RESUMO
In the European Alps, Rhododendron ferrugineum grows in silicate regions while Rhododendron hirsutum is restricted to limestone areas. At geologically mixed sites, also hybrids (Rhododendron × intermedium) can occur. We hypothesised that hydraulic properties would vary with the species' habitat requirements. Key hydraulic parameters (vulnerability to drought-induced embolism, stomata regulation) and related wood characteristics as well as diurnal courses of water potential (Ψ) and stomatal conductance were analysed on plants growing on a silicate, a limestone and a geologically mixed site. Highest embolism resistance[Ψ at 50% loss of conductivity (Ψ (50)), -3.24 ± 0.18 MPa] and the highest safety margin between the Ψ at stomata closure (Ψ (SC) at 10% of maximal leaf conductance) and Ψ (50) were observed in R. hirsutum at the limestone site (1.57 MPa). Like in R. ferrugineum, hydraulic parameters indicated less resistance at the geologically mixed site. Highest Ψ (50) (-1.95 ± 0.12 MPa), corresponding to wide conduits and a reduced conduit wall reinforcement, was found in R. × intermedium. Diurnal courses indicated a rapid stomata closure in response to low Ψ in R. hirsutum and R. × intermedium. The plasticity in drought adaptation of R. hirsutum corresponds to its ability to colonise dry limestone areas. In contrast, hydraulic limitations of R. × intermedium may explain restrictions to rather moist sites. This study provides insight into the role of xylem hydraulics and stomata regulation in shrub water relations, interspecific and site-specific differences in drought adaptation, as well as effects of hybridisation on plant hydraulics.