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@ARTICLE{VanAs:280605,
      author       = {Van As, H. and Windt, Carel and Homan, N. and Gerkema, E.
                      and Vergeldt, F. J.},
      title        = {{F}low {MRI} teaches us some lessons on hydraulic
                      conductivity in trees},
      journal      = {Magnetic resonance imaging},
      volume       = {25},
      number       = {4},
      issn         = {0730-725X},
      address      = {Amsterdam [u.a.]},
      publisher    = {Elsevier Science},
      reportid     = {FZJ-2016-00373},
      pages        = {586 - 587},
      year         = {2007},
      abstract     = {Hydraulic conductivity of long-distance xylem and phloem
                      transport in plants and trees is key information to validate
                      biophysical structure-function plant models. Such models are
                      in use to address water stress-induced effects and growth
                      limitations. In addition, such models are used to quantify
                      the contribution of plant evapotranspiration and carbon
                      exchange within global atmospheric circulation models [1].
                      In xylem and phloem, the effective flow conducting area and
                      the resistance within the vessel or tracheid connections
                      determine hydraulics. Using conventional methods, it is very
                      difficult to determine the active flow conducting area and
                      to study the dynamics (e.g., day–night, stress responses)
                      therein.A very promising and attractive method for providing
                      detailed quantitative information on effective
                      flow-conducting area in intact plants is flow magnetic
                      resonance imaging (MRI) based on PFG methods. Because the
                      diameter of flow-conducting vessels and tracheids are small
                      in comparison to the pixel size, a crucial step in
                      quantification of the flow and effective flow conducting
                      area is to discriminate stationary and flowing water within
                      a single pixel. The fact that the propagator for stationary
                      water is symmetrical around zero is used to separate the
                      stationary from the flowing water. The signal in the nonflow
                      direction is mirrored around zero displacement and
                      subtracted from the signal in the flow direction to produce
                      the displacement distribution of the flowing and the
                      stationary water. Using this approach, for each pixel in the
                      image, the following flow characteristics are extracted in a
                      model-free fashion: total amount of water, amount of
                      stationary water, amount of flowing water (or flow
                      conducting area), average velocity (including the direction
                      of flow) and volume flow per pixel. A propagator flow
                      imaging method was developed that allowed the flow profile
                      of every pixel in an image to be recorded quantitatively,
                      with a relatively high spatial resolution, while keeping
                      measurement times down to 15–30 minutes [2].Phloem
                      transport in plants is particularly difficult to measure
                      quantitatively. The slow flow velocities and the very small
                      flowing volumes in the presence of large amounts of
                      stationary water make it difficult to distinguish the slowly
                      flowing phloem sap from freely diffusing water. We have
                      optimized the MRI hardware (3-T vertical-bore, intact-plant
                      MRI) and the propagator-fast imaging method to
                      quantitatively measure, for the first time, detailed flow
                      profiles of phloem flow in large and fully developed plants,
                      including trees [3]. In this way, the dynamics in phloem and
                      xylem flow and flow conducting area are studied. The
                      observed differences for day and night in flow-conducting
                      area, which directly relate to xylem and phloem hydraulics,
                      are one of the most striking observations, which
                      demonstrates the potential of the method to study hydraulics
                      in intact plants under normal and stress conditions.Here, we
                      discuss the accuracy of the method to determine the
                      effective flow-conducting area and present results of
                      dynamics in effective flow-conductive area under different
                      conditions. Some striking lessons emerge from the results,
                      which demonstrate that trees are dynamic, unsaturated porous
                      media.},
      cin          = {IBG-2},
      ddc          = {610},
      cid          = {I:(DE-Juel1)IBG-2-20101118},
      pnm          = {899 - ohne Topic (POF3-899)},
      pid          = {G:(DE-HGF)POF3-899},
      typ          = {PUB:(DE-HGF)16},
      doi          = {10.1016/j.mri.2007.01.105},
      url          = {https://juser.fz-juelich.de/record/280605},
}