Thursday, August 6, 2009

PS 71-139: Water transport properties in leaves of the chaparral shrub, Heteromeles arbutifolia, in a post-fire environment

Gabriella M. Orona1, Aaron R. Ramirez2, Lawren Sack3, Iolana N. Kaneakua4, Christine Scoffoni5, Athena McKown3, and Stephen D. Davis4. (1) Whittier College, (2) California State University Bakersfield, (3) UCLA, (4) Pepperdine University, (5) University of Bordeaux

Background/Question/Methods Leaf hydraulic conductance (Kleaf) is a measure of how efficiently water is transported through a leaf. In theory, Kleaf should decrease with increasing water stress and should be higher for post-fire resprouts than pre-fire adults. These predictions were tested for a fire-prone member of the chaparral shrub community of California, Heteromeles arbutifolia (toyon; Rosaceae). Branchlets were excised before dawn, rehydrated for 2 hr, and subsequently dehydrated for increasing lengths of time before determination of Kleaf by the evaporative flux method, using a Scholander-Hammel pressure chamber was used to estimate leaf water potential (Ψ). As Ψ decreased, Kleaf declined, allowing the construction of a vulnerability curve to water stress. Standard pressure-volume-curves were also constructed to determine the Ψ at turgor loss and the osmotic potential at full hydration. Stomatal conductance (gs) of post-fire resprouts and pre-fire adults were also compared to help interpret observed differences in Kleaf. Results/Conclusions For H. arbutifolia, leaves of resprouts had higher values for gs, Ψ at turgor loss point and osmotic water potential at full hydration than adults (P < 0.003). Consistent with these patterns, Kleaf was significantly higher for resprouts than adults and leaf hydraulic failure occurred at a higher Ψ for resprouts than adults (-5 and -7 MPa respectively). Our results suggest that a luxurious supply of water to resprout leaves, characteristic of the post-fire environment, is associated with greater water transport efficiency but reduced safety against water stress-induced loss of conductance. This association may be an adaptation to facilitate rapid canopy expansion and reestablishment after wildfire, albeit, at the expense of tolerance of water stress.