Particle-Based Imaging Tools Revealing Water Flows in Maize Nodal Vascular Plexus

Abstract

In plants, water flows are the major driving force behind growth and play a crucial role in the life cycle. To study hydrodynamics, methods based on tracking small particles inside water flows attend a special place. Thanks to these tools, it is possible to obtain information about the dynamics of the spatial distribution of the flux characteristics. In this paper, using contrast-enhanced magnetic resonance imaging (MRI), we show that gadolinium chelate, used as an MRI contrast agent, marks the structural characteristics of the xylem bundles of maize stem nodes and internodes. Supplementing MRI data, the high-precision visualization of xylem vessels by laser scanning microscopy was used to reveal the structural and dimensional characteristics of the stem vascular system. In addition, we propose the concept of using prototype "Y-type xylem vascular connection" as a model of the elementary connection of vessels within the vascular system. A Reynolds number could match the microchannel model with the real xylem vessels.

Keywords: Particle Image Velocimetry; Zea mays L.; contrast-enhanced magnetic resonance imaging; internodes; lab-on-a-chip; laser scanning microscopy; nodal plexus; plant 3D imaging; systems biology; vascular system.

Figure 1

MRI methods for mapping the water distribution in maize stem tissues: $T_2$-weighted MRI (a); proton-weighted maps of non-aqueous (b) and aqueous (c) proton distributions; diffusion-weighted MRI (d) and velocity map of water flow through a virtual slice (e). (f) $T_1$-weighted MRI before and after root exposure to MnCl${}_2$ solution (10 mM); the arrows indicate the locations of manganese ion accumulation; pseudo-staining reflects the intensity distribution of the MRI signal on the scan; MRI scans are axially oriented. (g) $T_2$-weighted MRI image of the growth zone of leaves and stem of maize; a longitudinal section of the stem (g) and some transverse sections (h–l) located in internodes (in, in the figure) (for (i,k)) and nodal plexuses (np, in the figure) (for (j,l)) are indicated; the pixel size was 56 × 56 µm, and the slice thickness was 0.5 mm. Turbo Rapid Imaging with Refocused Echoes (TurboRARE) method.

Figure 2

Contrast-enhanced MRI study of gadolinium chelate distributing inside nodal plexuses (nps) and internodes (ins) of the maize stem. (a) The MRI-studied area marked by the red frame was at the bottom of the maize stem, from the root/shoot nodal plexus (np) to a height of 18 mm. The location of the internode (in) section for high-resolution LSM-imaging is marked with a blue arrow. (b) Temporal series of MRI slices. (c) Matching of internodal vascular bundles in the MRI slice and in the high-resolution LSM-image. (d) Space–time distribution of MRI signal reflecting Gd penetration into the maize stem xylem vascular system. The red arrow indicates the roots/stem connection region, which can be interpreted as a source of gadolinium chelate.

Figure 3

Particle Image Velocimetry. (a) General scheme of the experimental stand. (b) An example of xylem vessel connections (marked by red circles) corresponding to microchannel prototype “Y-type xylem vascular connection”. The image of xylem vessels in a maize leaf was obtained after 2 h of enzymatic degradation of tissues under bright light on an Olympus BX53 microscope (Olympus corporation, Japan) with 20× magnification. (c) Double image of tracer particles in model microchannel. (d) Scheme of prototype “Y-type xylem vascular connection”. (e) Instantaneous velocity field obtained for the double image superimposed on the tracer particle image.

Figure 4

Longitudinal component of the average flow velocity in prototype “Y-type xylem vascular connection” resulted from different ratios of inlet flow rates and equal total flow rate in the outlet channel. (a) $Re=3$ equaled to 0.1 mm per second; (b) $Re=10$ equaled to 0.25 mm per second; (c) $Re=47$ equaled to 1.2 mm per second.

Figure 5

LSM image-based determination of numerical characteristics for Reynolds number-based matching between the xylem vessels in the maize stem and the microchannels in the lab-on-a-chip model. (a) The main stages of LSM image processing for obtaining data on the spatial distribution of the dimensional characteristics of the xylem vessels (metaxylem (Mx) and protoxylem (Px)) in the internode of the maize stem, including expert manual segmentation, contour extraction, calculation of cross-sectional areas, and ellipse fitting. (b) The location of the xylem vessels on the cross section of the maize stem in the I, II, III, or IV “rings”; the coordinates were recalculated into the polar system relative to the nominal center of the stem; the color indicates the cross section of the vessel area. (c) LSM images for typical xylem bundles located in the I, II, III, and IV “rings”. (d) For xylem bundles located in the I, II, III, and IV “rings”, the distribution of vessels relative to the center of the stem, the total cross-sectional areas, and violin plots for Reynolds numbers ($Re$).