Oxygen and the Ecology of Armillariella Elegans Heim

Abstract

Oxygen is important in the ecology of A. elegans because it affects both the rate of growth and form ofrhizomorphs. Maximum growth is dependent on high rates of oxygen diffusion within the central canals of rhizomorphs but partial pressures of oxygen in excess of 0·04 atm in contact with the outside surfaces of rhizomorphs are inhibitory.In long rhizomorphs, the rate of diffusion is maintained because of repeated connections that are made between the central canals and substrate-air interfaces. In the absence of such interfaces, rhizomorphs grow for about 20 cm when the rate of diffusion of oxygen to the apices becomes limiting. Then the rhizomorphs flatten and become plaque-like and their growth slows. Such rhizomorphs are well adapted for absorbing nutrients from a host but not for spreading from host to host.Partial pressures of oxygen in excess of about 0·04 atm in contact with the outsides of rhizomorphs stimulate the activity of p-diphenol oxidase. This enzyme catalyses the formation of a brown pigment in the rhizomorphs and circumstantial evidence indicates that when this pigment forms in the apices of rhizomorphs it inhibits their growth. This pigment overlays the walls of the cells and probably prevents either the uptake of nutrients or the disposal of waste products by the cells.In soil, oxygen partial pressures are usually above 0·04 atm but rhizomorphs continue to grow if water films form around their apices. Oxygen diffuses slowly in these films and the rhizomorphs reduce the partial pressure of oxygen at their surface to below 0·04 atm. The formation of such water films depends on the moisture content, texture, and bulk density of the soil. By altering these properties and preventing the pathogen spreading from host to host, the disease caused by A. elegans may be controlled.Carbon dioxide had little effect on the initiation and growth of rhizomorphs except that partial pressures between about o· Ol and 0·07 atm were slightly stimulatory. Because this is the range of partial pressures normally found in soil and hosts, carbon dioxide is unlikely to significantly affect A. elegans.

Association of Rhizomorph Formation with Laccase Activity in Armillaria spp.

“…Our initial observations suggested a close association between laccase activity and rhizomorph formation. The results reported here indicate that laccase may be responsible not only for eventual inhibition of rhizomorphs, as Smith & Griffin (1971) suggested, but also for their induction and/or growth. (Vahl : Fr.…”

“…Such treatment reduced laccase activity (as indicated by less browning of gallic acid under the same conditions). Laccases of wood decay fungi, including Armillaria spp., are known to be more sensitive to O2 deprivation than is mycelial growth (Smith & Griffin, 1971;Worrall & Parmeter, 1983). Inverting the agar and overlaying cultures with water agar had a similar effect which was probably not due to C 0 2 accumulation as C 0 2 has been shown to have very little or a slight stimulatory effect on Armillaria spp.…”

“…O2 is required for the initial stage of morphogenesis of pseudo-or microsclerotia, which precede rhizomorph formation (Lopez-Real & Swift, 1977). Higher O2 concentrations at the point of origin of rhizomorphs support greater growth (Smith & Griffin, 1971), and rhizomorph production in soil is less when the O2 content of the atmosphere above the soil is reduced (Rishbeth, 1978).…”

“…Indeed, Smith & Griffin (1971) found that O2 partial pressures above 0.04 atm at the rhizomorph surface are inhibitory, although growth requires high concentrations of O2 at the rhizomorph origin or at connections which occasionally form between the rhizomorph canal and substrate-air interfaces. A brown pigment, formed by the action of laccase (EC 1 .…”

“…l o . 3.2), was thought to inhibit growth in the presence of high O2 concentrations by blocking either nutrient assimilation or excretion of waste products (Smith & Griffin, 1971).…”