to either cross the cuticle or to enter through the stomata. Therefore, cutin, cuticular
waxes and other cellular lipids act as a lipophilic barrier that might absorb different
substances. A correlation could furthermore be detected between the surface wax
concentration and the resistance to foliar penetration [76]. Although spray irrigation
with treated wastewater contaminated with pharmaceuticals serves the possibility
that these molecules are deposited on plants’ leaves and could therefore be taken up,
we are not aware of studies about the leaf penetration of these chemical contaminants. Some hints for the possibility of pharmaceuticals uptake by leaves are given
[77]. Comparing the bioaccumulation in roots and leaves of a submerged and a freefloating plant species, differences in allocation of several pharmaceuticals could be
detected. Highest concentrations of these chemicals were found in the plant tissue,
which was exposed to the contaminated environment. Free-floating common water
hyacinth (Eichhornia crassipes) having their roots exposed to different pharmaceuticals in water exhibited a higher concentration in the roots rather than leaves, except
for carbamazepine which is known to be translocated to the leaves very fast [20]. For
the submerged plant, burhead (Echinodorus horemanii), where leaves are
surrounded by contaminated water, the tested compounds accumulated in the leaves
in a higher proportion compared to roots. Even though submerged plants show
differences compared to higher terrestrial plants (e.g. no transpiration, reduced
xylem, thin cuticle), this study gives useful initial information about the possible
uptake of pharmaceuticals by plant leaves.
Many pharmaceuticals are susceptible to photodegradation, which is an advantage in the wastewater treatment process to degrade them by UV treatment
[78, 79]. As leaves are exposed to intensive light intensities, photodegradation
within plants is theoretically possible, although no evidence about photodegradation
of pharmaceuticals in plants is available till now.
2.4 Role of Biotransformation in the Translocation
of Pharmaceuticals
The biotransformation of pharmaceuticals plays an important role in their translocation and risk assessment. From the intensive research about herbicide resistance in
weeds, herbicide detoxification in crops and the removal of organic xenobiotics by
phytoremediation, it has been known that plants possess an elaborate detoxification
system for organic xenobiotics and agrochemicals, comprising of a metabolic
cascade proceeding in three phases [80–82] (see Fig. 3). During phase I, xenobiotics
can be activated by oxidation, reduction or hydrolysis depending on their molecule
structure. The activated molecules can be conjugated to reactive groups, such as
amino acids, glutathione or sugars by specific enzymes like glutathione S-transferases or glycosyltransferases to reduce the compounds reactivity and increase their
water solubility during the consecutive phase II. Conjugated metabolites can afterwards be sequestered in vacuoles during phase III (vacuolar sequestration) or form
114
Y. Bigott et al.
waxes and other cellular lipids act as a lipophilic barrier that might absorb different
substances. A correlation could furthermore be detected between the surface wax
concentration and the resistance to foliar penetration [76]. Although spray irrigation
with treated wastewater contaminated with pharmaceuticals serves the possibility
that these molecules are deposited on plants’ leaves and could therefore be taken up,
we are not aware of studies about the leaf penetration of these chemical contaminants. Some hints for the possibility of pharmaceuticals uptake by leaves are given
[77]. Comparing the bioaccumulation in roots and leaves of a submerged and a freefloating plant species, differences in allocation of several pharmaceuticals could be
detected. Highest concentrations of these chemicals were found in the plant tissue,
which was exposed to the contaminated environment. Free-floating common water
hyacinth (Eichhornia crassipes) having their roots exposed to different pharmaceuticals in water exhibited a higher concentration in the roots rather than leaves, except
for carbamazepine which is known to be translocated to the leaves very fast [20]. For
the submerged plant, burhead (Echinodorus horemanii), where leaves are
surrounded by contaminated water, the tested compounds accumulated in the leaves
in a higher proportion compared to roots. Even though submerged plants show
differences compared to higher terrestrial plants (e.g. no transpiration, reduced
xylem, thin cuticle), this study gives useful initial information about the possible
uptake of pharmaceuticals by plant leaves.
Many pharmaceuticals are susceptible to photodegradation, which is an advantage in the wastewater treatment process to degrade them by UV treatment
[78, 79]. As leaves are exposed to intensive light intensities, photodegradation
within plants is theoretically possible, although no evidence about photodegradation
of pharmaceuticals in plants is available till now.
2.4 Role of Biotransformation in the Translocation
of Pharmaceuticals
The biotransformation of pharmaceuticals plays an important role in their translocation and risk assessment. From the intensive research about herbicide resistance in
weeds, herbicide detoxification in crops and the removal of organic xenobiotics by
phytoremediation, it has been known that plants possess an elaborate detoxification
system for organic xenobiotics and agrochemicals, comprising of a metabolic
cascade proceeding in three phases [80–82] (see Fig. 3). During phase I, xenobiotics
can be activated by oxidation, reduction or hydrolysis depending on their molecule
structure. The activated molecules can be conjugated to reactive groups, such as
amino acids, glutathione or sugars by specific enzymes like glutathione S-transferases or glycosyltransferases to reduce the compounds reactivity and increase their
water solubility during the consecutive phase II. Conjugated metabolites can afterwards be sequestered in vacuoles during phase III (vacuolar sequestration) or form
114
Y. Bigott et al.