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Adaptive variation in the vulnerability of woody plants to xylem cavitation

Maherali, H, WT Pockman, RB Jackson
Journal Volume/Pages: 
85: 2184–2199

The ability of plants to supply water to their leaves is intimately associated with survival. Water supply to leaves depends on maintaining an intact water column in the xylem from the roots to shoots. Because this hydraulic pathway is under tension, it is vulnerable to breakage through the induction of air emboli (cavitation). Although the physiological benefit of resistance to water stress induced xylem cavitation for desiccation tolerance is clear, there is considerable interspecific variation within and across climates. To understand the adaptive significance of this variation and the potential tradeoff with water transport, we compiled a database of 167 species from 50 seed plant families and examined relationships among resistance to xylem cavitation, water transport capacity (as determined by the specific conductivity of xylem [KS]), and climate. Relationships were evaluated using standard cross-species correlations (r). Because inferences about the adaptive significance of these correlations can be biased by the potential similarity of closely related species, we also analyzed our data using phylogenetically independent contrast correlations (PIC) calculated over a range of alternate seed plant phylogenies. Resistance to cavitation, expressed as the xylem tension at which 50% of hydraulic conductivity was lost (Y50), ranged from -0.18 to -9.9 MPa for angiosperms and from -1.5 to -14.1 MPa for conifers. Conifers were most resistant to cavitation, with average Y50 80% more negative than angiosperms (P

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