Magnetic resonance microscopy of changes in water content in stems of transpiring plants.

Abstract

Differences in water content and degree of binding in the various stem tissues of Pelargonium hortorum were observed by magnetic resonance imaging. 'H images were obtained with a resolution of 100 ,um in the transverse plane and a slice thickness of 1250 pum. It was possible to distinguish the principal tissues of the stem by differences in their proton density or apparent water content and spin lattice relaxation time (T,) or degree of water binding. Measurements were made while the plant was slowly and actively transpiring. In the slowly transpiring plant, T, of various tissues ranged from an average of 659 to 865 ms with a proton density variation offrom 72 to 100%. In the actively transpiring plant, T, ranged from an average of 511 to 736 ms, and the proton density was reduced, ranging between 62 and 88% of the peak value found in the slowly transpiring plant. The fibrous sheath surrounding the vascular tissue and the epidermal region was found to have the highest spin density and T1. This paper describes an investigation of the use of magnetic resonance imaging (MRI) for nondestructive research on the content and binding of water in plant tissues. Since the initial demonstration of spatially localized magnetic resonance by Lauterbur (1), it has been clear that MRI has potential as a valuable tool in the study ofplants. Several early researchers demonstrated images of fruits and nuts (2-4), and MRI has been applied to studies of intact plants (5, 6). Studies have been limited by picture volume elements (voxels) with sampling volumes of >10 mm3. But the resolution has been extended into the microscopic domain with voxels as small as 0.012 mm3 with a sufficiently high signal to noise ratio to distinguish subtle differences in signal intensity (7-11). This permits production of images that distinguish the various tissues in a plant stem.A magnetic resonance image is generated by placing a sample in a strong magnetic field that causes the protons (primarily in water) to precess synchronously about the field at the Larmor frequency. Externally applied radio frequency pulses at the Larmor frequency can be used to manipulate the precessing protons away from their equilibrium position (precessing about the field). In the process the protons absorb energy that is later reemitted. The reemitted signal can be localized in space to create a digital image by application of magnetic gradients. For the partial saturation or the spin echo sequence (12), the signal S at any point in the image is given by Eq. 1 as S = NO41 -2e(-TR-TE/2)/T1 + e-TRIT1ie-TE/T2[1]where TR is the repetition time between pulse sequences, TE is the time between the 900 radio frequency pulse and the formation of the spin echo signal, NH is the proton density, and T1 and T2 are the spin lattice and spin-spin relaxation times of the sample. For a more complete description of the technology see ref. 13. One application of MRI in plant science would be to monitor water movement through plant tissue, using change in NH as a gauge of water content...

Nuclear-magnetic-resonance imaging of leaves ofMesembryanthemum crystallinum L. plants grown at high salinity

“…Although this view is accepted by many authors (Bottomley et al 1984;Aguayo et al 1986;Eccles and Callaghan 1986;Johnson et al 1987), so far the resolution of NMR images has been inferior to that obtainable by optical microscopy. However, the results reported here demonstrate that the use of high magnetic field strengths could bring the resolution of the NMR images to the level of a few cells and that additional variation of the spin-echo time (TE) allows the qualitative distinction between water concentration and water binding in different tissue cells.…”

“…1987;Johnson et al 1987). However, the field strengths and gradients used in these studies were not high enough to achieve an in-plane resolution that approaches the dimensions of a single cell in a plant tissue.…”

“…With these magnets the spatial resolution is only of the order of I mm 3. However, the resolution has recently been extended to volume elements of less than 0.012 mm 3 by using a modified whole-body NMR-imaging system (Johnson et al 1987). Magnetic resonance microscopy on a tissue or even at a cellular level is, therefore, conceivable (Aguayo et al 1986;Bottomley et al 1986).…”

Study of the Consequences of Global Warming in Water Dynamics During Dormancy Phase in Temperate Zone Fruit Crops

“…MR techniques have been used to study the sap flow in xylem (Johnson et al, 1987;Köckenberger et al, 1997;Scheenen et al, 2007;Van As, 2007;Wistuba et al, 2000) as well as in its embolism occurrence (Clearwater & Clark, 2003;Fukuda et al, 2007;Holbrook et al, 2001;Umebayashi et al, 2011;Utsuzawa et al, 2005). In contrast to MRI, images obtained by synchrotron X-ray has higher spatial resolution, with possibility to obtain in real time.…”

Observation of the Oxygen Diffusion Barrier in Soybean (Glycine max) Nodules with Magnetic Resonance Microscopy

“…Due to the noninvasive nature of the technique, the same material can be repeatedly observed under different treatment or experimental conditions (3,12,14). Detailed structural information as well as Til and T2 relaxation times can be repeatedly acquired on the same specimen, providing the ability to nondestructively test for specific treatment effects.…”