title: Phloem as capacitor: radial transfer of water into xylem of tree stems occurs via symplastic transport in ray parenchyma.
abstract: The transfer of water from phloem into xylem is thought to mitigate increasing hydraulic tension in the vascular system of trees during the diel cycle of transpiration. Although a putative plant function, to date there is no direct evidence of such water transfer or the contributing pathways. Here, we trace the radial flow of water from the phloem into the xylem and investigate its diel variation. Introducing a fluorescent dye (0.1% [w/w] fluorescein) into the phloem water of the tree species Eucalyptus saligna allowed localization of the dye in phloem and xylem tissues using confocal laser scanning microscopy. Our results show that the majority of water transferred between the two tissues is facilitated via the symplast of horizontal ray parenchyma cells. The method also permitted assessment of the radial transfer of water during the diel cycle, where changes in water potential gradients between phloem and xylem determine the extent and direction of radial transfer. When injected during the morning, when xylem water potential rapidly declined, fluorescein was translocated, on average, farther into mature xylem (447 ± 188 µm) compared with nighttime, when xylem water potential was close to zero (155 ± 42 µm). These findings provide empirical evidence to support theoretical predictions of the role of phloem-xylem water transfer in the hydraulic functioning of plants. This method enables investigation of the role of phloem tissue as a dynamic capacitor for water storage and transfer and its contribution toward the maintenance of the functional integrity of xylem in trees.
authors.affiliation: Hawkesbury Institute for the Environment, University of Western Sydney, Penrith, New South Wales 2751, Australia (S.P., M.G.T.);École Normale Supérieure, 75005 Paris, France (J.R.); andConfocal Bio-Imaging Facility, Hawkesbury Campus, Deputy Vice-Chancellor (Research and Development) Division, University of Western Sydney, ‪‪Penrith South, ‪New South Wales 1797, Australia (A.S.)‬‬‬‬‬‬ s.pfautsch@uws.edu.au.
authors.forename: Sebastian
authors.identifier: http://orcid.org/0000-0002-4390-4195
authors.initials: S
authors.lastname: Pfautsch
authors.affiliation: Hawkesbury Institute for the Environment, University of Western Sydney, Penrith, New South Wales 2751, Australia (S.P., M.G.T.);École Normale Supérieure, 75005 Paris, France (J.R.); andConfocal Bio-Imaging Facility, Hawkesbury Campus, Deputy Vice-Chancellor (Research and Development) Division, University of Western Sydney, ‪‪Penrith South, ‪New South Wales 1797, Australia (A.S.)‬‬‬‬‬‬
authors.forename: Justine
authors.initials: J
authors.lastname: Renard
authors.affiliation: Hawkesbury Institute for the Environment, University of Western Sydney, Penrith, New South Wales 2751, Australia (S.P., M.G.T.);École Normale Supérieure, 75005 Paris, France (J.R.); andConfocal Bio-Imaging Facility, Hawkesbury Campus, Deputy Vice-Chancellor (Research and Development) Division, University of Western Sydney, ‪‪Penrith South, ‪New South Wales 1797, Australia (A.S.)‬‬‬‬‬‬
authors.forename: Mark G
authors.initials: MG
authors.lastname: Tjoelker
authors.affiliation: Hawkesbury Institute for the Environment, University of Western Sydney, Penrith, New South Wales 2751, Australia (S.P., M.G.T.);École Normale Supérieure, 75005 Paris, France (J.R.); andConfocal Bio-Imaging Facility, Hawkesbury Campus, Deputy Vice-Chancellor (Research and Development) Division, University of Western Sydney, ‪‪Penrith South, ‪New South Wales 1797, Australia (A.S.)‬‬‬‬‬‬
authors.forename: Anya
authors.initials: A
authors.lastname: Salih
mesh_terms: D001692:Biological Transport; D005052:Eucalyptus / Q000033:anatomy & histology / Q000648:ultrastructure; D005456:Fluorescent Dyes / Q000378:metabolism; D052585:Phloem / Q000378:metabolism*; D018547:Plant Stems / Q000378:metabolism*; D013997:Time Factors; D014197:Trees / Q000378:metabolism*; D014867:Water / Q000378:metabolism*; D014934:Wood / Q000033:anatomy & histology / Q000648:ultrastructure; D052584:Xylem / Q000378:metabolism*
chemical_list: D005456:Fluorescent Dyes; D014867:Water