The biology of mycorrhiza in the orchid Goodyera repens BR
Investigations have been carried out into the mycorrhizal association of the orchid Goodyera repens with particular reference to the role of infection in the photosynthetic plant. Infection was present in field plants throughout the year. Fluctuations in the level of infection were influenced more by the rate of growth of the plants than by external factors. Population dynamics of G.repens were similar to those of many other autotrophic perennial plants. Fungi were isolated from the roots of field plants. Most of the isolates were similar in culture and were identified as Rhizoctonia goodyerae-repentis. The remainder were not identified. Only isolates of R. goodyerae-repentis were able to enhance the growth of protocorms. There was a wide range of symbiotic activity within these isolates, with some isolates causing rapid growth of a large number of protocorms and others causing rapid growth of very few. The average size of the large protocorms produced by each isolate also varied. After one or two years the ability of all the isolates to enhance the growth of protocorms declined, regardless of whether they had been maintained in pure culture or in association with the host. Fungicide treatment was a satisfactory method of producing plant material similar to mycorrhizal material but lacking external mycelium. Mycorrhizal plantlets and plants treated this way had a greatly reduced growth rate, lower P and N contents and lower net assimilation rates compared with untreated plantlets and plants. R.goodyerae-repentis was able to translocate 32p both in pure culture and to host plants over distances of up to 9 cm. The inflow rate of P was much greater in untreated mycorrhizal plants than in uninfected or fungicide treated plants. A growth stimulation due to the presence of external mycelium was only found in plants under conditions of P stress where uptake of P was limited by its rate of diffusion to the root surface. This enhancement of P uptake by the mycorrhiza is likely to be of ecological importance. 14C was translocated into protocorms and plantlets, but not into mature plants. The growth rate of plants was unaffected by the presence of carbon in the medium. No significant movement of carbon from host to fungus was detected when the shoots of large plants were fed 14CO2. The selective transfer of P but not carbon into plants suggests that the transfer mechanism is biotrophic rather than necrotrophic. This, together with the enhancement of both growth and mineral nutrition is similar to the situation in other (i.e. non-orchidaceous) mycorrhizal plants. Further work on the movement of carbon between partners is required to clarify the true nature of this symbiosis.