Nuclear magnetic resonance imaging of water motion in plants

This Thesis treats one of the new techniques in plant science i.e. nuclear magnetic resonance imaging (NMRi) applied to water motion in plants. It is a challenge, however, to measure this motion in intact plants quantitatively, because plants impose specific problems when studied using NMRi. At high magnetic field strength air-filled intercellular spaces in the plant tissue cause susceptibility-related local magnetic field inhomogeneities, which are much smaller at low magnetic field strength. The inherently low signal-to-noise ratio at low magnetic fields is compensated by the possibility to record a long train of spin-echoes, since generally the spin-spin relaxation time T 2 at low magnetic field is longer than at high magnetic field.In this Thesis the spin echo train is used to shorten the time to produce an NMR image. As a result, time-dependent flow phenomena can be followed at a physiologically relevant time scale using dynamic NMRi employing either a pulsed field gradient (PFG) spin echo sequence (for fast flow, Chapter 2) or a PFG stimulated echo motion-encoding sequence (for slow flow, Chapter 3). Using the quantification method presented in this Thesis (Chapter 4) a number of flow characteristics can be determined for every pixel in an image of a plant stem:the total amount of water,the amount of stationary water,the amount of flowing water,the mean linear flow velocity of the flowing water andthe volumetric flow.These flow characteristics, together with the water density (or total amount of water) and the T 2 value per pixel (measured with quantitative T 2 imaging), were studied in the stem of a cucumber plant as a function of the day-night cycle and cooling of the root system. Root cooling results in inhibition of the water uptake and xylem- and phloem transport, and causes severe wilting of the plant leaves. Following root cooling, during recovery of the plant from its wilted state, the T 2 -values of tissue around the vascular bundles strongly decrease, which may indicate an increased membrane permeability for water of the tissue cells in this period (Chapter 5).During root cooling, large negative pressures in the plant xylem cause cavitations in the vessels, blocking further water transport. In this Thesis the first direct in vivo observations of refilling of cavitated xylem vessels are presented (Chapter 6). This refilling takes many hours and occurs while nearby vessels are under tension and are transporting water. This finding has important implications for the mechanism underlying the refilling process: water entering the refilling vessel must be hydraulically isolated from flowing water in nearby vessels.The strategy (Chapter 7) and methodology of quantitative flow and T 2 NMR imaging, discussed in this Thesis has opened new ways to find answers to longstanding questions in plant science.