Title: Weak Polymers, Strong Impact: How Pectin Hydrogels Control Embolism Spread in Plant Water Transport
Abstract: Across biological systems, structural integrity often emerges from fiber-reinforced composites that have stiff fibers embedded within softer matrices. In plants, this design is exemplified by the cell wall, where cellulose microfibrils are embedded in a matrix of polymers such as pectin and hemicellulose. In plant xylem, water transport operates under negative pressure, making it vulnerable to embolism, characterized by the formation and spread of air bubbles that block water flow. Pit membranes are micron-scale porous structures that separate adjacent xylem conduits and prevent the spread of embolism. While current models attribute embolism resistance primarily to cellulose network architecture, this dissertation tests the hypothesis that the soft pectin network plays a critical mechanical role in stabilizing cellulose and limiting air entry.
In Chapter 1, I use Acer rubrum as a model system to test how chemical modification of pectin influences embolism resistance. By enzymatically removing pectin or disrupting calcium crosslinks and combining with different surface tension conditions, I show that pectin removal dramatically decreases embolism resistance and increases pit membrane compliance, supporting the idea that pectin restricts the cellulose network during embolism spread.
In Chapter 2, I extend this approach across three angiosperm species -- Betula nigra, Acer rubrum, and Ostrya virginiana -- that span a gradient of drought resistance. I find that removing pectin consistently increases embolism vulnerability across all species, while disrupting calcium crosslinks produces species-specific responses. These results suggest that pectin chemistry is central to controlling thresholds for embolism spread.
In Chapter 3, I introduce Chara corallina as a macroscopic mechanical analogue for pit membranes to visualize air propagation through primary cell walls. Using this system, I identify three distinct modes of air transport: diffusion, tunneling, and bursting. I demonstrate that pectin removal both weakens cell wall integrity under uniaxial loading and promotes pore expansion under biaxial stress, linking wall mechanics directly to embolism dynamics.
Together, this work challenges the prevailing view that embolism resistance is controlled solely by cellulose network structure, and instead highlights the critical role of soft polymer networks in constraining deformation and regulating failure. By integrating plant hydraulics with material science, this dissertation provides new insight into the physical basis of drought resistance and offers inspiration for engineering drought tolerant crops and designing fiber-hydrogel composites in engineered systems.
Committee: N. Michele Holbrook (Advisor), Andrew Biewener, Ned Friedman (Chair), Zhigang Suo
LOCATION: STATUS:CONFIRMED DTSTART:20260430T160000Z DTEND:20260430T170000Z END:VEVENT END:VCALENDAR