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College of Engineering

Back to Aristotle: Bringing hydraulics into plants

Presented by: Dr. Gabriel G. Katul from Duke University

Date: January 31, 2025

Time:  2:00 pm

Location:  HM Comer 1026

Abstract:  

Life requires movement – an observation that has been attributed to Aristotle (4th century BC). Curiously, optimality principles pertaining to movement such as Fermat’s principle of least travel time, the principle of least action, or flows take the path of least resistance have all been originally proposed for physical systems devout of life. Despite the connection between movement and life, the use of optimality theories in plant hydraulics has and continues to be under-utilized. Attention to water movement in plants is not new and has formed the cornerstone for the so-called ‘vitalists’ view.

In 1898, Francis Darwin noted that “transpiration is stomatal rather than cuticular, so that other things being equal, the yield of watery vapor depends on the degree to which stomata is open, and may be used as an index to their condition”. Curiously, Darwin was a harsh critic of cohesion-tension theory in plant xylem – a theory proposed by Dixon and Joly and despised by vitalists back then. Darwin noted that “To believe that columns of water should hang in the tracheals like solid bodies, and should, like them, transmit downwards the pull exerted on them at their upper ends by the transpiring leaves, is to some of us equivalent to believing in ropes of sand”.

The developments of these ideas in parallel with progress in statistical mechanics, xylem network mapping, and sucrose transport in phloem are reviewed and linked together using optimality theories. This concept has been particularly successful at explaining the form and function of terrestrial vegetation from eco-hydrological and carbon-economy perspectives, and across spatial and temporal scales. Any optimality model is based on three key ingredients: an objective function that describes the gain that needs to be maximized or loss to be minimized, a control variable that shifts the dynamics in the desired direction, and a set of constraints that account for environmental conditions and conservation laws bounding the system. All three ingredients are difficult to define and quantify – especially in ecological systems.

Despite these difficulties, optimality approaches may complement process-based approaches when mechanistic knowledge is scarce – resembling ‘closure models’ in turbulence theories. A blueprint of how these theories can be bridged to explain widely used empirical formulations of stomatal conductance, the shape of xylem vulnerability curves, and capacity of the phloem to transmit sugars are discussed.

Bio:

Gabriel G. Katul received his B.E. degree in 1988 at the American University of Beirut (Beirut, Lebanon), his M.S. degree in 1990 at Oregon State University (Corvallis, OR) and his Ph.D degree in 1993 at the University of California in Davis (Davis, CA). He currently holds a distinguished Professorship in Hydrology and Micrometeorology at the Department of Civil and Environmental Engineering at Duke University (Durham, NC). He was a visiting fellow at University of Virginia (USA) in 1997, the Commonwealth Science and Industrial Research Organization (Australia) in 2002, the University of Helsinki (Finland) in 2009, the FulBright-Italy Distinguished Fellow at Politecnico di Torino (Italy) in 2010, the École polytechnique fédérale de Lausanne (Switzerland) in 2013, Nagoya University (Japan) in 2014, University of Helsinki (Finland) in 2017, the Karlsruher Institute for Technology (Germany) in 2017, Princeton University (USA) in 2020, and CzechGlobe (Brno – Czech Republic) in 2023.

He received several honorary awards, including the inspirational teaching award by the students of the School of the Environment at Duke University (in 1994 and 1996), an honorary certificate by La Seccion de Agrofisica de la Sociedad Cubana de Fisica in Habana (in 1998), the Macelwane medal and became thereafter a fellow of the American Geophysical Union (in 2002), the editor’s citation for excellence in refereeing from the American Geophysical Union (in 2008), the Hydrologic Science Award from the American Geophysical Union (in 2012), the John Dalton medal from the European Geosciences Union (in 2018), the Outstanding Achievements in Biometeorology Award from the American Meteorological Society (in 2021) and later became an elected fellow of the American Meteorological Society (in 2024), and the recipient of the American Meteorological Society hydrologic science medal (in 2025).

Katul was elected to the National Academy of Engineering (in 2023) for his contributions in eco-hydrology and environmental fluid mechanics. He served as the Secretary General for the Hydrologic Science Section at the American Geophysical Union (2006-2008). His research focuses on micro-meteorology and near-surface hydrology with emphasis on heat, momentum, carbon dioxide, water vapor, ozone, particulate matter (including aerosols, pollen, and seeds) and water transport in the soil-plant-atmosphere system as well as their implications to a plethora of hydrological, ecological, atmospheric and climate change related problems.

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