Unusual Water Relations in the CAM Atmospheric Epiphyte Tillandsia usneoides L. (Bromeliaceae)

Past studies have reported two unusual aspects of the water relations of the atmospheric CAM epiphyte Tillandsia usneoides L. (Bromeliaceae): a drought stimulation of nocturnal CO2 uptake, and nocturnal absorption of water vapor. Contrary to past reports, a 10-d drought did not stimulate nocturnal CO2 uptake in this species. On the other hand, previous reports of nocturnal water vapor absorption were confirmed in situ throughout a year, although tissue hydration from this source was insufficient to offset daytime water loss. Deposition of dew on the plants was never observed in the field. It is hypothesized that the unusual nature of the water relations of T. usneoides is attributable to the interactions between two "pools" of water and the external atmosphere. The dense indumentum of trichomes obscuring the surface of this epiphyte comprises one pool and is most likely responsible for rapid hydration early in the night and dehydration early in the day. In addition, stomata control water loss from the living mesophyll cells, the second pool, for the remainder of the night. The high rates of water loss observed throughout the day when stomata are closed probably result from leakage through the trichomes.