Distance-related semi-quantitative estimation of the extramatrical ectomycorrhizal mycelia of Cortinarius obtusus and Tylospora asterophora

Extramatrical mycelia of ectomycorrhizae (ECM) can markedly differ in their density, organization and extension as well as in their biomass. As these mycelia are very important for nutrient uptake, are sinks for carbohydrates and probably act as potential agents to interfere with mycelia of other ECM, a method allowing calculations of density, extension and amount of extramatrical mycelium could further ecological studies at different scales. As a first step synthesized ECM of Cortinarius obtusus and Tylospora asterophora, which represent two different ‘exploration types’ — T. asterophora belongs to the 'short distance exploration type’ and C. obtusus to the ‘medium distance exploration fringe subtype’ — were compared with respect to their extramatrical mycelia. To investigate the mycelial distribution in detail, flat rhizotrons filled with peat were used to synthesize the ECM on roots of Picea abies (Norway spruce). As both species form white mycelia, they contrast conspicuously to the dark brown peat substrate. This made it favourable to use CD-camera photographs and a consecutive image analysis of the bright mycelium in comparison to the dark background. For that, nine distance areas have been defined as the regions between nine concentric lines placed around the mantle surface of the ectomycorrhizal root (EMR). The distances between these lines were fixed in a regular manner as the square multiples of roughly one diameter of the EMR (0,27 mm). This resulted in lines representing distances of 1, 4, 9, 16, 25, 36, 49, 64, and 81 multiples of the EMR diameter. The distance of 81 EMR diameters equals roughly a real distance of app. 20 mm from the surface of the EMR. The studies showed highly significant differences in mycelial density between the four innermost distance areas, i. e. areas up to a distance of 16 EMR diameters. This applied for both species. Significant differences were found in the same areas when both species were compared. It was shown that C. obtusus still forms a considerable amount of mycelium beyond the 49 distance line, where the substrate was covered by 15–20 %. However, a limited data set did not allow any statistical calculations at these positions. In contrast to C. obtusus, the amount of extramatrical mycelium of T. asterophora decreased more rapidly with increasing distance. Mycelium could not be detected with this method beyond the 49 distance line. We conclude that these two species differ considerably in the amount and extension of their mycelia and this may be in line with their assignment to the respective exploration types. Further studies are needed to substantiate these results with additional species representing the full set of known exploration types. Envisaged correlation studies of these results with mycelial biomass by direct measurement of hyphal length or via quantitative PCR, can possibly result in an estimation of extramatrical mycelia of ECM in natural soil.