provide a fail-safe approach for ensuring seed set occurs in
restored orchid populations [18].
Mycorrhizal biology
Orchid-mycorrhizal biology has a long scientific history
beginning in the 19th century, with several wide-ranging
reviews of the biology, ecology and fungal associates of
orchids (e.g. Refs [19–21]). Whereas most orchids investigated
have an association with fungi, only some studies
demonstrate ecological specificity or the ecological reliance
of the orchid for a fungal associate. The nature of this
association, although historically considered mutualistic,
hasmore recently been recognised as a ‘unilateral’ relationship
in favour of the plant [22]. Understanding orchidmycorrhizal
relationships and the functionality of the
association involves research based on isolation of fungus
fromsingle pelotons or tissues and subsequent growth of the
fungus in axenic culture [23]. In vitro germination experiments
[24] and fungal ‘baiting’ assays [25] provide evidence
of potential and ecological specificity of the fungus.
Linked with traditional fungal morphological assessment
[26] or, more recently, the application of molecular
approaches [27,28], particularly phylogenetics, inferences
can now be made into the nature and breadth and/or
specificity of the fungal associates. However, some caution
is needed in extrapolating from assessing presence of a
fungus to ascribing functional importance to the same
organism. For example, direct polymerase chain reaction
(PCR) from infected tissues or sequencing of pelotons
reveals the phylogenetic breadth of the associated fungus
but provides limited indication of ecological function or
benefit of the mycorrhiza to the plant. Specificity must
assume functionality, which can only be determined
through testing of Koch’s postulates. The methods most
commonly used to determine fungal identity, diversity,
presence and distribution are summarised in Table 1.
We suggest that to obtain a more accurate picture of
mycorrhizal specificity and functional significance, these
methods should be integrated to illustrate functional
benefit to the plant of the mycorrhizal fungus.
The link between specificity and fungal diversity,
reinforced by the use of molecular data, is a potent tool
for conservation planning in orchids. Recent research
reveals the monophagous nature of terrestrial orchids
[24] whereas other studies have demonstrated that some
orchids have a wide variety of host choices spanning
different fungal families (see Ref. [29]). Furthermore, the
ectomycorrhizal associations of achlorophyllous orchids
highlight the pivotal ecological role of certain heterobasidiomycete
associates [21,30].