Root water uptake uphill of water potential gradients: did vascular plants drop thermodynamics classes at school?

Couvreur, Valentin;Heymans, Adrien;Lobet, Guillaume;Bennett, Malcom;Draye, Xavier
(2021) ISRR - Rooting 2021 — Location: Hanovre (24.May.2021)

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As active transmembrane water pumps have not been discovered in plants (water flows passively across channels called aquaporins), the current paradigm of root radial water flow concludes that water only flows “downhill” of pressure (Ψp) and total water potential (Ψtot) gradients. For instance, solute loading in xylem lowers its Ψtot, indirectly facilitating water uptake. However, numerous studies report that plant roots may absorb water “uphill” of both Ψp and Ψtot gradients, a curiosity. This contradicts the current paradigm of root water acquisition, and seemingly goes against the second law of thermodynamics. We recently found a physiological mechanism solving this enigma with a micro-hydrological model of water flow across roots, called MECHA. Osmotic gradients between living cells generate pressure gradients driving water through plasmodesmata, with a surprising result: subcellular flow “downhill” of local water potential gradients may translate into flow “uphill” of both total and pressure potential gradients between root surface and xylem. This water pumping mechanism means any vascular plant may reduce its xylem water tension and increase its water availability beyond predictions of the current theory. Here, we mathematically demonstrate the water pumping mechanism by solving water flow equations analytically on a triple-cell system. Then we show that the associated upscaled equations hold in 2- and 3-D maize and Arabidopsis hydraulic anatomies, and that water may flow “uphill” of water potential gradients toward xylem as observed experimentally.
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Couvreur, V., Heymans, A., Lobet, G., Bennett, M., & Draye, X. (2021). Root water uptake uphill of water potential gradients: did vascular plants drop thermodynamics classes at school? ISRR - Rooting 2021, Hanovre. https://hdl.handle.net/2078.5/251323