2 Plant Uptake from Hydroponic Solutions
Hydroponic exposure, where pharmaceuticals are dissolved in nutrient solutions and
plants grow suspended either in the solution or in non-reactive media like glass
beads, can offer a mechanistic understanding of plant uptake without the complexities of soil-plant-pharmaceutical interactions. In addition to being ideal systems for
uptake studies, hydroponic studies maintain environmental relevance especially as it
relates to phytoremediation of contaminated water systems.
2.1 Mechanistic Uptake
One of the earliest mechanistic uptake studies was conducted with the tetracycline
antibiotic oxytetracycline in alfalfa (Medicago sativa) [37]. This study revealed that
the uptake into alfalfa from solution followed the Michaelis-Menten equation which
relates the rate of uptake and substrate concentration with a measure of substratebinding affinity called the Michaelis constant (K M ). The uptake of a structurally
different sulphonamide antibiotic, sulfamethazine, in alfalfa was investigated by
Kurwadkar et al. [38], which revealed that the greatest concentrations were detected
in the roots, whilst there was some translocation to other plant portions including
shoots and sap. Interestingly, they found that higher concentrations were measured
in the upper plant tissues compared to lower shoots. Mathews et al. [39] investigated
the uptake and translocation of the antimicrobials triclocarban and triclosan in
11 food crops. Their study revealed that translocation of these two compounds
was limited with maximum translocation from root to shoot of 1.9% and 3.7% for
triclocarban and triclosan, respectively. Concentrations in tubers were also less than
the concentrations in roots.
The use of chemical inhibition of various plant processes has also provided
insights into the uptake mechanisms of antibiotics. Kong et al. [37] followed up
their initial oxytetracycline experiments by showing that decreased uptake
corresponded to plant metabolic inhibition using 2,4-dinitrophenol and that
aquaporin competition with glycerol and silver ions (Ag
+
) had no impact on oxytetracycline uptake. Furthermore, cellular stress resulting from mercury (Hg
2+ )
exposure also reduced uptake. These results suggest that uptake of oxytetracycline
is not passive, rather an energy-dependent process. More recently, Zhang et al. [40]
used the respiration inhibitors salicylhydroxamic acid and sodium azide (NaN 3 ) and
the aquaporin blocker mercuric chloride (HgCl 2 ) to show that the uptake of three
veterinary antibiotics (chlortetracycline, sulphamethoxazole, sulfathiazole) was also
an active process and that the uptake of chlortetracycline and sulfamethoxazole was
associated with aquaporin activity.
14 C radiolabelled compounds have been used to provide detailed insights into the
distribution of pharmaceuticals in hydroponic systems. The benefits of using
radiolabelled compounds include the ability to overcome poor ionisation of some
Uptake and Effects of Pharmaceuticals in the Soil-Plant-Earthworm System
185
Hydroponic exposure, where pharmaceuticals are dissolved in nutrient solutions and
plants grow suspended either in the solution or in non-reactive media like glass
beads, can offer a mechanistic understanding of plant uptake without the complexities of soil-plant-pharmaceutical interactions. In addition to being ideal systems for
uptake studies, hydroponic studies maintain environmental relevance especially as it
relates to phytoremediation of contaminated water systems.
2.1 Mechanistic Uptake
One of the earliest mechanistic uptake studies was conducted with the tetracycline
antibiotic oxytetracycline in alfalfa (Medicago sativa) [37]. This study revealed that
the uptake into alfalfa from solution followed the Michaelis-Menten equation which
relates the rate of uptake and substrate concentration with a measure of substratebinding affinity called the Michaelis constant (K M ). The uptake of a structurally
different sulphonamide antibiotic, sulfamethazine, in alfalfa was investigated by
Kurwadkar et al. [38], which revealed that the greatest concentrations were detected
in the roots, whilst there was some translocation to other plant portions including
shoots and sap. Interestingly, they found that higher concentrations were measured
in the upper plant tissues compared to lower shoots. Mathews et al. [39] investigated
the uptake and translocation of the antimicrobials triclocarban and triclosan in
11 food crops. Their study revealed that translocation of these two compounds
was limited with maximum translocation from root to shoot of 1.9% and 3.7% for
triclocarban and triclosan, respectively. Concentrations in tubers were also less than
the concentrations in roots.
The use of chemical inhibition of various plant processes has also provided
insights into the uptake mechanisms of antibiotics. Kong et al. [37] followed up
their initial oxytetracycline experiments by showing that decreased uptake
corresponded to plant metabolic inhibition using 2,4-dinitrophenol and that
aquaporin competition with glycerol and silver ions (Ag
+
) had no impact on oxytetracycline uptake. Furthermore, cellular stress resulting from mercury (Hg
2+ )
exposure also reduced uptake. These results suggest that uptake of oxytetracycline
is not passive, rather an energy-dependent process. More recently, Zhang et al. [40]
used the respiration inhibitors salicylhydroxamic acid and sodium azide (NaN 3 ) and
the aquaporin blocker mercuric chloride (HgCl 2 ) to show that the uptake of three
veterinary antibiotics (chlortetracycline, sulphamethoxazole, sulfathiazole) was also
an active process and that the uptake of chlortetracycline and sulfamethoxazole was
associated with aquaporin activity.
14 C radiolabelled compounds have been used to provide detailed insights into the
distribution of pharmaceuticals in hydroponic systems. The benefits of using
radiolabelled compounds include the ability to overcome poor ionisation of some
Uptake and Effects of Pharmaceuticals in the Soil-Plant-Earthworm System
185