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Unformatted text preview: Intact Plant Magnetic Resonance Imaging to Study Dynamics in Long-Distance Sap Flow and Flow-Conducting Surface Area 1 T.W.J. Scheenen 2 , F.J. Vergeldt, A.M. Heemskerk 3 , and H. Van As* Laboratory of Biophysics and Wageningen Nuclear Magnetic Resonance Centre, Department of Agrotechnology and Food Sciences, Wageningen University, 6703 HA Wageningen, The Netherlands Due to the fragile pressure gradients present in the xylem and phloem, methods to study sap flow must be minimally invasive. Magnetic resonance imaging (MRI) meets this condition. A dedicated MRI method to study sap flow has been applied to quantify long-distance xylem flow and hydraulics in an intact cucumber ( Cucumis sativus ) plant. The accuracy of this MRI method to quantify sap flow and effective flow-conducting area is demonstrated by measuring the flow characteristics of the water in a virtual slice through the stem and comparing the results with water uptake data and microscopy. The in-plane image resolution of 120 3 120 m m was high enough to distinguish large individual xylem vessels. Cooling the roots of the plant severely inhibited water uptake by the roots and increased the hydraulic resistance of the plant stem. This increase is at least partially due to the formation of embolisms in the xylem vessels. Refilling the larger vessels seems to be a lengthy process. Refilling started in the night after root cooling and continued while neighboring vessels at a distance of not more than 0.4 mm transported an equal amount of water as before root cooling. Relative differences in volume flow in different vascular bundles suggest differences in xylem tension for different vascular bundles. The amount of data and detail that are presented for this single plant demonstrates new possibilities for using MRI in studying the dynamics of long-distance transport in plants. Sap flow and hydraulic conductivity of long-distance xylem and phloem transport provide key informa- tion to validate biophysical structure-function plant models based on integrated carbon and water alloca- tion. Such models are currently used to address plant performance and stress-induced effects (e.g. Daudet et al., 2002; Tardieu, 2005), as well as to quantify the contribution of plant evapotranspiration and carbon exchange within global atmospheric circulation models (Sellers et al., 1997). In xylem and phloem, the flow- conducting surface area and the resistance within the vessel or tracheid connections determine hydraulic conductivity (Sperry et al., 2003). Using conventional methods, it is difficult to mea- sure transport within intact plants or to determine the active flow-conducting area. In the phloem, xylem, and tissue surrounding them, complex and fragile gradients in pressure and osmotic potential exist that are easily disturbed by invasive experimentation (Verkman, 2000; Steudle, 2001; Koch et al., 2004)....
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This note was uploaded on 05/28/2010 for the course WE BIOL000000 taught by Professor Laurychaerle during the Spring '10 term at Ghent University.
- Spring '10