Gross mycelial contact is a 'catch-all' term covering interactions that do not involve parasitism, hyphal interference or interaction at a distance. When mycelia meet in agar or soil microcosms, dramatic changes in mycelial morphology occur, both in the vicinity of the antagonist and often elsewhere in the mycelium (Figures 1 and 3; Boddy, 2000; Donnelly and Boddy, 2001). Such changes include production of aerial tufts, barrages, mycelial cords (Figure 1d) and pigment (Figure 1c). Replacement interactions could be divided into those in which the following features are observed: (1) lysis occurs ahead of the advancing mycelium (Figure 1d); (2) overgrowth and through-growth of mycelium with more or less simultaneous death of the weaker antagonist; (3) overgrowth by aggregated mycelial structures with subsequent death of the weaker competitor. Overgrowth should not necessarily be equated with replacement, and must be confirmed, e.g. by isolation. Some fungi are able to overgrow others at the surface (e.g. of agar or compressed soil), but the overgrown fungus remains viable. This is particularly common with Armillaria species which may be overgrown but remain viable within a covering of PSPs. Overgrowth can itself sometimes be an important aspect of antagonism. For example, some cord-forming fungi (Chapter 1), which deadlock with opponents on soil, are able to grow over the opposing mycelium to reach the organic resource from which it is growing (Figure 1a and b). Confrontation within the resource can
Figure 3 Effect of the wood decay fungus Phanerochaete velutina, growing from a piece of wood (PV), on allocation of carbon to the extra-radical mycelium of the Mycorrhizal fungus Paxillus involutus growing in association with Betula pendula (a and b), and on Suillus bovinus in association with Pinus sylvestris in soil microcosms (d and f). (a) and (b) show ectomycorrhizal mycelial growth in the absence of the saprotroph. Plants were pulse-labelled with 14C, and this was quantified in a 20 x 24 cm below-ground area by digital autoradiography (indicated by a white line in b). Autoradiographs are shown in (b), (e) and (f) with the radioactivity scale indicated in (b). In the interaction with P. involutus there were two patches of litter (L) in the microcosm to provide resources for the EM fungus. Note the truncation and browning of the P. velutina cords in contact with P. involutus (small arrow heads) and the deflection of the growth of the saprotroph on the right of the wood block (large arrow head). The mycorrhizal fungus allocates the carbon it receives from the host plant away from the area of territorial combat, and its growth is locally stopped by contact with P. velutina. There was a 60% reduction in 14C allocated to mycelium of S. bovinus when interacting with P. velutina, up to 30 h after pulse labelling. Presence of 14C (0.03%) was detected in P. velutina after 5 days. (a) and (b) Modified from Leake et al. (2002) and (c)-(f) modified from Leake et al. (2001) with permission from Elsevier and Blackwell. (See Colour Section)
then result in replacement within the resource and the subsequent demise of extra-resource mycelium.
Since morphological changes are many and varied, it is likely that there is an equally wide variety of antagonistic mechanisms. Morphological changes during interactions are certainly correlated with differences in physiology, enzyme and toxin production (Griffith et al, 1994a, 1994b, 1994c; Rayner et al, 1994; see above). Changes may also occur distant from the site of interaction irrespective of morphology, and have major impacts on nutrient uptake, distribution and loss, and hence on ecosystem functioning (see below).
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