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.
Intellectual Merits 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 drought conditions. Protection against hydraulic failure is important for survival of most plants, including crops and horticultural plants, but the mechanisms underlying such protections are poorly understood. 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 conferred by membranes between water-filled and air-filled cells that are impenetrable to gas, except under very large pressure. Embolism reparability depends on activities of living cells in the xylem, which are thought to provide the water and energy for refilling. Repair can occur even while the xylem is under negative pressure, a phenomenon known as embolism repair under tension. Redundancy reflects the number of root to leaf conducing pathways and the degree of their interconnectedness (hydraulic integration). Much research to date has focused specifically on resistance. This research project added investigations of reparability and redundancy to resistance. The main research question was whether the relative amounts of different types of protection change with increasing habitat aridity in dominant shrub species from multiple lineages along transcontinental transects in North and South America. We also asked if there are trade-offs between the three types of protection. Shrubs were chosen as study objects because they occur in almost all environments, from deserts to forests, and thus are suitable for research along aridity gradients. A total of 80 shrub species were studied for this research in the United States and Argentina, and 21 of these in California, Texas, Georgia, and North Carolina were intensively studied for their structural wood traits and their hydraulic functions. This study was the first to simultaneously examine the relative roles of redundancy, resistance and repair in protecting plants from hydraulic failure. A major contribution of this research for the plant sciences was the finding that embolism repair under tension is a very widespread phenomenon in shrubs and that it is associated with leaf transpiration, usually during the night. Contrary to previous thoughts, embolism repair does not require closure of leaf pores (stomata) and appears to require nighttime transpiration, possibly because the resulting nocturnal sap flow efficiently removes dissolved gas from the xylem. Our research thus provided evidence for an important physiological function of nighttime transpiration in plants, a process that otherwise would appear to be wasteful, and which has remained to date. The research also showed that living xylem cells are actively involved in embolism repair and provide pathways for water that is used for refilling gas-filled vessels. Finally, the research fully supported the hypothesis that plants have at least three strategies to combat drought-induced failure of their hydraulic systems. Previous research had focused on resistance to embolism formation, but repair and redundancy were shown to be common strategies as well. Broader Impacts The research involved active participation of more than 30 students at Cal State Fullerton and funded the development of teaching units about plant water relations at Cal State Fullerton, which have been taught to more than 50 students to date. The research resulted in six peer-reviewed publications to date and in 31 conference presentations, including many with undergraduate students as co-authors. Equipment purchased for this grant benefited teaching in plant ecology and independent research by undergraduate and graduate students, including many from groups under-represented in science, as Cal State Fullerton is a minority-serving institution. Results from this study will aid researchers and land managers in understanding how hydraulic protection influences shrub survival during global climate change and desertification. Equipment purchased and techniques developed during this project are now applied in research on fruit trees in order to develop new plant-based measures to create better irrigation schedules that will increase water conservation in horticulture.
