Collaborative Research: Resistance, Repair and Redundancy: Traits that protect shrubs against drought-induced hydraulic failure

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In engineering terms, the hydraulic system of a woody plant is a "negative pressure flow system" in which conducting cells in the wood (xylem) are linked from roots to leaves. This type of hydraulic system is prone to failure due to the introduction of air bubbles, which create embolisms. Plants frequently develop embolisms in their hydraulic systems, especially under water limited conditions. Whether man-made or natural, a pressure flow system can be protected from failure in three ways: resistance, reparability, and redundancy. Resistance to hydraulic failure in plants is fairly well understood and involves the thickness of cells walls in conducting cells and in the surrounding matrix of thick-walled cells (fibers). Reparability after failure depends on living cells in and adjacent to xylem. Redundancy reflects the number of root to leaf conducing pathways and the degree of their interconnectedness (hydraulic integration). The unique arrangements of conducting cells, supporting fibers, and living cells in different species suggest that tradeoffs occur among these three types of protections. To date, interactions among them are virtually unknown. The proposed research questions whether the relative amounts of different types of protection change with increasing habitat aridity in dominant shrub species from multiple lineages along transcontinental aridity gradients in North America, South America, and Australia. Methods will involve standard techniques used to study the structure of plant xylem and the use of dye tracers to characterize three-dimensional pathways of water transport. Xylem function studies will quantify resistance to embolisms and reparability of experimentally induced embolisms. This study will be the first to simultaneously examine the relative roles of redundancy, resistance and repair in protecting plants from hydraulic failure. Results from this study will aid researchers and land managers in understanding how hydraulic protection influences shrub survival during global climate change and desertification. Students of different ages, ethnic, and cultural backgrounds will experience the scientific process as they participate in every aspect of the research.