In plants, hydraulic failure in the water transport system, the xylem, is caused by the introduction of gas bubbles and formation of embolisms in individual conducting cells. The fact that plants can repair xylem embolisms even when their hydraulic systems are under considerable tension has remained one of plant science's most puzzling and important mysteries - the mechanisms underlying this repair are essentially unknown despite evidence the phenomenon occurs in a wide variety of plant species. This research will test a novel hypothesis about a physical mechanism of embolism repair under tension. The research will be conducted with shrub species from field sites across North America. Measurements to be conducted include the percent loss of conductance due to embolism formation, gas contents in root and stem xylem in the field and under controlled conditions in the lab, as well as dye-tracer and anatomical studies to determine the pathways of water movement during embolism formation and repair. The research could potentially be transformative for plant biology by establishing mechanisms by which plants sustain water transport under high tension in the presence of abundant gas in their water transport cells. The proposed activity will help to fund a Ph.D. student and a postdoctoral position. At Cal State Fullerton, the project will include active involvement of undergraduate students in the research, including a large number of students from underrepresented groups. The research potentially will have implications for application in horticulture and agriculture, most directly for sustainable irrigation management practices of woody fruit crops.
When a man-made hydraulic system needs repair, the system has to be shut down by relieving the pressure that provides the energy source for driving the system. In plants, hydraulic failure in the water transport system, the xylem, is caused by the introduction of gas bubbles and formation of embolisms in individual conducting cells. The fact that plants can repair xylem embolisms even when their hydraulic systems are under considerable tension has remained one of plant science’s most puzzling and important mysteries — the mechanisms underlying this repair were essentially unknown when this EAGER proposal was submitted, despite evidence the phenomenon occurs in a wide variety of plant species. Embolism repair is an important adaptation to drought in plants, and a better understanding of the underlying mechanisms would constitute an important advance in plant biology. It also could have potential implications for irrigation, crop selection, and plant breeding in agriculture and horticulture. This research tested a novel hypothesis about a physical mechanism of embolism repair under tension, using shrub species from field sites across North America. Measurements included the percent loss of conductance due to embolism formation, gas contents in root and stem xylem in the field and under controlled conditions in the lab, as well as dye-tracer and anatomical studies to determine the pathways of water movement during embolism formation and repair. We confirmed that living cells in the wood play a crucial role in the refilling of plant hydraulic systems with water, but also found that the mechanism of air removal is purely physical in nature. Leaf transpiration and sap flow during the night was found to play an important role for efficient air removal from the hydraulic system, but it was not found to be required for embolism repair under tension, as originally hypothesized. The findings throw new light on the biophysics of the gas and liquid phases in plant hydraulic systems. At the University of Connecticut, three Ph.D. students and a postdoctoral researcher were supported through this grant, and four undergraduate students gained research experience. At Cal State Fullerton, the project supported a research associate and two undergraduate students and actively involved a total of 12 undergraduate students in the research, including a large number of students from groups underrepresented in science. Findings were presented in 16 conference presentations at regional, national, and international scientific meetings and will be published in peer-reviewed journals in the near future.
