title: Temperature effects on xylem sap osmolarity in walnut trees: evidence for a vitalistic model of winter embolism repair.
abstract: We studied the effect of temperature on the carbohydrate status of parenchyma cells during winter in relation to the efflux and influx of sugars between parenchyma cells and xylem vessels in 1-year-old twigs of walnut (Juglans regia L.). The mechanism of sugar transfer between contact cells and vessels was also investigated. We obtained new insights into the possible osmotic role of sugars, particularly sucrose, in stem pressure formation and winter embolism repair. Accumulation of sucrose in the xylem sap during winter was mainly influenced by: (1) abundant conversion of starch to sucrose in the symplast at low temperatures; (2) sucrose efflux into the apoplast at low temperatures (1 degrees C); and (3) inefficient sugar uptake at low temperatures, although efficient sugar uptake occurred at 15 degrees C. We hypothesize that a diethyl pyrocarbonate (DEPC)-sensitive protein mediates facilitated diffusion of sucrose from parenchyma cells to xylem vessels (efflux) in walnut. We discuss the possible occurrence of active H+-sucrose symports and the coexistence of both influx and efflux processes in walnut in winter and the modulation of the relative importance of these flows by temperature.
authors.affiliation: U.M.R. PIAF (INRA - Université Blaise Pascal), Site INRA de Crouelle, 234 Avenue du Brezet, F-63039 Clermont-Ferrand Cedex 2, France. ameglio@clermont.inra.fr
authors.forename: Thierry
authors.initials: T
authors.lastname: Améglio
authors.forename: Mélanie
authors.initials: M
authors.lastname: Decourteix
authors.forename: Georges
authors.initials: G
authors.lastname: Alves
authors.forename: Vincent
authors.initials: V
authors.lastname: Valentin
authors.forename: Soulaiman
authors.initials: S
authors.lastname: Sakr
authors.forename: Jean-Louis
authors.initials: JL
authors.lastname: Julien
authors.forename: Gilles
authors.initials: G
authors.lastname: Petel
authors.forename: Agnes
authors.initials: A
authors.lastname: Guilliot
authors.forename: André
authors.initials: A
authors.lastname: Lacointe
mesh_terms: D002241:Carbohydrates / Q000502:physiology; D031324:Juglans / Q000502:physiology*; D008954:Models, Biological; D009994:Osmolar Concentration; D018547:Plant Stems / Q000502:physiology; D012621:Seasons; D013696:Temperature; D014197:Trees / Q000502:physiology*
chemical_list: D002241:Carbohydrates