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| Home > Publications database > Flow MRI teaches us some lessons on hydraulic conductivity in trees |
| Home > Publications database > Flow MRI teaches us some lessons on hydraulic conductivity in trees |
| Journal Article | FZJ-2016-00373 |
; ; ; ;
2007
Elsevier Science
Amsterdam [u.a.]
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Please use a persistent id in citations: doi:10.1016/j.mri.2007.01.105
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.
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