Environmental Engineering Reference
In-Depth Information
(1b) Facultative metallophytes: genotypes or ecotypes/subspecies of common
species with a specific tolerance to metals. They also occur in distinct non-
metal-enriched phytogeographical areas. The highly specialised ecotype, sub-
species or genotype is dependent on the occurrence of specific metals in the
soil. Examples are: Armeria maritima s.l. (Baumbach & Hellwig
2007
; Baumbach &
Schubert
2008
), Minuartia verna s.l., Silene vulgaris (Ernst
1974
) and Thlaspi
caerulescens s.l. (Koch et al.
1998
).
(2a) Associated metal-tolerant species: matrix species that are associated with
the related plant association with a large ecological amplitude. They are either
called 'pseudo-metallophytes' or 'accompanying species' of the true metallo-
phyte vegetation. These species are moderately tolerant of heavy metals
in soil, but not dependent on their presence. Examples of such species which
are both common and have a wide geographic distribution are: Achillea
millefolium, Campanula rotundifolia, Euphrasia spp., Plantago lanceolata, Polygala
vulgaris, Ranunculus acris, Rumex acetosella, Thymus pulegioides, Agrostis capillaris,
Holcus lanatus and Phragmites australis.
(2b) Associated non-metal-tolerant species from related associations, with
little or no metal tolerance, the so-called 'indifferent' or 'accidental' species:
these are usually weedy species, often annuals, showing neither vigour nor
persistence on metalliferous soils.
Metallophytes can occur as a mosaic of patches in other vegetation classes:
especially in nutrient-poor grasslands. The once very extensive alluvial tertiary
metal vegetation in the Geul valley of the Netherlands and Belgium, for
example, is a mixture with the association of Festucetum-Thymetum serpilli, char-
acteristic for sandy soils, within the class Koelerio-Corynephoretea (Weeda et al.
2002
), whereas the calaminarian grassland grows on clay soil on the riverbank
(cf. Ernst
1978
).
Ecophysiology of metallophytes
Up to now, no investigations have detected any specific metabolites in metal-
tolerant ecotypes. Metal tolerances are due to differential gene activities which
are up- or down-regulating enzymes. In the case of metal uptake into the roots,
there is a down-regulation of the high-affinity phosphate transporter in
arsenic-tolerant plants (Macnair & Cumbes
1987
) or the elevated expression of
Zn transporter genes (Assun¸˜o et al.
2001
). Once the metal is in the cell, metal-
tolerant plants have modified the activity or the metal affinity of enzymes in
such a way that a surplus of heavy-metal ions is rapidly removed from the plant
cell metabolism to prevent physiological damage. These processes are metal-
specific (Ernst et al.
1992
,
2008
; Clemens
2001
). Examples are the over-expression
of the metallothionein gene MT2b in Cu-tolerant ecotypes of Silene vulgaris and
S. paradoxa (Van Hoof et al.
2001
;Mengoniet al.
2003
), cis-regulatory changes and
triplication of the heavy-metal ATPase gene HMA4 in the Zn-hyperaccumulator
