Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 149

a high tolerance to the contaminants present enables a cover of vegetation to be
re-established. This can be of particular importance for exposed sites, minimising
the effects of wind erosion, wash off or soil leaching which otherwise can signif-
icantly hasten the spread of pollutants around and beyond the affected land itself.
Organic Phytoremediation
A wide variety of organic chemicals are commonly encountered as environmental
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 2m
of the surface. Accordingly, since they are effectively in direct contact with the
rhizosphere, they are a good example of ideal candidates for phytoremediation.
However, recent work has shown that plant phylogeny is an important influence
on both the tolerance of a potential effector species to organic pollutants and its
ability to take them up (Collins and Willey, 2009). In addition, there seems to be
no correlation between the uptake of organic pollutants and heavy metals; further
work on this could prove invaluable for phytoremediation scheme design as well
as minimising the risk or failure when restoring previously contaminated sites.
The mechanisms of organic phytoremediation are typically phytodegradation,
rhizodegradation and phytovolatilisation.
Phytodegradation
Phytodegradation, which is sometimes known by the alternative name of
phytotransformation, involves the biological breakdown of contaminants, either
internally, 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 extra-cellular enzyme route has allowed
this technique to be successfully applied to the remediation of chemicals as
varied as chlorinated solvents, explosives and herbicides. Since this process
depends on the direct uptake of contaminants from soil water and the accu-
mulation of resultant metabolites within the plant tissues, in an environmental
application, it is clearly important that the metabolites which accumulate are
either non-toxic, or at least significantly less toxic than the original pollutant.
Rhizodegradation
Rhizodegradation, which is also variously described as phytostimulation or
enhanced rhizospheric biodegradation, refers to the biodegradation of contami-
nants in the soil by edaphic microbes enhanced by the inherent character of the
rhizosphere itself. This region generally supports high microbial biomass and