Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 343

transport, chelation and sequestration, in order to maintain the concentrations of
essential elements within the physiological limits and to minimize the detrimen-
tal effects of non-essential elements. The exact mechanisms are only starting to be
unravelled. Most molecular insights obtained so far are on the cellular level, and
very little is known about mechanisms controlling metal distribution on the level
of the plant (Clemens 2001 ). The transport of metal(loid)s from roots to shoots is
probably largely through the xylem. Guo and Marschner ( 1995 ) showed that the
cadmium and nickel concentrations in the shoot dry matter were positively corre-
lated with the concentrations in the xylem sap. The soluble fraction of cadmium in
the roots was much larger for maize than for the other plant species tested, which
was in agreement with its higher mobility in the plant (Table 8.5 ). Similarly, the
higher soluble fraction of nickel in the roots extracts of bean compared with maize
was in accordance with the higher nickel mobility in bean plants.
Phytochelatins (PCs) are metal-complexing peptides that play an important role
in metal tolerance. The possible roles of PCs in heavy metal detoxification and
homeostasis have been reviewed by Cobbett and Goldsbrough ( 2002 ). Phytochelatin
synthesis is induced upon exposure to a variety of metals and metalloids (Grill et al.
1989 ). Overexpression of phytochelatin synthase was found to increase tolerance
to cadmium, mercury and arsenate (Vatamaniuk et al. 1999 ). In some plants, sul-
phides also seem to play a role in the detoxification of cadmium by PCs. It was
shown that phytochelatin-cadmium complexes of tomato grown at high cadmium
concentration (100 M) contained cadmium-S-peptide aggregates of ca. 2 nm diam-

eter that consisted of a CdS crystallite core coated with PCs (Reese et al. 1992 ).
Phytochelatins may be sequestered in the vacuole (Salt and Rauser 1995 ; Vogeli-
Lange and Wagner 1990 ), but they may also be transported in the xylem (Gong
et al. 2003 ). Translocation of cadmium in the xylem has been found to be indepen-
dent of PC production (Florijn et al. 1993 ; Salt et al. 1995 ). Limited translocation of
cadmium from shoot to root seems therefore not to be related to the presence of PCs,
but is most likely due to sequestration of metals in the vacuoles of root cells, either
as complexes with PC, as free ion or in another form. The transport from leaves to
other plant tissues (e.g. grains, tubers) can occur in phloem only. For instance, cad-
mium in potato tubers and peanut kernels is not directly taken up from the soil, but is
first transported in the xylem to the shoot, and then back down through the phloem
(Popelka et al. 1996 ;Reidetal. 2003 ). Page and Feller ( 2005 ) showed that nickel
and zinc were redistributed from older to younger leaves in wheat plants, indicating
high phloem mobility, whereas Mn remained in the old leaves. Also split-root exper-
iments and foliar application of 65 Zn demonstrated significant phloem transport of
zinc from leaves to other plant parts (Haslett et al. 2001 ).
Differences in translocation of metals are an important factor determining the
concentrations in plant tissues. The selection of plant species or cultivars with rela-
tively small or large (in case of essential elements) concentrations in the harvested
products can be used to manage metal concentrations in food crops. For instance,
McLaughlin et al. ( 1994a ) showed that potato cultivars grown commercially in
Australia exhibited significant, nearly two-fold, differences in tuber cadmium con-
centration. Plant-breeding can also be an important tool to reduce the concentrations

μ