A wide variety of organic chemicals are commonly encountered as environ-mental pollutants including many types of pesticides, solvents and lubricants. Probably the most ubiquitous of these across the world, for obvious reasons, are petrol and diesel oil. These hydrocarbons are not especially mobile, tend to adhere closely to the soil particles themselves and are generally localised within 2 metres of the surface. Accordingly, since they are effectively in direct contact with the rhizosphere, they are a good example of ideal candidates for phytoreme-diation. The mechanisms of action in this respect are typically phytodegradation, rhizodegradation, and phytovolatilisation.
Phytodegradation, which is sometimes known by the alternative name of phyto-transformation, involves the biological breakdown of contaminants, either inter-nally, having first been taken up by the plants, or externally, using enzymes secreted by them. Hence, the complex organic molecules of the pollutants are subject to biodegradation into simpler substances and incorporated into the plant tissues. In addition, the existence of the extracellular enzyme route has allowed this technique to be successfully applied to the remediation of chemicals as var-ied as chlorinated solvents, explosives and herbicides. Since this process depends on the direct uptake of contaminants from soil water and the accumulation of resultant metabolites within the plant tissues, in an environmental application, it is clearly important that the metabolites which accumulate are either nontoxic, or at least significantly less toxic than the original pollutant.
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