Hydrophobic trichome layers and epicuticular wax powders in Bromeliaceae

The distinctive foliar trichome of Bromeliaceae has promoted the evolution of an epiphytic habit in certain taxa by allowing the shoot to assume a sig nificant role in the uptake of water and mineral nutrients. Despite the pro found ecophysiological and taxonomic importance of this epidermal structure , the functions of nonabsorbent trichomes in remaining Bromeliaceae are not fully understood. The hypothesis that light reflection from these trichome layers provides photoprotection was not supported by spectroradiometry and fluorimetry in the present study; the mean reflectance of visible light fr om trichome layers did not exceed 6.4% on the adaxial surfaces of species r epresenting a range of ecophysiological types nor was significant photoprot ection provided by their presence. Several reports suggesting water repelle ncy in some terrestrial Bromeliaceae were investigated. Scanning electron m icroscopy (SEM) and a new technique-fluorographic dimensional imaging (FDI) -were used to assess the interaction between aqueous droplets and the leaf surfaces of 86 species from 25 genera. In the majority of cases a dense lay er of overlapping, stellate or peltate trichomes held water off the leaf ep idermis proper. In the case of hydrophobic tank-forming tillandsioideae, a powdery epicuticular wax layer provided water repellency. The irregular arc hitecture of these indumenta resulted in relatively little contact with wat er droplets. Most mesic terrestrial Pitcairnioideae examined either possess ed glabrous leaf blades or hydrophobic layers of confluent trichomes on the abaxial surface. Thus, the present study indicates that an important ances tral function of the foliar trichome in Bromeliaceae was water repellency. The ecophysiological consequences of hydrophobia are discussed.