Uptake of Organic Contaminants from Soil into Vegetables and Fruits - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 396

The role of the plant enzyme system is detoxification, and there is an upper
limit, beyond which plants suffer from toxic effects and probably die (Trapp et al.
2007b ). This contamination of plants may limit activities in gardening, agriculture
and forestry on contaminated sites.
Metabolism by plants has been described by the green liver concept, because
plant metabolism rather resembles the processes in the animal liver than the bacterial
metabolism (Sandermann 1994 ). The first step (phase I reaction) is typically an oxy-
genation with cytochrome P-450, followed by conjugation reactions (phase II) with
glutathione-S-transferases (GST) (Barret 1995 ; Pflugmacher and Schröder 1995 ).
Unlike animals, plants are not able to excrete conjugates via the urine. Instead, phase
III of plant xenobiotic metabolism involves storage of soluble conjugates in the vac-
uole and of insoluble conjugates in the cell wall (Komossa et al. 1995 ). This may
lead to so-called bound residues. These bound unextractable residues resist solubi-
lization in common laboratory solvents and are therefore not accessible to standard
residue analysis. It was found that bound residues can be present in larger amounts
than the parent contaminant and could therefore represent a source of significant
consumer exposure (Sandermann 2004 ).
Little is known about metabolism rates of contaminants by vegetation. Cyanide
(HCN) was used as model contaminant to study the variation of rates among plant
species. Even though inorganic, cyanide behaves like an organic contaminant in
terms of lipid solubility, volatility and metabolism. The removal of free cyanide
followed Michael-Menten kinetics (Larsen et al. 2004 ). Adding Michaelis-Menten
kinetics to the mass balance equation for roots (Eq. 9.18 ) leads to the following
non-linear equation:

k R

dC R
dt

Q
M R ×

Q
M R ×

v max × C R

= C W −

K RW +

× C R − (9.38)

K M × K RW + C R

where v max (mg kg − 1 d − 1 ) is the maximal metabolism velocity of the contaminant
and K M (mg L − 1 ) is the half-saturation constant and (Larsen et al. 2005 ).
At a low external concentration in soil pore water, all contaminants that are taken
up are metabolized (Fig. 9.10 ). At higher concentrations, however, the enzyme sys-
tem is overloaded. Then, uptake is linearly related to the external concentration.
This was shown experimentally for free cyanide (HCN) by Larsen et al. ( 2005 ). A
non-linear BCF relation indicating enzymatic activity of plants was found repeat-
edly, e.g. for phenol (Ucisik and Trapp 2006 ) and salt, NaCl (Trapp et al. 2008 ). In
these cases, the BCF was low at a low external concentration in soil pore water, but
increased at higher external concentrations.
The Michaelis-Menten parameters K M and v max varied with plant species, but
less than expected. Values of v max of 12 species from nine plant families were found
in a relatively narrow range between 6.7 and 21.9 mg CN kg plant − 1 h − 1 and were
normally distributed with a mean of 13 mg CN kg plant − 1 h − 1 (Yu et al. 2004 ). The
authors concluded that the variation of metabolism rates between plant species was