plant surface microbiology.pdf

19 Functional Diversity of Arbuscular Mycorrhizal Fungi on Root Surfaces 339
4.1 Diversity of Arbuscular Mycorrhizal Fungi Inside Roots
The benefit to plants through enhanced P uptake is expected to be markedly
altered according to the effectiveness of dominant AM fungi inside roots.
However, species diversity of AM fungi has generally been examined from the
perspective of presence or absence of fungi in soil, not in roots (van der Heijden
et al. 1998a; Bever et al. 2001). There is little evidence of a simple relationship
between the relative abundance of morphotypes of AM fungi inside roots
and spores in soil (Scheltema et al. 1987; Merryweather and Fitter 1998a). The
effectiveness of AM fungi in taking up P in these studies is generally not considered,
but neither is it easy to determine, particularly as effectiveness can
change for the same fungus depending on the P status of the soil and plant
(Thomson et al. 1986). The diversity of AM fungi needs to be investigated further
in relation to their relative abundance inside the roots (Abbott and Gazey
1994). In addition, the methods applied to selecting combinations of species
of AM fungi in studies of the role of species diversity need to be considered
carefully (Wardle 1999). Another consideration is that AM fungi form associations
with plants that differ markedly in their growth habits and susceptibility
to colonisation, and this makes assessment of the impact of diversity of
AM fungi even more difficult to predict or measure at an ecosystem level.
The diversity of AM fungi could be relevant to communities of AM fungi in
the same way that high plant species diversity may help stabilise plant community
structure and ecosystem processes (Klironomos et al. 2000; Tilman
1996). However, high diversity can also lead to competitive exclusion and
cause a reduction in the number of co-existing species (Huston 1994). For AM
fungi, this may only reduce the abundance of some species below levels of
detection (Bever et al. 2001). The competitive ability of species of AM fungi in
roots is likely to be an important factor in determining the dominance of AM
fungi inside roots as well as in soil. This is compounded by seasonal changes
in infectivity of AM fungi (Merryweather and Fitter 1998b), which is influenced
by fungal life cycles (Abbott and Gazey 1994) such as sporulation and
associated changes in infectivity of the hyphae (Pearson and Schweiger 1993).
As AM fungi occur as communities in soil and in roots, the extent to which
they are likely to collectively contribute to P uptake (Jakobsen et al. 2001;
Solaiman and Abbott 2003) depends on the mycorrhiza dependency of the
host plant (van der Heijden et al. 1998b). Although there have been intensive
studies of single AM fungus function, there have been few investigations of
the contributions of communities of AM fungi. Reconstructed communities
of AM fungi in soil can promote plant growth (Daft 1983; Daft and Hogart
1983), or have no effect on plant growth (Sylvia et al. 1993). A correlation
between the occurrence of AM fungal morphotypes and seasonal P uptake for
AM fungi in a natural ecosystem was observed (Merryweather and Fitter
1998b), which may be related to differences among fungi in the functional
characteristics of hyphae they form in soil (Smith et al. 2000).