The Orchidaceae is one of the largest families in the plant kingdom, with more than 20,000 species identified, most distributed in the tropics and subtropics. Of the European species, of which there are around 90, the genera’s Orchis, Ophrys, Epipactis and Dactylorhizas are particularly well represented.
This diversity is perhaps surprising given that at some point in their life cycle all orchids are reliant upon mycorrhizal fungi. Whether they are chlorophyllous or achlorophyllous as adults, all orchids have a stage where they are non-photosynthetic and therefore dependent on external sources of nutrients. In the vast majority of cases, it is just the seedling stage that is obligately mycorrhizal. Orchid seeds are very small (around 0.3 – 14 mg per seed) and contain little nutrient reserves.
They contain small amounts of high energy protein and lipids, but little sugar, though some species also contain small starch granules.
Mycorrhizal fungi can provide the nutrients, and particularly carbohydrates, needed to grow, and in fact, most orchid seeds will not germinate unless they have been infected by an appropriate fungus. This factor was a problem in the early 20th century to horticulturalists trying to propagate orchids for a highly lucrative market. It is now known that many species can be cultured axenically (i.e. without mycorrhizal infection) if they are supplied with an exogenous source of sugar.
The mycorrhizal fungi in orchids are Basidiomycot, and in particular Rhizoctonia species with which many orchids are associated. Some Rhizoctonia species are known to form ectomycorrhizal associations, but whether this is a common occurrence is still unknown.
The infection of an orchid seed by fungi occurs after the embryo takes up water and swells, rupturing the seed coat. The embryo emerges and produces a few root hairs, which hyphae rapidly colonise. As hyphae penetrate a cell of the embryo, the plasma membrane of the orchid cell invaginates, and the hypha becomes surrounded by a thin layer of cytoplasm. An orchid embryo consists of only a few hundred cells and the fungi spread quickly from cell to cell.
Within cells, hyphae form coils called pelotons (Figs 3 & 4), which greatly increase the interfacial surface area between orchid and fungus. Each intracellular peloton has a short life-span, lasting only a few days before it degenerates and is digested by the orchid cell. In fact hyphae in the orchid have short life-spans too; older hyphae develop large vacuoles and thick cell walls, and the cytoplasm degenerates. The hyphal cells eventually collapse and are consumed by the orchid cell. During this process, the plant cell remains functional and can be recolonised by any surviving hyphae, or by fungi invading from adjacent cells.
Infection by mycorrhizal fungi does not necessarily result in the germination and growth of an orchid. Upon the association, three outcomes are possible:
a mycorrhizal interaction, as described above;
parasitic infection, in which the orchid cells are invaded and the embryo dies;
the orchid cells reject the fungal infection.
All three of these interactions may occur in a population of protocorms, highlighting the relatively unstable nature of the association.
A successful fungal infection results in the germination of an orchid seed. The fungus may be the sole source of nutrition during the first years of life. Most orchid species develop chlorophyll in their adult stage and become less dependent on the mycorrhiza. However, most still have mycorrhizal roots, and gain nitrogen and phosphorus from the fungus. Whether they still induce the fungi to transfer carbon to them is unknown.
Around 200 species of orchids remain achlorophyllous throughout their lifetimes. Species such as Galeola, Gastrodia, Corallorhiza, Rhizanthella and many others continue to gain carbon from their mycorrhizal fungi. Even some chlorophyllous species, such as Cephalanthera rubra (Red Helleborine, one of the rarest orchids in Britain) spend several years underground before producing overground flowering scapes.