Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 344

of potentially harmful elements, such as cadmium. For instance, low-cadmium
durum wheat cultivars and sunflower hybrids have been developed for this purpose
(Grant et al. 2008 ).

8.3.2 Foliar Uptake of Metals

In air, non-gaseous metals and metalloid are associated with small particulates
(<10 m) that can be deposited on plants. These aerosols may adsorb to plants

or eventually also be absorbed and the metals and metalloids translocated. The
mechanisms of the absorption processes are still unclear. Washing or even peel-
ing vegetables in preparation for cooking does not fully remove the airborne metals
(Dalenberg and Van Driel 1992 ). As a result, airborne metals can be a significant
source of metals in the food chain. As shown below, this fraction can be even more
important than that derived from soil.
Metals may also enter plants and the food chain through gaseous exposure
through soil emissions (e.g. mercury). Little is known of the potential for uptake
of metals by plants through gaseous routes of exposure (as opposed to aerosol or
particulate exposure). Mercury (Hg) can be present in air as gaseous forms (Hg ◦ )
or as particulates (Hg-P). Plants can absorb mercury from soil via root uptake, from
airasofHg ◦ via stomatal uptake and from air via adherence of Hg-P similar to
the mechanisms described above. Plants can also be a source of mercury, releasing
mercury when grown in low air mercury and high soil mercury. As a result, there is
a so-called compensation point, the air concentration where no net flux of mercury
vapour occurs and this point increases as soil mercury concentration increases (e.g.
Ericksen and Gustin 2004 ).
There are very few data on uptake of airborne metals or metalloids and the
methodologies to assess these are limited. Harrison and Chirgawi ( 1989 ) estimated
the atmospheric contribution from the differences in metal (cadmium, chromium,
nickel, lead and zinc) concentrations between plants grown in cabinets with either
filtered or unfiltered air. An alternative method uses soils enriched with stable or
radioactive isotopes and the isotope dilution in the crop as the basis for estimating
airborne metal contribution (Mosbaek et al. 1989 ; Tjell et al. 1979 ). Dalenberg and
Va n D r i e l ( 1990 , 1992 ) combined both methods to reduce the uncertainty related
to the isotope ratio of bioavailable metals in soil. Finally, surveys of soil, plant
and atmospheric concentrations combined with statistical tools allows a statistical,
indirect, estimate of the fractions metal derived from atmosphere and soil (Voutsa
et al. 1996 ). A compilation of data for cadmium and lead shows that most (gen-
erally >80%) of the lead in above-ground plant tissues of crops derives from the
atmosphere (even in washed plant tissue), whereas variable results are obtained for
cadmium (Table 8.6 ). The larger percentages for lead than for cadmium are related
to the lower availability of soil lead relative to cadmium (largest sorption for lead).
The airborne percentages of nickel and chromium are intermediate between cad-
mium and lead whereas the percentages for zinc are similar to those of cadmium
(Harrison and Chirgawi 1989 ). The statistical method applied in an industrialized

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