Historically, the orchidacean type mycorrhiza was described as a certain group of organ- and tissue-specific fungal infections exhibiting definite histological picture. Fungi occurring in subterranean organs other than sensu stricto roots, or in tissues other than the secondary cortex were excluded per definitionem.

The trouble started when the French scientist Noël Bernard found old, mouldy pods of the orchid Neottia nidus-avis L. in which seeds were germinating. He jumped into the conclusion that infection by fungi present in orchid roots is an essential precondition to orchid seed germination.

With all the respects due to Noël Bernard, his observations on the germination of Neottia nidus-avis L. made on 3rd May in the Fontainebleau forest and published on 15th May 1899, i. e. mere twelve days later, cannot be taken as proven proofs of a hypothesis he drew from them.

Especially not if one happens to know that starting in 1876 with A. de Bary, who coined the term symbiosis precisely to describe what he saw in the roots of this orchid and, in spite of not reproducible claims in the academic literature, nobody was able to isolate the microbiont of Neottia nidus-avis L. to date. Which, in turn, means that in the very orchid playing cardinal role in the history of orchid mycorrhiza research, Bernard’s hypothesis experimentally proven is not - even today.

The primitive methods used in the early attempts to isolate microbionts from other orchids yielded a wide variety of microscopical fungi. Based on the presupposition that a proper orchid symbiont isolated from the roots of an adult plant should promote germination and/or development of the same orchid species, "symbiotic germination experiments" were performed.

Although it is questionable why should the clearly organ- and tissue-specific infections of adult plants behave exactly in the same ways in minute orchid protocorms, this question was glossed over. It should not have been, because a number of isolates failed to live up to the expectations. From not penetrating protocorms at all to killing them outright in no time, a colorful spectrum of "specific", "not-so-specific", "symbiotic" and "pathogenic" combinations plus lengthy disputes resulted from these experiments. The dismal results did not deter devotees and experiments of this kind are dutifully repeated time to time ever since.

Because fortunately or unfortunately, some isolates did and do behave in the way they were expected to.  Specifically, in the experiments of Noël Bernard, three  Rhizoctonia  isolates, namely R. lanuginosa, R. mucoroides and R. repens did promote the germinaton and early development of some orchids.

Under the microscope, the appearance of the typical, so-called orchidacean type mycorrhiza is quite characteristic. This microphotograph shows the cross-section of the root of Ophrys sphegodes Mill. - at a relatively low, about 100 times magnification.

Orchid roots are covered with a special tissue called velamen occurring only in a very few non-orchid plant families.

This special tissue is followed by a layer of cells called endodermis (= "internal skin") then comes a layer consisting of cells featuring thickened cell walls. In cross-sections these thick cell walls appear to be U-shaped, open towards the external root surface.

The fourth layer is called primary cortex. The relatively thin tissue areas mentioned in this paragraph are located between the velamen and the next , in our present context the most relevant layer called secondary cortex. This is the region where the symbiotic fungi form coils called pelotons - the darker spots on the image.

Finally, in the middle there is the central vascular bundle, transporting water and nutrients to the upper organs (stem, leaves and flowers) of the plant.

In a normal orchidacean type of mycorrhizal association the fungal coils disintegrate, leaving behind only a relatively small amount of materials, which cannot be metabolized by the orchid host cell.

To put it differently, in a mycorrhizal association the fungal partner dies without causing any damage to the higher plant visible to the naked eye (!), in contrast with parasitic associations where fungi visibly damage at least certain tissue regions of the higher plant's root.

On these images stages of this disintegration can be seen, numbered 1 - 6 in an enlarged part of the same Ophrys sphegodes root cross-section.

To the right pelotons already isolated from the root cells of Epipactis helleborine offer a perhaps clearer impression of this process.

The disintegration of fungal coils in root cells followed by the absorption of materials released from the pelotons by the root cells is usually termed as "digestion". In this case "digestion" is an utterly unfortunate, anthropocentric and, hence, misleading concept - eminently responsible for the sorry state of mycorrhiza research up to today.

The intricate role of this phenomenon in the early stages of orchid seed germination are far beyond the scope of this brief treatment, but inquiring into the problem in case of an adult orchid might shed some light on the true nature of orchidacean mycorrhiza.

Studying the very same Ophrys root in a SEM (Scanning Electron Microscope) equiped with EDXRA (Energy Dispersive X-Ray Analysis) facility apart from providing additional visual information concerning the finer morphology of infected cells and fungal pelotons, measuring the concentrations of elements possible in volumes as small as 1 cubic micron.

(In the current example this means that in a single, roughly 40x40x40 microns root cell at least theoretically 64,000 points could be analysed!)

Results of energy dispersive X-ray analysis performed on a single, representative cross-section of this Ophrys sphegodes root are presented in simplified form on the graph here.

The non-infected secondary cortex and the central vascular boundle show distribution patterns of elements common in higher plants. The presence of aluminium (Al) and silicon (Si) should be noticed here: on EDXRA spectra they cannot be missed, both elements are normally present in higher plants in non-negligible concentrations.

Note also the "absence" of the important micronutrient zinc (Zn) in these areas although in the soil surrounding the root the concentration of zinc was quite normal, around ~16 µg Zn / g soil.
In the infected area zinc is present in an amount nearly equalling the amount of Fe (iron), Ca (calcium) content is high while K (potassium) content is low. The actual potassium/calcium ratios were 1.21 in the non-infected secondary cortex, 1.56 in the central vascular boundle and 0.31 in the infected area.

All the differences listed above are in accordance with the sub-cellular morphological features of infected cells, especially with the heavily influenced membrane structures. The virtual "leakiness" of the mycorrhiza-harbouring area is pronounced in both ways, i.e. potassium-loss and easy penetration of chloride ions (Cl) was observable. (The high amount of chlorine in the sample is an artefact to be attributed to the sterilisation procedure and, at the same time, a proof of the increased permebility as well.)

In the infected area, with the exception of K (potassium) all the other elements were present in amounts higher than in the non-infected tissue zones. The net result was that the overall mineral content - in this actual case - was higher by about 40% in the mycorrhiza-harbouring tissue regions than in the non-infected areas.

While certainly neither the actual figures measured in this experiment nor probably the changes in ratios of macronutrients can be generalised the definitely increased rate of nutrient transport, i. e. the marked increase in metabolic activity of tissue regions harbouring mycorrhiza resulring in significantly higher nutrient uptake certainly true in other orchid/fungus associations as well.

Using EDXRA methods nitrogen, one of the major plant nutrients always present in NPK fertilizer mixes cannot be reliably measured, however, consequences of mycorrhizal association on the uptake of other important plant nutrients are quite revealing.

Finally, take note of data on S (sulphur) in the graph above: this element is often present in a relatively or absolutely short supply in the substrate of cultivated orchids causing abnormally developed leaves with dry, black spots and leaf tips.

© 1997 - 2004 Dr. T. Pátkai.