Young and thievish – Insights into C and N gains of orchids in their juvenile stage
P 1.9 in Ecosystem Function
About 10 % of all vascular plants are thought to be in their early subterranean growth phase entirely dependent on nutrient supplies from associated mycorrhizal fungi. This mode of nutrition is called initial mycoheterotrophy. Due to minute seeds lacking sufficient endosperm for germination, all species of the family Orchidaceae belong to this group of initially mycoheterotrophic plants. This investigation quantifies for the first time the carbon (C) and nitrogen (N) pools in the seeds and the development of C and N gains from the fungal hosts in the very early seedling phase of three orchid species having different nutritional modes as adults and being associated with different types of fungi: (1) Neottia nidus-avis, a fully mycoheterotrophic orchid associated with fungi forming ectomycorrhizas, (2) Epipactis helleborine, a partially mycoheterotrophic orchid associated with fungi forming ectomycorrhizas and (3) Serapias parviflora, an autotrophic orchid exclusively associated with Rhizoctonias. The comparison between seeds and seedlings shows an increase in C pools by a factor up to 100 within one year. Isotope abundance data confirm the entire C gain from the fungal source for each of the investigated orchid species. The same is indicated for N, where initial N amounts in the seeds are multiplied by a factor of up to 270 within one year. Thus, seed reserves are, indeed, almost negligible for the development of orchid seedlings in comparison to C and N gained from fungal hosts and different fungal hosts behave identical as nutrient supplies.