and sulphuric acid. It is non-phytotoxic to most plants. It is a powerful and persistent insecticide, used most effectively to control mosquitoes in countries where malaria is a problem. It is stored in the bodies of animals and birds. 

For most crops, other than rice, urea in the soil must first undergo hydrolysis to ammonia and then nitrification to nitrate before it can be absorbed by plant roots. One problem is that in relatively cool climates these processes are slow thus plants may be slow to respond to urea fertilization. Another problem, more likely in warmer climates, is that ammonia formed in the soil hydrolysis step may be lost as vapor. This problem is particularly likely when surface appHcation is used, but can be avoided by incorporation of the urea under the soil surface. Another problem that has been encountered with urea is phytotoxicity, the poisoning of seed by contact with the ammonia released during urea hydrolysis in the soil. Placement of urea away from the seed is a solution to this problem. In view of the growing popularity of urea, it appears that its favorable characteristics outweigh the extra care requited in its use. 

Atmospheric Toxicity. The only known atmospheric toxicity effect of phthalates is the phytotoxicity arising from the use of DBP plasticized glazing bars in greenhouses. However, the higher phthalates such as DEHP are not phytotoxic. General atmospheric concentrations of phthalates are extremely low and it is concluded that they pose no risk to plants or animals. 

The ECOTOXicology database is a source for locating single chemical toxicity data for aquatic life, terrestrial plants and wildlife. ECOTOX integrates three toxicology effects databases AQUIRE (aquatic life), PHYTOTOX (terrestrial plants), and TERRETOX (terrestrial wildlife). These databases were created by the U.S. EPA, Office of Research and Development (ORD), and the National Health and Environmental Effects Research Laborator) (NHEERL), Mid-Continent Ecology Division... 

As indicated earlier, heavy contamination can be buried, sealed or removed. Burying of the material should be well below the root growth zone, and this is normally taken as 3.0 m below the final ground-surface level. Sealing for heavy contamination to prevent vertical or lateral leaching through groundwater flow can be with compacted clay or proprietary plastic membranes. Removal from site of the contaminants is normally only contemplated in a landscaped scheme where the material, even at depth, could be a hazard to public health directly or phytotoxic to plant life. 

In areas where particular crops are grown continuously, decreases in production with time have been noted. The condition is usually species speciAc, and the disorders which result are frequently referred to as soil-sickness or replant problems. Fruit trees are especially sensitive and the problem has been encountered with apples, peaches, grapes, cherries, plums, and citrus. In most situations, phytotoxicity has been related to the formation of toxic materials as a consequence of the microbial decomposition of plant remains. 

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. 

The application of various antibiotics such as rifampicin/tetracycline (63), cefatoxime/trimethoprim (64), or bacteriostatic compounds such as Micropur (Roth, Karlsruhe, Germany) (65) used for root pretreatment or added to collection media is another strategy to prevent biodegradation during root exudate collection. However, depending on dosage and plant species, also phytotoxic effects of antibiotics have been reported (Table 3). Antibiotics in the soil environment... 

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