MOSSES RESIST BUCKLING AND CAVITATION BIOLOGY AND HEALTH SCIENCES In vascular plants a critical limitation on water transport is that the SYNCHROTRON SOLEIL HIGHLIGHTS 2020 continuous water column contained within the non-living pipeline of tubular xylem cells becomes meta-stable when the water potential falls below the equilibrium vapor pressure. Thus, plants exposed to drying soil or strong evaporative demand become increasingly vulnerable to PSICHE BEAMLINE bubble nucleation (loosely termed “cavitation”[3]). We first using cryoSEM determined whether the hydroids (water-conducting cells) of Polytrichum Associated publication commune could resist buckling when exposed to the range of tensions Advanced vascular function discovered used by vascular plants to extract water from the soil. Hydroids were found in a widespread moss. to sustain liquid water in the lumen without buckling or deformation when exposed to water potentials as low as -1.45 MPa. Having determined T. J. Brodribb, M. Carriquí, S. Delzon, that the vascular system of Polytrichum operates under tension, we S. A. M. McAdam, N. M. Holbrook. expected that it would also be prone to bubble formation under water Nature Plants 6, 273 (2020). stress. We used x-ray micro-computed tomography at the PSICHE beamline to examine hydroids of Polytrichum during imposed water stress (Figure 1), to test whether dehydration caused cavitation in the References moss vascular system. Hydroids were found to resist cavitation until [1] P. Kenrick & P. R. Crane, Nature 389, water potentials fell below -0.8 MPa, at which point we recorded the 33 (1997). onset of cavitation in the central hydroid bundle (Figure 2). [2] J. A. Raven, Advances in Botanical Research 5, 153 (1977). DISCOVERY [3] M. T. Tyree & M. H. Zimmermann, M. H. Xylem structure and the We find that classical assumptions about the inability of non-vascular ascent of sap (Springer Science & plants to transport water using a powerful suction are incorrect. Our Business Media, 2013). results document a remarkable convergence between the functionality [4] N. Martin-StPaul et al., Ecology letters 20, 1437 (2017). of moss and vascular plants. Using new visual tools, we show that a common tall moss species not only transports water under tension, Corresponding author but also protects its vascular system from cavitation by regulating transpiration in a similar fashion to vascular plant [4]. Thus, we conclude Sylvain Delzon that despite the naming of a “vascular plant” division, the key Université de Bordeaux, innovations required for tree evolution are not associated with INRAE, UMR BIOGECO, water transport, butrather with efficient water use. All plant biology 33615 Pessac France sylvain.delzon@u-bordeaux.fr textbooks emphasize the evolution of internal water transport in vascular plants as a turning point in the history of the Earth. Our demonstration Captions of a complex and efficient vascular pipeline in mosses challenges this thinking, and by doing so impacts the entire spectrum of biologists from FIGURE 1: During water stress, cavitation events are localized to the central conducting tissue of the moss students to professors, and from paleo to molecular biologists. (black pixels localize cavitated hydroids). From left to right with the increased water stress, there is also a FIGURE 2 strong shrinkage of the water transport system which is completely reversible. FIGURE 2: The water transport system in Polytrichum commune shows a similar pattern of vulnerability to cavitation-induced failure as seen in vascular plants. Vulnerability curves obtained with three different techniques, including X-ray microCT, shows that during water stress, the water transport system of the moss resists cavitation under substantial water tension, with 50% water transport conduits cavitated at -1.8 MPa. 51