The Velamen Radicum of Orchids: A Special Porous Structure for Water Absorption and Gas Exchange

Abstract

Aerial roots of epiphytic orchids cannot absorb water from the soil but supply the plant by collecting atmospheric water together with dissolved nutrients. A special outer tissue layer, the velamen radicum (VR), consisting of dead cells, is crucial for water interception and absorption. In this contribution, the VR is explored as an intricate porous material whose surface and structure is evolutionarily fine-tuned for water capture and absorption. After wetting, the VR is usually completely drained after less than two hours, as demonstrated by MRI (Magnetic Resonance Imaging). Furthermore, the MRI results indicate that the VR is drained homogeneously in such a way that no isolated water-filled parts occur. Interspecific differences in drainage time can be explained at least partially by differences in thickness and structure of the considered taxa: dehydration time of an aerial root increases with increasing thickness of the VR. Drainage behavior is, however, not completely dependent on thickness alone. Also porosity of the VR walls and the size of the single pores are of relevance. Aerial roots of leafless orchids are particularly astounding, because they conduct two tasks, water absorption and photosynthesis and show a very thin VR. Here, the exodermis, situated underneath the VR, probably contracts during dehydration, thereby restricting evaporation. The variability of external layers of aerial roots, VR and exodermis, very likely played a major role during the successful radiation of epiphytic orchids into various niches and habitats, and many functional features and special adaptations are still not well understood or are even undetected.