NMR / MRI and porous (bio)systems

Current research focuses on:

One of the least well understood aspects of plant physiology is the integration and coordination of processes at the organ and whole plant level. This deficiency is scarcely surprising; technological developments that would make it possible to routinely and accurately measure performance of the whole plant have received little attention. Mechanism and control of water and solute fluxes in the xylem and the phloem are examples of such processes, essential for understanding important mechanisms such as elongation growth, fruit development and growth inhibition by e.g. drought stress and fungal infection. By being able to measure these fluxes in relation to the plant (cell) water balance and the cell-to-cell transport it will be possible to explicitly or implicitly examine many aspects of plant function.

Nuclear Magnetic Resonance (NMR) is a non-invasive and non-destructive technique which is increasingly applied to the study of plant cells, plant organs and living plants. The information content and contrast of the MR images can be manipulated to represent parameters of physical or metabolic processes. In this way a combination of anatomical and physiological (functional) information is obtained, referred to as functional imaging. An example of this functional information is the molecular displacement of water molecules. This type of imaging we refer to as dynamic imaging.

The advantage of NMR over any other method of analysis is its potential for the non-invasive and simultaneous measurement of the water relations at the whole organism, tissue and (sub)-cellular level, in intact undisturbed plants. 

The aim of this project is to develop a low field NMR imaging and microscopy technique for intact plant studies. 

Sub-projects within this theme are:

- Mechanism of drought tolerance in maize and pearl millet studied by functional NMR imaging and leaf fluorescence

NMR imaging of relaxation time, diffusion and water transport are used to study the mechanism of drought tolerance in intact plants, by comparing growth response and tissue adaptations in maize and pearl millet during osmotic stress. The NMR parameters are related to information on water distribution over cell compartments, water membrane permeability and cell-to-cell transport. This information will be collaborated to tissue hydraulics and the functioning of water channel forming proteins (aquaporins).

- Functioning of water pathways in cut flowers and intact plants in relation to air embolism and embolism repair in xylem

Vase life of cut flowers is often shortened by water stress: loss of turgidity results in leaf wilting. Air entering the xylem vessels via the cut surface of the stem at harvesting and/or during dry storage of cut flowers can seriously block water (and nutrient) uptake. So, removal of the air is assumed to be essential to avoid water stress and wilting. In intact plants air embolism can also occur due to cavitation of the xylem water column. 

Only indirect methods have been used to establish the role of emboli on the water uptake of flowers and the water balance in intact plants. The really non-destructive and non-invasive method is nuclear magnetic resonance imaging (MRI). Flowing and diffusing ('stationary') water can be measured and discriminated by tracing its displacement in a well defined time interval by pulsed field gradient (PFG) MRI. Recently, we realised a combination of PFG-NMR or (q-space imaging) with turbo spin echo (TSE) imaging to reduce the acquisition time of a complete displacement-encoded image set to less than 15 minutes. In addition, we developed a method to quantify both the self-diffusion constant and some flow characteristics of every pixel in an image, without assuming any model for the flow profile of the observed flowing spins. In this way images of the linear velocity, volume flow rate, cross sectional area available for flow and the diffusion coefficient can be calculated. This information has been successfully applied to study air embolism repair in cut flowers and intact plants (Cucumber).

- NMR and MRI of the functioning of xylem and phloem towards tomato fruits in relation to growth and fruit quality

NMR and MRI techniques will be used to study the dynamics of water and assimilate import in tomato fruits, in relation to growth and development, internal plant water balance and external factors. 

This project is part of a project directed towards the regulation of the quality of tomato fruits (STW, in cooperation with TPK)

- Residual water in desiccating plant systems and its impact on metabolism and spatial structure

- NMR investigation of temporally and spatially resolved dynamics in porous media (Marie-Curie Fellow Dr. Ulrich Tallarek, 1998-2001)

Based on motion-encoding nuclear magnetic resonance techniques, we have investigated a new approach for detailed studies of steady-state and transient stagnant mobile phase mass transfer in chromatographic media (Tallarek et al., J. Phys. Chem. B 1999, 103, 7654-7664). These fundamental studies compared different particle technologies and have allowed important conclusions about the role of the intraparticle tortuosity factor and the finite particle size distributions of these adsorbent media. Studies were made with analytical columns (4.6 mm i.d.), and the packing materials were provided by industrial companies.

A new NMR capillary configuration has been devised, built and employed successfully to study many aspects of the involved fluid dynamics in micro-columns (250 mm i.d.) used for capillary electrophoresis and capillary electrochromatography (Tallarek et al., Anal. Chem. 2000, 72, 2292-2301). We have demonstrated the high efficiency which in fact can be achieved with electrokinetically-driven fluid flows in open tubes and packed beds, the actual profile of the electroosmotic flow (as compared to conventional pressure-driven flow) and its stability. These factors belong to the most important aspects controlling reproducibility of chromatographic separations. With this setup, electroosmotic perfusive flow field in fixed beds has been demonstrated directly and for the first time (Tallarek et al., Angew. Chem. Int. Ed., submitted). The results demonstrate the huge potential this effect may have in separation science over the next decade, especially in the era of genomics and proteomics and very sensitive on-line coupling schemes with nano-electrospray-ionization-mass spectrometry.

- Investigations of mass transport processes involving liquids, gases and solids in channels and in porous media by spatially resolved NMR techniques.

Mid 2003 we will start this new project in cooperation with Dr. Igor V. Koptyug (International Tomography Center, Novosibirsk, Russia). This project is sponsored by the Dutch organisation for Scientific Research (NWO) in the framework of the Dutch-Russian Scientific coopration program.

- Enabling and accelerating the innovation of low water food products