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Biology of Dryland Plants

Jacobsen, Anna L. [1], Davis, Stephen D. [2], Pratt, R. Brandon [2], Ewers, Frank W. [1].

Convergence of xylem structure and function: California chaparral and South African fynbos.

Mediterranean-type climate regions are characterized by long, dry summers and cool, moist winters and are dominated by evergreen, sclerophyllous shrubs. The shrubs of Mediterranean-type climate regions are often cited as an example of convergent evolution although the data, based mostly on leaf morphology, are equivocal. We tested the hypothesis of convergence by comparing xylem structure and function between shrub species of southwestern South Africa and southern California. Species in both regions displayed a similar range of minimum seasonal water potentials. Xylem density, mechanical strength, and vessel and fiber wall thickness were correlated with minimum seasonal water potential in both regions and when analyzed using phylogenetically independent contrasts. Xylem hydraulic conductivity was not correlated to seasonal water stress or xylem biomechanics in either region. Our results suggest that similar environmental stresses, including a protracted summer drought, have resulted in similarly low seasonal water potentials among species in these two Mediterranean-type climate regions. In response to these low water potentials and concomitant risk of vessel collapse, xylem structure and function appears to have converged among these distantly related species supporting the hypothesis of convergent evolution.


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1 - Michigan State University, Department of Plant Biology, 166 Plant Biology, East Lansing, Michigan, 48824, USA
2 - Pepperdine University, Natural Sciences Division, 24255 Pacific Coast Highway, Malibu, California, 90263-4321, USA

Keywords:
chaparral
Fynbos
California
Xylem
Convergent evolution
water stress
South Africa.

Presentation Type: Symposium or Colloquium Presentation
Session: 49-13
Location: Salon K - Austin Grand Ballroom/Hilton
Date: Wednesday, August 17th, 2005
Time: 2:15 PM
Abstract ID:97


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