the roots easily, when compared to large-sized pharmaceuticals (molar mass >400 g/
mol).
In the living parenchymal tissue deeper inside the root, and towards the delicate
younger apical roots, the cell wall and the biomembrane (plasmalemma) may
function as filters (membrane permeation) limiting the uptake or movement of
organic molecules based on their size.
Besides that, physicochemical properties of the molecules, like lipophilicity and
ionic strength (polarity H bonding), will dictate their fate, even before uptake and
transport into the plant vascular translocation system (xylem and phloem) occur. A
significant proportion of pharmaceuticals are ionizable meaning that they can
assume neutral, cationic, anionic or zwitterionic form under different pH conditions
[22]. This means that the difference in lipophilicity between the neutral and ionic
forms varies within compounds and is difficult to predict. Usually, a single log K OW
value (also called P) is determined, reflecting only the lipophilicity of neutral species
[23]. So, it has been discussed that for ionic forms, log D OW seems to be more
appropriate to express the lipophilicity of these molecules because it accounts for pH
dependence (i.e. acid dissociation constant (pKa)) of a molecule in aqueous
solution [24].
In early research on the topic, Briggs and co-workers [25] established a linear
relationship between K OW of non-ionized chemicals and the observed root concentration. Albeit only shown for industrial pollutants and herbicides [26], this relationship seems to hold true also for other synthetic molecules like pharmaceuticals. It is
crucial to consider that pharmaceuticals have been specifically designed to penetrate
through biological borders and membranes, to ensure their rapid delivery at the site
of action. Wild and co-workers [27] pointed out that non-ionic organic chemicals
with log K OW > 4 seem to exhibit high retention in plant roots, while Cousins and
Mackay [28] suggested that for organic chemicals with log K OW < 2 and a Henry’s
Law constant of less than 100 cm
3 cm
À3 , the water filled intercellular space seemed
to be the main storage compartment [29]; the topic has been extensively covered by
Schröder and Collins [30].
2.2 Uptake of Pharmaceuticals by Plant Roots
In the first step, compounds from the surrounding medium or pore water (usable
water in soil for plants) become available for root uptake by diffusion, where
compounds properties like solubility, lipophilicity, molar mass, compound concentration and characteristics from the surrounding environment as temperature and soil
humidity (if the case) will influence the uptake performance [21] (Fig. 1). Here, soils
with high proportions of clay minerals might be a significant temporary sink for
charged molecules and build up local hotspots of organic pollutants. In a second
phase, compounds are available to root uptake: due to a negative water potential in
soils at field capacity, a net movement of pharmaceutics towards plant rhizospheres
might prevail. The root surface and its extensions are key compartments for uptake
108
Y. Bigott et al.
mol).
In the living parenchymal tissue deeper inside the root, and towards the delicate
younger apical roots, the cell wall and the biomembrane (plasmalemma) may
function as filters (membrane permeation) limiting the uptake or movement of
organic molecules based on their size.
Besides that, physicochemical properties of the molecules, like lipophilicity and
ionic strength (polarity H bonding), will dictate their fate, even before uptake and
transport into the plant vascular translocation system (xylem and phloem) occur. A
significant proportion of pharmaceuticals are ionizable meaning that they can
assume neutral, cationic, anionic or zwitterionic form under different pH conditions
[22]. This means that the difference in lipophilicity between the neutral and ionic
forms varies within compounds and is difficult to predict. Usually, a single log K OW
value (also called P) is determined, reflecting only the lipophilicity of neutral species
[23]. So, it has been discussed that for ionic forms, log D OW seems to be more
appropriate to express the lipophilicity of these molecules because it accounts for pH
dependence (i.e. acid dissociation constant (pKa)) of a molecule in aqueous
solution [24].
In early research on the topic, Briggs and co-workers [25] established a linear
relationship between K OW of non-ionized chemicals and the observed root concentration. Albeit only shown for industrial pollutants and herbicides [26], this relationship seems to hold true also for other synthetic molecules like pharmaceuticals. It is
crucial to consider that pharmaceuticals have been specifically designed to penetrate
through biological borders and membranes, to ensure their rapid delivery at the site
of action. Wild and co-workers [27] pointed out that non-ionic organic chemicals
with log K OW > 4 seem to exhibit high retention in plant roots, while Cousins and
Mackay [28] suggested that for organic chemicals with log K OW < 2 and a Henry’s
Law constant of less than 100 cm
3 cm
À3 , the water filled intercellular space seemed
to be the main storage compartment [29]; the topic has been extensively covered by
Schröder and Collins [30].
2.2 Uptake of Pharmaceuticals by Plant Roots
In the first step, compounds from the surrounding medium or pore water (usable
water in soil for plants) become available for root uptake by diffusion, where
compounds properties like solubility, lipophilicity, molar mass, compound concentration and characteristics from the surrounding environment as temperature and soil
humidity (if the case) will influence the uptake performance [21] (Fig. 1). Here, soils
with high proportions of clay minerals might be a significant temporary sink for
charged molecules and build up local hotspots of organic pollutants. In a second
phase, compounds are available to root uptake: due to a negative water potential in
soils at field capacity, a net movement of pharmaceutics towards plant rhizospheres
might prevail. The root surface and its extensions are key compartments for uptake
108
Y. Bigott et al.