Phytotechnology and Photosynthesis 149on 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.
Rhizodegradation
Rhizodegradation, which is also variously described as phytostimulation
or enhanced rhizospheric biodegradation, refers to the biodegradation of
contaminants in the soil by edaphic microbes enhanced by the inherent character
of the rhizosphere itself. This region generally supports high microbial biomass
and consequently a high level of microbiological activity, which tends to increase
the speed and efficiency of the biodegradation of organic substances within
the rhizosphere compared with other soil regions and microfloral communities.
Part of the reason for this is the tendency for plant roots to increase the soil
oxygenation in their vicinity and exude metabolites into the rhizosphere. It has
been estimated that the release of sugars, amino acids and other exudates from
the plant and the net root oxygen contribution can account for up to 20% of
plant photosynthetic activity per year (Foth 1990), of which denitrifying bacteria,
Pseudomonas spp., and general heterotrophs are the principal beneficiaries.
In addition, mycorrhizae fungi associated with the roots also play a part in
metabolising organic contaminants. This is an important aspect, since they have
unique enzymatic pathways that enable the biodegradation of organic substances
that could not be otherwise transformed solely by bacterial action. In principle,
rhizodegradation is intrinsic remediation enhanced by entirely natural means,
since enzymes which are active within 1 mm of the root itself, transform the
organic pollutants, in a way which, clearly, would not occur in the absence of the
plant. Nevertheless, this is generally a much slower process than the previously
described phytodegradation.
Phytovolatilisation
Phytovolatilisation involves the uptake of the contaminants by plants and their
release into the atmosphere, typically in a modified form. This phytoremediation
biotechnology generally relies on the transpiration pull of fast-growing trees,
which accelerates the uptake of the pollutants in groundwater solution, which are
then released through the leaves. Thus the contaminants are removed from the
soil, often being transformed within the plant before being voided to the atmo-
sphere. One attempt which has been explored experimentally uses a genetically
modified variety of the Yellow Poplar,Liriodendron tulipifera, which has been
engineered by the introduction of mercuric reductase gene (mer A) as discussed
in Chapter 9. This confers the ability to tolerate higher mercury concentrations
and to convert the metal’s ionic form to the elemental and allows the plant
to withstand contaminated conditions, remove the pollutant from the soil and