Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 337

Table 8.4 The net statistical effect of soil pH on cadmium concentrations in vegetables based on
a survey of crops and associated soils in contaminated and non-contaminated soils in Belgium and
the Netherlands with pH roughly between 5-7. The effects given are all statistically significant at
P ≤

0.05

Average factor decrease in crop Cd
per unit pH increase at constant
soil Cd Crop n

Potato tuber 239 1.2

Endive 76 2.1

Leek 139 1.9

Bean 48 2.1

Scorsonera 52 2.8

Lettuce 170 1.4

Spinach 96 1.1

Carrots 192 1.3

Maize 185 1.2

Celery 103 1.6

Grass 907 1.1

Overall 1.1-2.8, mean 1.6

Jansson et al. ( 2007 )

differs dramatically from that in the bulk soil, in terms of concentrations of met-
als, accumulation of dissolved organic matter, accumulation of Ca 2+ ions and pH
gradients (McLaughlin et al. 1998 ). Secondly, nutrient solution culture has also its
limitations as a model system for separately studying ion interactions: the ion activ-
ity ratios in nutrient solution are not necessarily identical to those at the root surface
where the free metal ion activity is a complex function of uptake rate, diffusion
and metal-ligand dissociation rate (see above). Thirdly, interactions in soil are usu-
ally complex with several factors involved. The effects of pH are interpreted and
modelled as H + :M 2+ competition at the biological membrane, while the physiol-
ogy behind this relationship is unclear. Modifying soil pH alters the availability of
many elements that also can compete with the metal uptake process and it becomes
tedious (and difficult) to take all such interactions into account. Finally, BLM mod-
elling is not yet of practical use for Risk Assessment, both because it lacks precision
(Antunes et al. 2006 ) and it requires numerous parameters (see below).

8.3.1.4 Translocation of Metals and Metalloids in the Plant

The long distance transport of solutes in plants takes place in the vascular system of
xylem and phloem. Translocation is an important process in determining trace metal
concentrations in plant tissues. For instance, Florijn and Van Beusichem ( 1993a )
found that internal distribution rather than root uptake explained the genotypic dif-
ferences in cadmium accumulation in shoots of maize inbreds. Figure 8.5 shows
three typical patterns of metal partitioning between shoots and roots. In Fig. 8.5a ,the
root and shoot concentrations increase proportionally and are of similar magnitude