Fróðskaparrit - 01.07.2004, Blaðsíða 32

30
HABITAT PREFERENCES OF THE ARBUSCULAR MYCORRHIZAL FUNGI
OF THE FAROES COMPARED WITH OTHER PLACES
tection against pathogens (Newsham et al.,
1995a). For the fungi the benefits are obvi-
ous. The AM fungi are unable to obtain car-
bohydrates from other sources than from
the plant root, because they are obligate
symbionts. They have an internal and an
extemal phase. The internal phase consists
of hyphae, arbuscules and vesicles. Arbus-
cules are “tree-like” structures and consist
of highly branched very fine hyphae. Vesi-
cles are storage organs, and are swollen ei-
ther hyphal ends or attached laterally to hy-
phae. The external phase consist of hyphae
and spores. The external hyphae can grow
several centimetres from the plant root, al-
lowing transport of phosphate from outside
the zone of depleted soil that surrounds the
root, and this is considered as the main ben-
efít for the plant. Terrestrial systems are of-
ten limited by phosphorus, which is one of
the most immobile nutrients in soil. There-
fore, under phosphorus limited conditions,
it is very valuable for the plant to be con-
nected to some fungi that with their fine hy-
phae are able to explore the surrounding
soil for nutrients much better than the
plants relative coarse roots can.
AM fungi disperse with external hyphal
growth. However they also produce spores,
and while the external hyphae can be con-
sidered as dispersal through space, then the
spores can be considered as dispersal
through time (Fitter and Garbaye, 1994).
One of the confusing things about AM
fungi is the taxonomy. By definition the
taxonomy is done from spores which can
be found in the soil. However, several stud-
ies have shown, that the spores extracted
from the soil do not reflect the fungi found
colonising the roots (Clapp et al., 1995;
Merryweather and Fitter, 1998). Some fun-
gi produce many spores, other few or none.
Therefore in ecological studies the question
of diversity might be of more importance
than the taxonomy.
The accepted method of identifying
these fungi is to describe their spores, and
hitherto are less than 200 AM fungal
species in total described (Morton and Ben-
ny, 1990). This is probably the main reason
behind the belief that the AM fungi are gen-
eralists, and able to colonise nearly whatev-
er plant species is present. However, more
and more evidence is revealed indicating
that this is not the case. For example AM
fungi, if not specifíc in their host choice,
are at the least selective (Fitter et al., 2004).
Further, a profound difference has been
found in the occurence of the AM fungal
species at low and high altitude in the
Faroes (this study).
If only 200 AM fungal species are able to
colonise around 200,000 plant species we
would indeed have generalistic fungi. If it
is so we should expect the same fungi to be
found worldwide, with only slightly con-
sideration to habitat. The preliminary fínd-
ings from my molecular work is that the
fungal types found in a root varies with
both host plant, altitude and aspect; or host
and habitat, but which or these is the main
factor determining the arbuscular fungal
tlora?
Material and Methods
Material
The planl species used in this study is