Successful
implementation of a stabilization technique for any metal within the soil
environment lies heavily on a thorough understanding of the chemistry of the
root zone, root exudates, contaminants, and fertilizer or soil amendments, to
prevent unintended effect that might increase contaminant solubility and
leaching (Pivetz, 2001). The Knowledge
of rhizospheric processes mediated by root exudates has not developed to a
definitive level; likewise, the chemo-diversity of root exudates is largely
unexplored and it is an obvious place to search for novel biologically active
compounds that could enhance phytostabilization. Mucilage, a viscous, high molecular weight, insoluble,
polysaccharide-rich material, secreted
by root caps and epidermal cells, in addition to other functions,
provides protection to plants from desiccation (Walker et al., 2003). It has
been speculated that as the soil dries up, exudates will also begin to lose
water to soil, and as a result, their viscosity increases. Likewise, the
resistance to movement of soil particles in contact with exudates will also
increase, and a degree of stabilization within the rhizosphere will be achieved
(Walker et al., 2003). This speculation implies that exudates (mucilage) play a
major role in the maintenance of root-soil contact, and suggest the possibility
of this process in limiting plant nutrient uptake or rhizostabilization of
contaminants. According to the Stokes-Einstein relation, diffusion is inversely
proportional to fluid viscosity (Sposito, 2008) hence, as the viscosity of the
mucilage increases due to loss of water to surrounding soil particles;
permeability of the mucilage layer by ions in solution should also be limited, and
consequently the amount of contaminant taken up by a plant. Also, the increase
in resistance to movement of soil particles in contact with the mucilage could
as well imply reduced exposure of roots to the soil matrix from where more
metals could be solubilized; at the same time, field water regime of crops
could be optimized to exploit this potential. Therefore, possible research question that
could be raised include:
1. To
what extent does the mucilage (galacturonic acid polymers with high viscosity)
covering the surface of apical roots limits the diffusion of metal-organic
complexes into the root apoplasm?
2. Is
there any difference in plant uptake of a metal with change in the viscosity of
the root cap mucilage layer?
3. How
can field water regime be managed to maintain an optimum soil resistivity so as
to limit root-soil contact, and thus metal uptake by plant roots?
The role of organic molecules that provide exchange sites for the metals
in the root zone is ill defined. An assessment of these molecules especially in
relation to their interaction with microbial exudates in influencing the
chemistry of the rhizosphere could provide vital information on the mechanism of
metal exclusion by plants. The negatively charged sites provided by these molecules
upon proton dissociation by the acid groups provide complexation site for
metals, including Cd. The possibility of this functional group binding Cd in
octahedral coordination with the oxygen ligands could result in complexes of
high stability which could precipitate out of solution due to soil solution
saturation. Equally important is the effect of protons released by the organic
acids on solubilization of metals bound to the residual fractions of the soil.
This is of concern in soils derived from parent materials with high enrichment
factors for heavy metals; our ability to quantify the effect of rhizospheric
exudates on this metal fraction could be very important in predicting
contaminant transfer to the food chain and the extent of ground water pollution
over time. Exogenous materials such as kerogen and black carbon have received
less attention in the study of organic compounds in the rhizosphere, a thorough
understanding of the reactivity and roles of such materials could reveal vital
information on how metal stabilization could be enhanced.
Due to the influence of root morphology on uptake of solutes, the
mechanism controlling the variable uptake observed in different plant is worth
investigating. The role of root hairs present in sub-apical root zones as it
pertains to the increased release of protons and organic compounds in this zone
needs a more complete evaluation. Furthermore, because the selectivity in uptake
of an ion over another reflect more of physico-chemical similarity rather than
essentiality (Marschner, 1995), it is important to understand the extent of
this selectivity (for Cd) under varied supply of another physico-chemically
similar metal (e.g. Zn). The extent of this inhibition could hint on possible ways
to limit plant uptake of Cd.
Other possible questions with
regards metal stabilization in the rhizosphere include:
i.
To what extent do changes in rhizospheric
processes affect metal mobility?
ii.
What are those stress factors that influence
plant root modification as an adaptation mechanism to exploit a larger soil
volume for improved acquisition of nutrients, and how can such information be
used in reducing plant uptake of heavy metals?
A profound understanding
of these points is fundamental to successful metal stabilization within the
soil environment.
Kirkham
M. B. 2006. Cadmium in plants on polluted soils: Effects of soil factors, hyperaccumulation,
and amendments. Geoderma 137, 19–32
Marschener, H.
Mineral Nutrition of Higher Plants. Academic Press. London, UK.
Pivetz, B. E. 2001. Phytoremediation of Contaminated Soil and
Ground Water at Hazardous Waste Sites. Ground Water Issue. EPA/540/S-01/500
Walker S. T.,
Bais H. P., Grotewold, E. and Vivanco, J. M. 2003. Root Exudation and Rhizosphere Biology. Plant Physiology. 132, 1 44-51 .doi: http://dx.doi.org/10.1104/pp.102.019661
