Monday, August 3, 2009 - 1:50 PMOOS 4-2: The role of wood fibers in hydraulic systems of shrubs
H. Jochen Schenk1, Alejandra N. Mendez1, Susana Espino1, Hugo Martinez-Cabrera2, and Cynthia S. Jones2. (1) California State University Fullerton, (2) University of Connecticut
Background/Question/Methods Plant hydraulic systems under drought conditions are prone to failure due to introduction of embolisms. Plants differ in their abilities to resist formation of such embolisms, repair them, and prevent their spread within the hydraulic system. In angiosperms, a high degree of connectedness among vessels, as measured by the area of inter-vessel pits per vessel, has been associated with higher vulnerability to embolism spread. Other wood traits that increase the degree of hydraulic integration within vessel-bearing wood therefore may also increase vulnerability to embolism. In a previous study of 61 shrub species from North and South America we found decreasing degrees of hydraulic integration with increasing aridity of the environment. Many shrubs from arid and semi-arid environments were structurally and/or functionally divided into independent hydraulic units. The degree of hydraulic integration increased with vessel density and decreased with increasing wall-thickness of imperforate tracheary elements (ITE) in the wood. Other studies have documented a positive relationship between the amount of ITE cross-sectional wall area and vulnerability to embolism. We hypothesized that the degree of hydraulic integration would differ between different types of ITEs and that air content in thick-walled fibers would be associated with increasing hydraulic isolation of vessels.
Results/Conclusions We tested these hypotheses by comparing hydraulic integration in our 61 species database between shrubs with ITE matrices consisting predominantly of tracheids, fiber tracheids, or libriform fibers. As hypothesized, thick-walled libriform fibers were associated with the highest degree of hydraulic isolation of vessels. For five shrub species from coastal sage scrub vegetation in southern California, Salvia mellifera, Salvia apiana, Artemisia californica, Eriogonum fasciculatum, and Malosma laurina, we tested the hypothesis that increasing air content in libriform fibers would account for increasing hydraulic isolation of vessels as the summer dry season progresses. Water and air content in fibers were quantified using a combination of gravimetric measurements and a new water-insoluble dye tracer technique. Water contents in fiber lumens of four out of five shrub species decreased from near 100% at the peak of the spring growing season to 0% during the summer dry season, but stayed constant in the fifth species, Malosma laurina, the species with the most thin-walled fibers relative to fiber diameter. Our findings support the hypothesis that air-containing libriform fibers contribute to the hydraulic isolation of vessels, which raises the intriguing possibility that spread of air among vessels may be inhibited by a matrix of air-containing fibers.
Results/Conclusions We tested these hypotheses by comparing hydraulic integration in our 61 species database between shrubs with ITE matrices consisting predominantly of tracheids, fiber tracheids, or libriform fibers. As hypothesized, thick-walled libriform fibers were associated with the highest degree of hydraulic isolation of vessels. For five shrub species from coastal sage scrub vegetation in southern California, Salvia mellifera, Salvia apiana, Artemisia californica, Eriogonum fasciculatum, and Malosma laurina, we tested the hypothesis that increasing air content in libriform fibers would account for increasing hydraulic isolation of vessels as the summer dry season progresses. Water and air content in fibers were quantified using a combination of gravimetric measurements and a new water-insoluble dye tracer technique. Water contents in fiber lumens of four out of five shrub species decreased from near 100% at the peak of the spring growing season to 0% during the summer dry season, but stayed constant in the fifth species, Malosma laurina, the species with the most thin-walled fibers relative to fiber diameter. Our findings support the hypothesis that air-containing libriform fibers contribute to the hydraulic isolation of vessels, which raises the intriguing possibility that spread of air among vessels may be inhibited by a matrix of air-containing fibers.