Lead [Pb, 82]
175
Plants may uptake Pb from water, soils, and air, and its contents in plants is highly
influenced by environmental factors. Lead is slightly available to plants, and strongly
governed its species in soils and plant factors. Several soil properties, such as SOM,
cation exchangeable capacity (CEC), pH, and granulometric composition, as well
as root surface areas and root exudates play a significant role in the Pb phytoavailability. Lead uptake by plants is passive, mainly by root hair, and it is stored to a
considerable degree in root cell walls. Nevertheless, Pb contents of plants highly correlate with its levels in soils, and thus its plant content is very useful for geochemical
prospecting (Kovalevsky vide Kabata-Pendias 2011). A variable Pb absorption from
its atmospheric deposition by plants was observed, which was three times higher in
spruce forest (90 mg/kg) than in pasture grass (30 mg/kg).
Although there is no evidence that Pb is essential to plants, there are some reports
on the stimulating effects on plant growth of some Pb salts, mainly Pb(NO 3 ) 2 .
Nevertheless, some plant species, ecotypes, and bacterial strains are able to develop
Pb-tolerance mechanism. This tolerance seems to be associated with the properties
of membranes. Lead may be strongly bound to cell walls, mainly due to the sorption
by pectic acid (Lane et al. vide Kabata-Pendias 2011). Also, the galacturonic acid
reveals an affinity to fix Pb (Polec-Pawlak et al. 2007). Lead has a marked influence
on the elasticity and plasticity of cell walls, resulting in their increased rigidity.
Plants can relatively easily uptake Pb from soil and nutrient solutions. Large
amounts of Pb are concentrated in the roots, but some amounts, at about 3% of
Pb root content, are transported to the above-ground biomass. Pb uptake by roots
is mainly intracellular and it may be aggregated in vacuoles (Meyers et al. 2008).
The main process responsible for Pb accumulation in root tissues is its deposition
along cell walls, as Pb pyrophosphate and other Pb precipitates. Similar Pb deposits
observed in roots, stems, and leaves suggest that it is transported and deposited in a
similar manner in all plant tissues. Lead accumulated by plants, after the decay of
this plant biomass, may be more easily available to other plants. Availability of Pb
in soil to plants increases from soils with low soil pH and CEC, as well as with low
contents of SOM, clay minerals, Fe hydroxides, and P compounds. Plants grown on
Zn–Pb waste deposits containing Pb up to 22,265 mg/kg contain this metal up to
117 mg/kg (Wójcik et al. 2014).
There are several interactions between Pb and both major and trace elements. The
most serious interactions are as follows:
r Pb–Zn, antagonism adversely affects the translocation of each element
from roots to tops.
r Pb–Cd, stimulating effects of Pb on Cd uptake by roots may be a secondary
effect of the disturbance of transmembrane systems.
r Pb–Ca, occurs due to Pb inhibition of some enzymes associated with Ca.
r Pb–P, low Pb phytoavailability is due to the formation of insoluble
Pb–phosphate compounds
r Pb–S, Pb transfer from roots to top is higher at low S contents.
Airborne Pb, its major source of pollution, is also readily taken up by plants through
foliage. Thus, its concentrations are always higher in older parts of plants than in