depend on plant species and shoot height. Environmental factors are also influencing
the daily transpiration rates. As it has been mentioned before, the molecular size of
pharmaceuticals can determine their diffusion rate through root cell membranes. A
good example of a pharmaceutical being translocated by xylem flow is carbamazepine. The uncharged compound with intermediate hydrophobicity (log K OW 3.64) is
known to be frequently detected in higher concentrations in aerial parts rather than in
roots [20, 52, 53]. Moreover, carbamazepine was detected through the whole plant in
xylem sap and even found in transpiration waters in the ambient air [10, 54].
Pharmaceuticals could be also transported via sieve tubes of the phloem, as
shown already for several herbicides [55, 56]. Compared to the unidirectional flow
from roots to leaves in the xylem, compounds in phloem can be translocated in two
directions: together with photosynthates (photosynthetically derived carbohydrates)
from leaves to the plant below (branch, shoot, root) and above (young developing
leaves, apical meristem, fruits). As generally alleged, phloem mass flow is driven by
an osmotically generated pressure gradient by the accumulation (active loading) of
sugars in the photosynthetically active leaves (source) and their deliverance
(unloading) to the place of consumption (sink). Therefore, it is hypothesized that
neutral compounds, which are mainly translocated by water flow (xylem), can be
generally found in higher concentrations in mature leaves [53], in contrast to
xenobiotics being transported via phloem to younger leaves, as suggested by Hsu
and Kleier [57]. In this respect the abovementioned carbamazepine, which is known
to be transported by xylem, was detected in higher concentrations in old leaves
compared to young leaves of cucumber plants. In contrast, the anionic antibiotic
tetracycline was quantified in similar concentrations in both kinds of leaves [58].
However, for non-ionic compounds, like the insecticide fipronil or some
neonicotinoids, the ion trap theory does not apply, and the active ingredient can
move freely between phloem and xylem according to its membrane permeability
[59]. Herbicides with high ability to cross membranes may equilibrate between
phloem and xylem but are preferentially transported by xylem because of the higher
water flow [60]. Although only described for agrochemicals, this concept may as
well influence the pharmaceutical compounds transport in plants.
The transpiration stream concentration factor (TSCF) is a descriptor for the
quantitative uptake of contaminants. It is defined as a ratio of contaminant concentration in the xylem to the concentration in nutrient media, and this ratio varies
between 0 and 1 [61]. The hydrophilic compound caffeine had a higher TSCF value
than the more hydrophobic compounds triclocarban or endosulfan in zucchini
(Cucurbita pepo ssp. pepo), soybean (Glycine max L.) and squash (Cucurbita
pepo ssp. ovifera). Hence, hydrophilic pharmaceuticals, after passing the Casparian
strip, seem to be translocated faster than hydrophobic ones [62]. The TSCF can give
useful information about the translocation of compounds although not many studies
exist measuring the pharmaceutical concentrations in xylem sap. Thus, the translocation factor (TF) describing the ratio between the pharmaceutical concentrations in
the leaf compared to the root is often used to characterize the translocation of
compounds. However, it is not taken into account if compounds are translocated
by xylem or phloem.
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the daily transpiration rates. As it has been mentioned before, the molecular size of
pharmaceuticals can determine their diffusion rate through root cell membranes. A
good example of a pharmaceutical being translocated by xylem flow is carbamazepine. The uncharged compound with intermediate hydrophobicity (log K OW 3.64) is
known to be frequently detected in higher concentrations in aerial parts rather than in
roots [20, 52, 53]. Moreover, carbamazepine was detected through the whole plant in
xylem sap and even found in transpiration waters in the ambient air [10, 54].
Pharmaceuticals could be also transported via sieve tubes of the phloem, as
shown already for several herbicides [55, 56]. Compared to the unidirectional flow
from roots to leaves in the xylem, compounds in phloem can be translocated in two
directions: together with photosynthates (photosynthetically derived carbohydrates)
from leaves to the plant below (branch, shoot, root) and above (young developing
leaves, apical meristem, fruits). As generally alleged, phloem mass flow is driven by
an osmotically generated pressure gradient by the accumulation (active loading) of
sugars in the photosynthetically active leaves (source) and their deliverance
(unloading) to the place of consumption (sink). Therefore, it is hypothesized that
neutral compounds, which are mainly translocated by water flow (xylem), can be
generally found in higher concentrations in mature leaves [53], in contrast to
xenobiotics being transported via phloem to younger leaves, as suggested by Hsu
and Kleier [57]. In this respect the abovementioned carbamazepine, which is known
to be transported by xylem, was detected in higher concentrations in old leaves
compared to young leaves of cucumber plants. In contrast, the anionic antibiotic
tetracycline was quantified in similar concentrations in both kinds of leaves [58].
However, for non-ionic compounds, like the insecticide fipronil or some
neonicotinoids, the ion trap theory does not apply, and the active ingredient can
move freely between phloem and xylem according to its membrane permeability
[59]. Herbicides with high ability to cross membranes may equilibrate between
phloem and xylem but are preferentially transported by xylem because of the higher
water flow [60]. Although only described for agrochemicals, this concept may as
well influence the pharmaceutical compounds transport in plants.
The transpiration stream concentration factor (TSCF) is a descriptor for the
quantitative uptake of contaminants. It is defined as a ratio of contaminant concentration in the xylem to the concentration in nutrient media, and this ratio varies
between 0 and 1 [61]. The hydrophilic compound caffeine had a higher TSCF value
than the more hydrophobic compounds triclocarban or endosulfan in zucchini
(Cucurbita pepo ssp. pepo), soybean (Glycine max L.) and squash (Cucurbita
pepo ssp. ovifera). Hence, hydrophilic pharmaceuticals, after passing the Casparian
strip, seem to be translocated faster than hydrophobic ones [62]. The TSCF can give
useful information about the translocation of compounds although not many studies
exist measuring the pharmaceutical concentrations in xylem sap. Thus, the translocation factor (TF) describing the ratio between the pharmaceutical concentrations in
the leaf compared to the root is often used to characterize the translocation of
compounds. However, it is not taken into account if compounds are translocated
by xylem or phloem.
112
Y. Bigott et al.