Host–fungus relationships in orchid mycorrhizal systems

The understanding of the host–fungus relationships in the mycorrhizas of orchids is important in relation to the application of symbiotic methods to seed germination and seedling development, and also for re-establishment in natural conditions either from seed or tissue culture as one of the contributions to conservation. Despite extensive progress in knowledge of the role of fungi in the early stages of germination, the inherent difficulty in the use of the symbiotic technique has inhibited its application. Orchid fungi are extremely variable and relatively few root inhabitants are true mutualistic symbionts. The outcome of the relationship between the partners is a finely balanced one and many fungi isolated from orchid mycorrhizas may, after a period in culture, become incompatible with, or even pathogenic to, orchid protocorms. Mutualistically symbiotic fungi enhance the nutrition of germinating seeds by the transference of carbon compounds. Photosynthetically active seedlings and mature plants, however, may be quite independent of their fungal partners. Evidence suggests that in conditions of nutrient stress the fungal partner may mediate in the movement of phosphate and/or nitrogen compounds, as in other mycorrhizal systems. Introduction Orchids, whether epiphytic or terrestrial, generally grow and thrive in conditions of nutrient impoverishment. As with other higher plants growing in similar environments this may only be possible by the association of fungi with the roots and other subterranean parts – the mycorrhizal association .