concentration in bulk soil are not comparable amongst the studies using different
soils because of the varying affinities of pharmaceuticals to soils. For example, the
differences in soil-based BCFs between three soils for caffeine, carbamazepine and
lamotrigine in tomato (Solanum lycopersicum) or cucumber (Cucumis sativus) were
found to be 20, 7.8 and 245 times, respectively [10]. To follow this up, Li et al. [28]
applied the quasi-equilibrium partition model, first developed by Chiou et al. [59],
with extensive literature data and found that the root concentration factors (RCF)
correlated strongly with chemical-root lipophilic coefficients ( f lip K ow ).
3.1 Distribution Amongst Different Plant Organs
Accumulation in different plant organs of the same species can be explained by
movement of the chemicals in the xylem from the roots, upwards towards aerial
tissues which is driven by the transpiration stream [28]. Research to date underpins
that this is largely driven by chemical properties of the pharmaceuticals.
For example, ionisation of functional groups was shown to play a significant role
in the distribution of pharmaceuticals in the radish experiment by Li et al. [28]. Both
carbamazepine and lamotrigine have similar log D ow values (2.45 and 2.57, respectively) and molecular weights (236.27 and 256.10 g/mol, respectively) and based on
their pKa values were known to exist in their neutral form in the pore water.
However, the magnitude of pharmaceutical uptake towards the aerial tissues, measured by a translocation factor, was approximately four times larger for carbamazepine (~8.0) than lamotrigine (~2.0). This was explained by the fact that 41% of
lamotrigine became positively charged in the vacuoles, meaning that it became
trapped in the negatively charged cell walls and thus reduced the translocation of
lamotrigine to the leaf material. In comparison, carbamazepine has no ionisable
functional groups and would have had no ionic interaction with the cell walls. Ion
trapping has been previously shown to enhance the accumulation of organic
chemicals in plants due to the alteration of chemical speciation in cell organelles,
whereby cationic chemicals become attracted to negatively charged plant root cell
membranes [60].
Recently, Li et al. [61] used radioautographic analysis to understand the distribution patterns of
14 C labelled carbamazepine uptake by three edible plant species,
celery (Apium graveolens), pak choi (Brassica rapa subsp. chinensis) and carrot.
Whilst
14 C-carbamazepine was taken up by all three plants, a lower amount of
14 C
was visualised in the stems in comparison to the roots and leaves supporting earlier
findings that the movement of carbamazepine towards fruits and leaves is driven by
transpiration processes with stems serving as a pathway for transport of carbamazepine by mass flow [5]. Movement of carbamazepine via mass flow towards the
aerial parts of the plant has also been suggested to be responsible for the accumulation of this pharmaceutical in the nectar and pollen of flowering plants [62]. In this
study, residues of carbamazepine up to 371 ng/mL and 30 μg/g were detected in
nectar and pollen sampled from zucchini flowers (Cucurbita pepo) grown in
Uptake and Effects of Pharmaceuticals in the Soil-Plant-Earthworm System
189
soils because of the varying affinities of pharmaceuticals to soils. For example, the
differences in soil-based BCFs between three soils for caffeine, carbamazepine and
lamotrigine in tomato (Solanum lycopersicum) or cucumber (Cucumis sativus) were
found to be 20, 7.8 and 245 times, respectively [10]. To follow this up, Li et al. [28]
applied the quasi-equilibrium partition model, first developed by Chiou et al. [59],
with extensive literature data and found that the root concentration factors (RCF)
correlated strongly with chemical-root lipophilic coefficients ( f lip K ow ).
3.1 Distribution Amongst Different Plant Organs
Accumulation in different plant organs of the same species can be explained by
movement of the chemicals in the xylem from the roots, upwards towards aerial
tissues which is driven by the transpiration stream [28]. Research to date underpins
that this is largely driven by chemical properties of the pharmaceuticals.
For example, ionisation of functional groups was shown to play a significant role
in the distribution of pharmaceuticals in the radish experiment by Li et al. [28]. Both
carbamazepine and lamotrigine have similar log D ow values (2.45 and 2.57, respectively) and molecular weights (236.27 and 256.10 g/mol, respectively) and based on
their pKa values were known to exist in their neutral form in the pore water.
However, the magnitude of pharmaceutical uptake towards the aerial tissues, measured by a translocation factor, was approximately four times larger for carbamazepine (~8.0) than lamotrigine (~2.0). This was explained by the fact that 41% of
lamotrigine became positively charged in the vacuoles, meaning that it became
trapped in the negatively charged cell walls and thus reduced the translocation of
lamotrigine to the leaf material. In comparison, carbamazepine has no ionisable
functional groups and would have had no ionic interaction with the cell walls. Ion
trapping has been previously shown to enhance the accumulation of organic
chemicals in plants due to the alteration of chemical speciation in cell organelles,
whereby cationic chemicals become attracted to negatively charged plant root cell
membranes [60].
Recently, Li et al. [61] used radioautographic analysis to understand the distribution patterns of
14 C labelled carbamazepine uptake by three edible plant species,
celery (Apium graveolens), pak choi (Brassica rapa subsp. chinensis) and carrot.
Whilst
14 C-carbamazepine was taken up by all three plants, a lower amount of
14 C
was visualised in the stems in comparison to the roots and leaves supporting earlier
findings that the movement of carbamazepine towards fruits and leaves is driven by
transpiration processes with stems serving as a pathway for transport of carbamazepine by mass flow [5]. Movement of carbamazepine via mass flow towards the
aerial parts of the plant has also been suggested to be responsible for the accumulation of this pharmaceutical in the nectar and pollen of flowering plants [62]. In this
study, residues of carbamazepine up to 371 ng/mL and 30 μg/g were detected in
nectar and pollen sampled from zucchini flowers (Cucurbita pepo) grown in
Uptake and Effects of Pharmaceuticals in the Soil-Plant-Earthworm System
189