title: Dynamics of leaf gas exchange, chlorophyll fluorescence and stem diameter changes during freezing and thawing of Scots pine seedlings.
abstract: Boreal trees experience repeated freeze-thaw cycles annually. While freezing has been extensively studied in trees, the dynamic responses occurring during the freezing and thawing remain poorly understood. At freezing and thawing, rapid changes take place in the water relations of living cells in needles and in stem. While freezing is mostly limited to extracellular spaces, living cells dehydrate, shrink and their osmotic concentration increases. We studied how the freezing-thawing dynamics reflected on leaf gas exchange, chlorophyll fluorescence and xylem and living bark diameter changes of Scots pine (Pinus sylvestris L.) saplings in controlled experiments. Photosynthetic rate quickly declined following ice nucleation and extracellular freezing in xylem and needles, almost parallel to a rapid shrinking of xylem diameter, while that of living bark followed with a slightly longer delay. While xylem and living bark diameters responded well to decreasing temperature and water potential of ice, the relationship was less consistent in the case of increasing temperature. Xylem showed strong temporal swelling at thawing suggesting water movement from bark. After thawing xylem diameter recovered to a pre-freezing level but living bark remained shrunk. We found that freezing affected photosynthesis at multiple levels. The distinct dynamics of photosynthetic rate and stomatal conductance reveals that the decreased photosynthetic rate reflects impaired dark reactions rather than stomatal closure. Freezing also inhibited the capacity of the light reactions to dissipate excess energy as heat, via non-photochemical quenching, whereas photochemical quenching of excitation energy decreased gradually with temperature in agreement with the gas exchange data.
authors.affiliation: Department of Forest Sciences, University of Helsinki, PO Box 27, FI-00014 Helsinki, Finland Department of Physics, University of Helsinki, PO Box 64, FI-00014 Helsinki, Finland lauri.lindfors@helsinki.fi.
authors.forename: Lauri
authors.initials: L
authors.lastname: Lindfors
authors.affiliation: Department of Forest Sciences, University of Helsinki, PO Box 27, FI-00014 Helsinki, Finland.
authors.forename: Teemu
authors.initials: T
authors.lastname: Hölttä
authors.affiliation: Department of Forest Sciences, University of Helsinki, PO Box 27, FI-00014 Helsinki, Finland.
authors.forename: Anna
authors.initials: A
authors.lastname: Lintunen
authors.affiliation: Department of Forest Sciences, University of Helsinki, PO Box 27, FI-00014 Helsinki, Finland.
authors.forename: Albert
authors.initials: A
authors.lastname: Porcar-Castell
authors.affiliation: Department of Forest Sciences, University of Helsinki, PO Box 27, FI-00014 Helsinki, Finland.
authors.forename: Eero
authors.initials: E
authors.lastname: Nikinmaa
authors.affiliation: Department of Forest Sciences, University of Helsinki, PO Box 27, FI-00014 Helsinki, Finland Department of Physics, University of Helsinki, PO Box 64, FI-00014 Helsinki, Finland.
authors.forename: Eija
authors.initials: E
authors.lastname: Juurola
mesh_terms: D002734:Chlorophyll / Q000378:metabolism*; D005453:Fluorescence; D005615:Freezing; D041605:Pinus sylvestris / Q000254:growth & development* / Q000378:metabolism*; D018515:Plant Leaves / Q000378:metabolism; D018547:Plant Stems / Q000254:growth & development
chemical_list: D002734:Chlorophyll
keywords: extracellular freezing; living cells; phloem; stomatal conductance; water potential; xylem