0.7 and 3 T MRI and Sap Flow in Intact Trees: Xylem and Phloem in Action

Abstract

Abstract. Dedicated magnetic resonance imaging (MRI) hardware is described that allows imaging of sap flow in intact trees with a maximal trunk diameter of 4 cm and height of several meters. This setup is used to investigate xylem and phloem flow in an intact tree quantitatively. Due to the fragile gradients in pressure present in both xylem and phloem, methods to study xylem and phloem transport must be minimally invasive. MRI flow imaging by means of this hardware certainly fulfils this condition. Flow is quantified in terms of (averaged) velocity, volume flow (flux) and flow conducting area, either in imaging mode or as a nonspatially resolved total. Results obtained for one tree, imaged at two different field strengths (0.7 and 3 T), are compared. An overall shortening of observed T 2 values is manifest going from 0.7 to 3 T. Although some susceptibility artefacts may be present at 3 T, the results are still reliable and the gain in sensitivity results in shorter measurement time (or higher signal-to-noise ratio) with respect to the 0.7 T system. The results demonstrate that by use of dedicated hardware, xylem and phloem flow and its mutual interaction, can be studied in intact trees in relation to the water balance and in response to environmental (stress) conditions.

Light on phloem transport (an MRI approach)

“…Another method is fluorescent dye tracking (Jensen et al, 2011;Froelich et al, 2011), although this cannot be truly termed "non-invasive" as the tracking agent must be physically injected into the phloem vascular tissue. The only truly non-invasive technique with the ability to measure over 24 hours and for extended time-frames is magnetic resonance imaging (MRI) (Kockenberger et al, 1997;Windt et al, 2006;Van As, 2007;Van As et al, 2009;. A representative MRI image of a tomato plant stem is depicted in Figure 1.2.…”

“…However, in the last two decades MRI has also become an increasingly common technique to study plant water status as well as xylem and phloem long distance transport in plants ( Kockenberger et al, 1997;Peuke et al, 2001Peuke et al, , 2015Scheenen et al, 2002;Windt et al, 2006;Van As, 2007;Windt & Blumler, 2015). The principal advantage of MRI over other techniques for phloem and xylem flow measurement is that MRI is completely non-invasive.…”

“…Furthermore, the T2 of water protons is influenced by many other factors like for example the size of organelles, membrane permeability or proximity to a membrane (van der Weerd et al, 2002). A more comprehensive introduction to the theory of NMR relaxation can be found in dedicated literature on the subject (Bakhmutov, 2005) or (Van As, 2007) and (Van As & van Duynhoven, 2013).…”

“…The detailed formulation of this approach is fully described in the work of Scheenen and Windt (Scheenen et al, 2000, 2001, 2002, 2007Windt et al, 2006;Van As, 2007). This PFG-TSE flow measurement (when the whole q-space is acquired) should not be mistaken with MRI velocimetry, when essentially only two points in the q-space are recorded and from their slope the average flow velocity can be calculated.…”

“…So far, the only non-invasive and long-term applicable technique is magnetic resonance imaging (MRI) (Kockenberger et al, 1997;Windt et al, 2006;Van As, 2007;Van As et al, 2009;.…”