Cell structure of epiphytic orchids’ root cortex and exodermis shape fungal colonization

Epiphytic orchids unlike terrestrial species possess water storage elements as an adaptation to dry habitats. Two types of structures are present in roots and thus involved in interaction with mycorrhizal fungi that obligately colonize roots. Lignified water storage cells (WSC) are situated in cortex and perform the function of water storage. Among lignified exodermis cells thin-walled passage cells (PC) are present. These elements are necessary for water exchange between cortex and the environment. In this study we measure colonization rate of water storage cells and passage cells and discuss how these two functions interact in orchid root. We show that water storage cells may be colonized but less favorably than thin-walled cortex cells and support exiting data that passage cells are the only way for fungal hyphae to pass through exodermis. This evidence provokes discussion on water uptake and mycorrhiza of epiphytic orchids.