Magnetic resonance imaging of water movement through asparagus

Abstract

Asparagus spears are rapidly growing structures supplied with abundant vascular bundles. After extended periods of water loss, the stem surface becomes flaccid over a core of turgid tissue. We have used magnetic resonance imaging (MRI) coupled with a paramagnetic contrast reagent (Mn²⁺) to visualise water and ion movement within spear tissues. Using this technique we estimated the rate of water flow in the xylem and lateral movement out of the xylem into the surrounding tissues. Longitudinal flow rates of at least 3.7 mm min^–1 and lateral diffusion rates of at least 17 μm min^–1 were recorded. The outer parenchyma was difficult to label with vascular-supplied Mn²⁺, suggesting the presence of an apoplastic barrier. Stem shrinkage was greater in the outer parenchyma than in the inner cortex and pith parenchyma. Feeding the cut spear with 70 mM sucrose reduced the rate of lateral movement of Mn²⁺. The MRI technique highlighted differences in transport rates between adjacent vascular bundles and identified structural features with a resolution of 78 μm. This non-invasive technique is useful both for observing details of the transport paths in living tissue and for setting minimal estimates of transport rates.