Variable, life stage-dependent mycorrhizal specificity and its developmental consequences in the fully myco-heterotrophic orchid .

The degree of specificity between fully myco-heterotrophic (MH) orchids and mycorrhizal fungi is regarded as high, but some species undergo a mycobiont shift as they transition from juvenile to adult plant. We investigated morphological and physiological aspects of the interaction between the fully MH and its four known mycobionts to elucidate developmental consequences of variable, life-stage-dependent specificity. Of five randomly sampled germinated seeds co-cultured with each mycobiont, sp. '' colonized all, and colonized one, and subsp. colonized none. Whereas 16.2% of aseptically germinated seeds exhibited onward growth when co-cultured with , just 1.5-2.6% did so with the other species. Even so, colonized and formed internal rhizomorphs within rhizomes established with , suggesting that this mycobiont can replace and potentially can oust the latter. We infer that the orchid can associate with and mycobionts throughout its life, but that enhances early growth. However, because has a higher wood-rotting capacity than , rapid resource exhaustion may cause nutrient supply shortages. We hypothesize that secondary colonization by or species triggers displacement and that this mycobiont shift sustains orchid growth for longer.