nitrogen crops such as B. napus. Meanwhile, translocation rates of metformin to
fruits were observed to be influenced by distance from the root, with BCFs for the
first truss of mature tomato fruits (0.024) significantly lower than the fourth truss
(0.058). A corresponding, but not significant, trend for BCF for the first (0.122) and
fourth (0.182) fruits of squash was found.
3.3 Metabolism
A study by Carter et al. [36] on uptake of benzodiazepines, which also highlighted
the importance of the relationship between soil partitioning, ionisation of benzodiazepines and plant uptake, included an analysis of known benzodiazepine metabolites. Results revealed active in-plant metabolism of benzodiazepines, potentially
analogous to the known metabolic transformation pathway of benzodiazepines in
humans. Interestingly, the metabolites detected in the diazepam, temazepam and
chlordiazepoxide treatments were benzodiazepine parent compounds in their own
right. Significant concentrations of nordiazepam were detected in the diazepam- and
chlordiazepoxide-exposed plants, in both soil types, which were in excess of the
concentration reported for the parent compound [36].
Eggen et al. [63] also analysed plant samples for guanylurea, a known metabolite
of metformin. It was only detected in barley grains, bean pods, potato peel and small
potatoes in the range of 2.6–5.7 mg/kg with no relationship between high plant
metformin concentration and the detection of guanylurea. As with the benzodiazepine study, as guanylurea was not detected in the soil, this supports active in-plant
metabolism processes rather than root uptake of guanylurea from the soil. With a
predominant focus of assessing plant uptake and accumulation of pharmaceutical
parent compounds, these studies highlight the need for further research to elucidate
the metabolic pathways pharmaceuticals in plants and to determine whether the
resulting metabolite products retain their bioactive nature and thus pose a risk to
human and ecosystem health.
The last 15 years have generated a wealth of research from both greenhouse and
growth chamber studies where soils have been spiked with a range of pharmaceuticals. Such studies have shown clear differences in accumulation between plant
species and amongst various plant organs. However, where soils have been spiked to
replicate exposure in the environment, the concentrations used in these studies are
often in excess of measured environmental concentrations which have been reported
in the μg – low mg/kg range. For example, Boxall et al. [57], Li et al. [28],
Karnjanapiboonwong et al. [66] and Carter et al. [5, 36] all spiked soils at concentrations of approximately 1 mg/kg, whilst Eggen et al. [63], Eggen and Lillo [69] and
Ahmed et al. [70] reported nominal concentrations for their studies to range between
5 and 20 mg/kg. However, it is important to consider these studies are generally
designed to understand kinetic uptake mechanisms and explore potential fate processes such as metabolism, and therefore spiking at higher concentrations is essential
to ensure uptake into the plant to observe these changes.
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L. J. Carter et al.
fruits were observed to be influenced by distance from the root, with BCFs for the
first truss of mature tomato fruits (0.024) significantly lower than the fourth truss
(0.058). A corresponding, but not significant, trend for BCF for the first (0.122) and
fourth (0.182) fruits of squash was found.
3.3 Metabolism
A study by Carter et al. [36] on uptake of benzodiazepines, which also highlighted
the importance of the relationship between soil partitioning, ionisation of benzodiazepines and plant uptake, included an analysis of known benzodiazepine metabolites. Results revealed active in-plant metabolism of benzodiazepines, potentially
analogous to the known metabolic transformation pathway of benzodiazepines in
humans. Interestingly, the metabolites detected in the diazepam, temazepam and
chlordiazepoxide treatments were benzodiazepine parent compounds in their own
right. Significant concentrations of nordiazepam were detected in the diazepam- and
chlordiazepoxide-exposed plants, in both soil types, which were in excess of the
concentration reported for the parent compound [36].
Eggen et al. [63] also analysed plant samples for guanylurea, a known metabolite
of metformin. It was only detected in barley grains, bean pods, potato peel and small
potatoes in the range of 2.6–5.7 mg/kg with no relationship between high plant
metformin concentration and the detection of guanylurea. As with the benzodiazepine study, as guanylurea was not detected in the soil, this supports active in-plant
metabolism processes rather than root uptake of guanylurea from the soil. With a
predominant focus of assessing plant uptake and accumulation of pharmaceutical
parent compounds, these studies highlight the need for further research to elucidate
the metabolic pathways pharmaceuticals in plants and to determine whether the
resulting metabolite products retain their bioactive nature and thus pose a risk to
human and ecosystem health.
The last 15 years have generated a wealth of research from both greenhouse and
growth chamber studies where soils have been spiked with a range of pharmaceuticals. Such studies have shown clear differences in accumulation between plant
species and amongst various plant organs. However, where soils have been spiked to
replicate exposure in the environment, the concentrations used in these studies are
often in excess of measured environmental concentrations which have been reported
in the μg – low mg/kg range. For example, Boxall et al. [57], Li et al. [28],
Karnjanapiboonwong et al. [66] and Carter et al. [5, 36] all spiked soils at concentrations of approximately 1 mg/kg, whilst Eggen et al. [63], Eggen and Lillo [69] and
Ahmed et al. [70] reported nominal concentrations for their studies to range between
5 and 20 mg/kg. However, it is important to consider these studies are generally
designed to understand kinetic uptake mechanisms and explore potential fate processes such as metabolism, and therefore spiking at higher concentrations is essential
to ensure uptake into the plant to observe these changes.
192
L. J. Carter et al.