Another difference between xylem and phloem, which influence the translocation
of environmental contaminants (e.g. pharmaceuticals), is the pH. Phloem juice is
about 8.0, which is similar to cytoplasmic pH (6.9–7.6), but inside xylem vessels,
and also in the apoplast and intracellular spaces, the pH is about 5.0 [63]. Translocation of emerging contaminants is also interlinked to physical and chemical properties of the organic compounds. pKa values, influencing the charge of some
pharmaceuticals at a specific pH is highly relevant (see previous section about root
uptake). Accumulation of lamotrigine in leaves correlated with uncharged
lamotrigine in pore water; thus, the pH-dependent charge of the molecule in the
soil had an impact on its translocation to aerial parts of durum weed (Triticum
durum) [64]. Such as the pKa, also the lipophilicity of compounds plays a crucial
role, as moderately lipophilic neutral substances, with log K OW (1–3.5) or log D OW
(0.5–3), are preferably translocated [65, 66]. Collins and co-workers [33] pointed out
that for some uptake models, the lipid content (in their case, of the leaves) represents
the most sensitive input parameter for lipophilic chemicals. It has not yet been
investigated whether this is also valid for the root compartment, although several
experimental studies showed missing or very low translocation of lipophilic compounds to aboveground parts [67, 68], but an exception exists. Astonishingly,
zucchini is able to take up and translocate different highly hydrophobic
polychlorinated dibenzodioxins and furans (PCDD/F) congeners to leaves and to
the entire fruit, whereas for pumpkin and cucumber, contaminants were shown to be
restricted to the outer part of the fruit [69]. It was hypothesized that zucchini might
release a binding substance for PCDD/Fs with root exudates, which forms a hydrophilic complex with the pollutant to enable the uptake by the plants’ roots. Furthermore, molecules in leaf extracts and in the xylem sap of zucchini and melon
(Cucumis melo L.) were detected with the ability to increase the apparent aqueous
solubility of tetrachlorodibenzodioxin (TCDD) by forming a reversible binding
[70]. More recently, 17-kD proteins (probably major latex-like proteins (MLPs)) in
xylem sap of zucchini were suggested to influence the translocation of hydrophobic
organic contaminants, as the expression of the MLP-GR3 gene in C. pepo cultivars
correlated positively with the presence of the 17-kD proteins and BCFs of dioxins
and dioxin-like compounds [71]. The translocation of hydrophobic pharmaceuticals
to shoots was as well enhanced in zucchini plants compared to soybean and closely
related squash. Additionally, higher xylem sap solubilities of these chemicals were
detected in zucchini, leading to the hypothesis of an involvement of xylem sap
proteins in the enhanced translocation of pharmaceuticals to aerial tissues like for
other ECs [62].
Dilution by growth is another factor influencing the concentration in plant parts,
which is especially important for the prediction of the foliar uptake of organic
compounds [29]. The resulting increased plant biomass leads to a potential dilution
of the pharmaceutical concentration relative to the flux of their uptake. In contrast,
expanded plant leaf area provides a larger surface for the foliar uptake of emerging
contaminants from ambient air [30, 33]. The uptake of organic contaminants by
aerial tissues was shown for many pesticides, polycyclic aromatic hydrocarbons
(PAHs) or polychlorinated contaminants [72–75]. To enter the leaf, chemicals have
Uptake and Translocation of Pharmaceuticals in Plants:. . .
113
of environmental contaminants (e.g. pharmaceuticals), is the pH. Phloem juice is
about 8.0, which is similar to cytoplasmic pH (6.9–7.6), but inside xylem vessels,
and also in the apoplast and intracellular spaces, the pH is about 5.0 [63]. Translocation of emerging contaminants is also interlinked to physical and chemical properties of the organic compounds. pKa values, influencing the charge of some
pharmaceuticals at a specific pH is highly relevant (see previous section about root
uptake). Accumulation of lamotrigine in leaves correlated with uncharged
lamotrigine in pore water; thus, the pH-dependent charge of the molecule in the
soil had an impact on its translocation to aerial parts of durum weed (Triticum
durum) [64]. Such as the pKa, also the lipophilicity of compounds plays a crucial
role, as moderately lipophilic neutral substances, with log K OW (1–3.5) or log D OW
(0.5–3), are preferably translocated [65, 66]. Collins and co-workers [33] pointed out
that for some uptake models, the lipid content (in their case, of the leaves) represents
the most sensitive input parameter for lipophilic chemicals. It has not yet been
investigated whether this is also valid for the root compartment, although several
experimental studies showed missing or very low translocation of lipophilic compounds to aboveground parts [67, 68], but an exception exists. Astonishingly,
zucchini is able to take up and translocate different highly hydrophobic
polychlorinated dibenzodioxins and furans (PCDD/F) congeners to leaves and to
the entire fruit, whereas for pumpkin and cucumber, contaminants were shown to be
restricted to the outer part of the fruit [69]. It was hypothesized that zucchini might
release a binding substance for PCDD/Fs with root exudates, which forms a hydrophilic complex with the pollutant to enable the uptake by the plants’ roots. Furthermore, molecules in leaf extracts and in the xylem sap of zucchini and melon
(Cucumis melo L.) were detected with the ability to increase the apparent aqueous
solubility of tetrachlorodibenzodioxin (TCDD) by forming a reversible binding
[70]. More recently, 17-kD proteins (probably major latex-like proteins (MLPs)) in
xylem sap of zucchini were suggested to influence the translocation of hydrophobic
organic contaminants, as the expression of the MLP-GR3 gene in C. pepo cultivars
correlated positively with the presence of the 17-kD proteins and BCFs of dioxins
and dioxin-like compounds [71]. The translocation of hydrophobic pharmaceuticals
to shoots was as well enhanced in zucchini plants compared to soybean and closely
related squash. Additionally, higher xylem sap solubilities of these chemicals were
detected in zucchini, leading to the hypothesis of an involvement of xylem sap
proteins in the enhanced translocation of pharmaceuticals to aerial tissues like for
other ECs [62].
Dilution by growth is another factor influencing the concentration in plant parts,
which is especially important for the prediction of the foliar uptake of organic
compounds [29]. The resulting increased plant biomass leads to a potential dilution
of the pharmaceutical concentration relative to the flux of their uptake. In contrast,
expanded plant leaf area provides a larger surface for the foliar uptake of emerging
contaminants from ambient air [30, 33]. The uptake of organic contaminants by
aerial tissues was shown for many pesticides, polycyclic aromatic hydrocarbons
(PAHs) or polychlorinated contaminants [72–75]. To enter the leaf, chemicals have
Uptake and Translocation of Pharmaceuticals in Plants:. . .
113