Nutrient responses as a key factor to the ecology of orchid species

The availability of N and P plays a critical role in orchid growth and survival; the aim of the present review is to give an overview over the mechanisms which contribute to the critical response. In the first heterotrophic and subterranean phase of orchid development, growth is entirely dependent on mycorrhizal fungi (for reviews see Hadley 1982; Harley & Smith 1983; Hadley & Pegge 1989). The nutrient metabolism of developing orchid individuals is adapted to this symbiosis: reductions in orchid nitrogen metabolism are permitted which can be considered adaptations to the parasitic habit during at least this phase (cf. Press et al. 1986). Furthermore, also in later life the nitrogen and phosphate fluxes are thoroughly affected (Alexander & Hadley 1984; Alexander et al. 1984). As a consequence, nutrient effects on orchid growth may occur via the symbiotic association, and may originate by affecting growth of the symbiotic fungi, or by affecting the symbiotic interaction between orchid and fungi (Dijk & Eck 1995c). Changes in orchid nutrient metabolism during the course of development reflect changes in the physiology of the symbiosis. The resulting development in orchid nutrient metabolism forms a further need to distinguish between the various stages in the life cycle, most notably between the germination and seedling stage and the C-autotrophic adult stage!. Once above ground, orchids as slowly growing and often low, rosette-forming species are very sensitive to eutrophication, which favours more competitive species to a much larger degree than orchids (Dijk & Olff 1994; Silvertown et al. 1994).