Environmental Engineering Reference
In-Depth Information
Arabidopsis halleri (Hanikenne et al.
2008
), the over-production of histidine in
Ni-tolerant Alyssum spp. (Kramer et al.
1996
) and the enhanced phytochelatin
synthesis in As-tolerant Holcus lanatus (Bleeker et al.
2006
). A high rate of Cd and
Zn translocation from roots to shoots is essential for metal hyperaccumulation
and differs between ecotypes of Thlaspi caerulescens (Xing et al.
2008
). Within the
leaves, metals have to be allocated to different cell types, showing a preference
for epidermal cells (Chardonnens et al.
1999
). Finally, there is a restricted metal
transport into the seeds (Ernst
1974
), so that the young seedling is not already
loaded with metals. All these different aspects of metal metabolism can explain
that an exposure of metal-tolerant plants to metals results in the modification
of hundreds of enzymes, as evidenced by transcriptomes and proteomes
(Tuomainen et al.
2006
;Weberet al.
2006
;Hammondet al.
2006
). These multiple
reaction patterns indicate that there is still a long way ahead for understanding
all aspects of metal tolerance mechanisms (Clemens et al.
2002
).
As established by Mendelian genetics, the number of genes necessary for
tolerance to cadmium, copper and zinc per se are two for each element, with
many modifiers determining the degree of metal tolerance (Broker
1963
;
Macnair et al.
1993
; Schat et al.
1996
; Bert et al.
2003
). In addition to any
prevailing metal toxicities, metallophytes have also to adapt to other extreme
chemical and physical soil factors (Baker
1987
), such as dry soils, by structurally
enhanced proline levels (Schat et al.
1997
), differences in calcium status
(Zhao et al.
2002
), iron availability (Lombi et al.
2002
) and sulphur supply (needed
to synthesise adequate amounts of metal-binding compounds) (Ernst et al.
2008
).
The low availability of the major nutrients nitrogen and phosphorus (Ernst
1974
), characteristic of open oligotrophic environments, requires metal-tolerant
plants to evolve a high degree of major nutrient efficiency, especially on second-
ary and most tertiary metal-enriched sites. Heavy-metal-resistant ecotypes do not
occur on non-polluted soils. As most species in ecosystems with moderately
vegetated soils, metallophytes are sensitive to shade (Schubert
1953
;Kakes
1980
). The most shade-sensitive species is Minuartia verna, already disappearing
during vegetation succession on metalliferous soils (Ernst
1964
,
1974
,
1976
).
Examples of populations of Thlaspi caerulescens which have managed to maintain
stable populations on small metal-enriched, shadowed patches in woodlands
are at Aberllyn zinc mine (N. Wales) and at the Silberberg near Osnabruck
(Germany). Mechanisms of adaptation are energy-expensive, and plants that
can tolerate high concentrations of heavy metals are thus weak competitors
(Wilson
1988
;Ernstet al.
1992
).
Most species of heavy-metal plant communities have a symbiosis with arbus-
cular mycorrhizal (AM) fungi, which by binding metals in the fungal cells
prevents the host from damage (Griffioen et al.
1994
; Pawlowska et al.
1996
;
Hildebrandt et al.
1999
; Tonin et al.
2001
; Turnau & Mesjasz-Przybylowicz
2003
;
Whitfield et al.
2004
). Colonisation by AM fungi is almost absent in M. verna,
