Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 342

variety, external concentration of the element and solution composition (pH, other
elements). Below we briefly discuss these major trends.
Metal or metalloid type, concentration and speciation: The results of Guo and
Marschner ( 1995 ) clearly illustrate that translocation is element-specific. Nickel
showed strong translocation to shoots in bean, curly kale, and to lesser extent, rice,
and was excluded from the shoot of maize, while the opposite was observed for
cadmium. The available data indicate that translocation of mercury to shoots is gen-
erally limited. The results of Göthberg et al. ( 2004 ) suggest that translocation of
lead is relatively large at background concentrations. However, the much larger con-
centration in the leaves than in the stem suggests that lead in the leaves was mainly
derived from aerial deposition. Most likely, translocation of lead from roots to shoots
is limited, since the endodermis acts as a barrier (Sobotik et al. 1998 ), unless under
specific conditions, e.g. after addition of synthetic chelators (EDTA) at high con-
centrations (Luo et al. 2005 ; Tandy et al. 2006b ). Translocation of redox-sensitive
metals or metalloids may depend on the redox state of the element in the pore water.
Hopper and Parker ( 1999 ) found that translocation of selenium was much larger
when the plants were exposed to selenate (Se(VI), SeO 4 2 − ) compared with selen-
ite (Se(IV), HSeO 3 − ) (Table 8.5 ). Wang et al. ( 2002 ) showed that the uptake rate
of arsenic by Pteris vittata was smaller, but the translocation efficiency larger, for
arsenite (As(III), H 3 AsO 3 ) than for arsenate (As(V), HAsO 4 2 − /H 2 AsO 4 − ).
Plant species and variety: Table 8.5 illustrates the large variation of cadmium
translocation to shoots for plants that were grown under similar conditions (e.g. Guo
and Marschner 1995 ; Hatch et al. 1988 ). Gramineae (maize, barley, oat, ryegrass,
cocksfoot, rice) are often shoot cadmium excluders, though this cannot be gener-
alized. For instance, some maize inbred lines show large cadmium translocation to
shoot. Also for the Leguminosae (pea, beans, etc.), the translocation of cadmium to
the aerial parts is usually small, whereas the Solanaceae (potato, tomato), Asteraceae
(lettuce), Brassicaceae (cabbage, watercress) and Amaranthaceas (spinach) show in
general relatively large translocation to the shoot (Kuboi et al. 1986 ).
Pore water composition: As discussed above, the availability of an element will
affect its translocation. For instance, the translocation of essential metals (cop-
per, zinc) is usually larger at small external concentration than at large external
concentration (Fig. 8.6 ). The concentrations of other elements may also affect the
translocation. For instance, Florijn and Van Beusichem ( 1993b ) found that uptake of
cadmium increased with increasing pH, presumably due to diminishing competition
with H + ions for binding on the root surface. The translocation of cadmium to the
shoot increased with increasing pH in the excluder plant species, whereas transloca-
tion was unaffected in the non-excluders (Table 8.5 ). Hatch et al. ( 1988 ) also found
reduction of cadmium uptake with increasing pH, but found no consistent effect of
pH on the translocation factor of cadmium in excluder (grasses) or non-excluder
species. Increasing phosphate concentrations decreased the uptake of As(V), indi-
cating competition for uptake between phosphate and arsenate, but increased the
arsenic translocation to the shoot (Geng et al. 2005 ; Quaghebeur and Rengel 2003 ).
It is clear that many factors may affect the distribution of metals and metalloids
within plants. Plants have a complex network of homeostatic mechanisms, including