© 2007 Heron Publishing—Victoria, Canada
Regulation of transpirational water loss in Quercus suber trees in a Mediterranean-type ecosystem
D. O. Otieno (1, 2), M. W. T. Schmidt (3), C. Kurz-Besson (4), R. Lobo Do Vale (4), J. S. Pereira (4) and J. D. Tenhunen (3)
1. Department of Botany, Maseno University, P.O. Box 333, Maseno, Kenya / 2. Corresponding author (denotieno@yahoo.com) / 3. Department of Plant Ecology, University of Bayreuth, P.O. Box 95440, Bayreuth, Germany / 4. Instituto Superior de Agronomia, Tapada da Ajuda, 1394-017, Lisboa, Portugal / Received February 20, 2006; accepted December 15, 2006; published online May 1, 2007
Summary
Sap flux density in branches, leaf transpiration, stomatal conductance and leaf water potentials were measured in 16-year-old
Quercus suber L. trees growing in a plantation in southern Portugal to understand how evergreen Mediterranean trees regulate water loss
during summer drought. Leaf specific hydraulic conductance and leaf gas exchange were monitored during the progressive summer
drought to establish how changes along the hydraulic pathway influence shoot responses. As soil water became limiting, leaf
water potential, stomatal conductance and leaf transpiration declined significantly. Predawn leaf water potential reflected
soil water potential measured at 1-m depth in the rhizospheres of most trees. The lowest predawn leaf water potential recorded
during this period was –1.8 MPa. Mean maximum stomatal conductance declined from 300 to 50 mmol m–2 s–1, reducing transpiration from 6 to 2 mmol m–2 s–1. Changes in leaf gas exchange were attributed to reduced soil water availability, increased resistances along the hydraulic
pathway and, hence, reduced leaf water supply. There was a strong coupling between changes in soil water content and stomatal
conductance as well as between stomatal conductance and leaf specific hydraulic conductance. Despite significant seasonal
differences among trees in predawn leaf water potential, stomatal conductance, leaf transpiration and leaf specific hydraulic
conductance, there were no differences in midday leaf water potentials. The strong regulation of changes in leaf water potential
in Q. suber both diurnally and seasonally is achieved through stomatal closure, which is sensitive to changes in both liquid and vapor
phase conductance. This sensitivity allows for optimization of carbon and water resource use without compromising the root–shoot
hydraulic link.
Keywords:
drought, hydraulic conductance, stomatal conductance, transpiration.
