Thursday, August 9, 2007

PS 61-93: The evolutionary physiology of the Cupressaceous conifers: The structure-function trade-offs associated with resistance to drought-induced embolism

Jarmila Pittermann, Todd Dawson Integrative Biology, and Bruce Baldwin Integrative Biology. University of California

The Cupressaceae are the most broadly distributed of all conifers.  Members of this family are endemic to nearly all continents in both the northern and the southern hemisphere, but the Cupressaceae phylogeny suggests an evolutionary trajectory towards drier environments characteristic of temperate climates and deserts. Interestingly, recent Cupressaceae phylogenies show that the riparian-mesic conifers, such as Taxodium sp. and Sequoia are ancestral to the more derived, xeric-adapted taxa such as Juniperus sp. and Widdringtonia. In an effort to better understand the ancestral physiology, as well as the evolutionary physiological trajectory of the Cupressaceae, we examined structural and functional traits such as species’ xylem pressure causing a 50% loss of conductivity due to embolism (P50), the xylem conduit wall thickness to lumen span ratio (t/b)2, wood density, leaf-specific conductivity and 13C/12C ratios across the Cupressaceae phylogeny.  This study was conducted in local ‘common gardens’ so we could examine not only how traits vary across the phylogeny, but also control for genetic and environmental influences on character expression. We hypothesized that variation in physiological traits across the Cupressaceae has primarily been constrained by phylogeny and our preliminary results are consistent with this hypothesis. Even in a well-watered common garden setting, species’ xylem showed a broad range of drought resistance with P50’s ranging from -2.5 to -8 MPa where the taxa most vulnerable to embolism exhibited the lowest wood densities and highest leaf-specific conductivities, while the opposite was true of the most resistant taxa.  This suggests that species’ radiation and adaptation to drier climates was accompanied by trade-offs between hydraulic efficiency and safety against drought-induced embolism.  Notable exceptions to this trend were the southern hemisphere Cupressaceae whose wood density was decoupled from their vulnerability to embolism.

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