Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

Detoxification Mechanisms
Lead sequestration in cell walls may be the most important detoxification mechanism in
plants. Calcium may play a role in this detoxification by regulating internal Pb concentrations
through the formation of Pb-containing precipitates in the cell wall (Antosiewicz, 2005). Yang
et al. (2000) screened 229 varieties of rice (Oryza sativa) for tolerance or sensitivity to Pb and
found that the oxalate content in the root and root exudates was increased in Pb-tolerant varieties.
The authors suggested that the oxalate reduced Pb bioavailability, and that this was an important
tolerance mechanism (Yang et al., 2000). Sharma et al. (2004) found Pb-sulfur and Pb-sulfate in
the leaves, and Pb-sulfur in the roots of Sesbania drummondii (Rattlebox Drummond), a Pb
hyperaccumulator plant grown in Pb-nitrate solution. They hypothesized that these sulfur
ligands were indicative of glutathione and phytochelatins, which play a role in heavy metal
homeostasis and detoxification (Sharma et al., 2004).
Sea pinks (Armeria maritima) grown on a metal-contaminated site (calamine spoils more
than 100 years old) accumulated 6Η the concentrations of Pb in brown (dead and withering)
leaves than green leaves (Szarek-Lukaszewska et al., 2004). The concentration of Pb in brown
leaves was similar to that in roots. This greater accumulation of Pb into older leaves was not
observed in plants grown hydroponically in the laboratory. The authors hypothesized that this
sequestering of Pb into the oldest leaves was a detoxification mechanism (Szarek-Lukaszewska
et al., 2004).
Terrestrial invertebrates also mitigate Pb toxicity. Wilczek et al. (2004) studied two
species of spider, the web-building Agelena labyrinthica, and the active hunter wolf spider
Pardosa lugubris. The activity of metal detoxifying enzymes (via the glutathione metabolism
pathways) was greater in A. labyrinthica and in females of both species (Wilczek et al., 2004).
Marinussen et al. (1997) found that earthworms can excrete 60% of accumulated Pb very
quickly once exposure to Pb-contaminated soils has ended. However, the remainder of the body
burden is not excreted, possibly due to the storage of Pb in waste nodules that are too large to be
excreted (Hopkin, 1989). Gintenreiter et al. (1993) found that Lepidoptera larvae (in this case,
the gypsy moth Lymantria dispar) eliminated Pb, to some extent, in the meconium (the fluid
excreted shortly after emergence from the chrysalis).
Lead, in the form of pyromorphite (Pb5(PO4)3Cl), was localized in the anterior pharynx
region of the nematode Ceanorhabditis elegans (Jackson et al., 2005). The authors hypothesized
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