High-resolution X-ray microscopy is increasingly used for structural phenotyping studies for plant biology at macro-, micro-, and nanoscale levels. To demonstrate the benefits that arise from having one instrument with multiple detectors, we imaged an apple at three different scales using our Bruker SkyScan 2214 Multi-scale X-ray Nano-CT.

With other technologies, quantitative analysis of cells and intercellular spaces in plants has been challenging and largely based on manual image processing. Micro-CT and nanotomography are now used to study water transport in plants; for 3D imaging of plant tissues and organs; and for the study of plant development and organ morphogenesis, non-destructively and with minimal or no sample preparation.

Multi-scale X-Ray Microscopy Imaging of an Apple

For this article, we examined an apple at three different scales. First, the entire apple was captured using the flat panel detector. Next, an apple seed was extracted and imaged using the standard resolution detector. Finally, an extracted section of the apple stem was also examined at a submicron voxel size using the same standard resolution detector. As can be seen from the view in Figure 1 above, the versatile setup of the SkyScan 2214 allowed us to capture the full volume of the apple at standard resolution as an overview scan.

As shown in Figure 2, the overview imaging of the apple provided by the flat panel detector allows us to view hidden interior details such as fruit bruising, the location of the core, seeds, and the intricate vascular network responsible for transporting water and sugars through the xylem and phloem.

Since the vasculature network within the fruit contains concentrated sugars and other transported molecules, we observe natural contrast from the rest of the fruit. Utilizing a maximum intensity projection (MIP) view within CTVox provides us with an interactive 3D view of the complex paths mapped out by the vasculature (Figure 3).

After removing a seed from the larger fruit, the seed was imaged using the standard resolution CCD detector within the SkyScan 2214 nano-CT (Figure 4). While the seed does not have a significant amount of natural contrast, the fine structures present within are still visible within our results using an isotropic voxel size of 5µm.

Within DataViewer we also can interactively examine the contents of the extracted apple seed (Figure 5).

After removing the stem from the fruit, a portion of the stem was imaged using the standard resolution CCD detector within the SkyScan 2214 nano-CT at an isotropic voxel size of 700nm (Figure 6). Prior to imaging the stem, the sample was immersed in a phosphotungstic acid solution before chemical drying with HMDS to boost natural contrast within the sample.

DataViewer provides a clear view of the various vasculature bundles and void spaces within the  chemically dried stem sample (Figure 7). The center of the stem contains a circular bundle of vertically networked vascular connections and a large void space responsible for the transportation of vital nutrients from the larger tree out into the fruit. The cells along the edge of the stem are arranged horizontally to provide structural rigidity and strength to the stem.

Conclusion

The SkyScan 2214 nano-CT is a great match for research groups who need maximum versatility from their instrument. With the ability to equip the instrument with up to four detectors, the SkyScan 2214 provides a flexible, multi-scale solution to most imaging projects. For this reason, research groups with multiple research projects spanning a wide variety of disciplines may see the most benefit from utilization of the SkyScan 2214 nano-CT.

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Scan Specifications

These scans were completed on our SkyScan 2214 nano-CT system at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon while visualization and volumetric analysis of the 2D and 3D results were completed using Dataviewer, CTVox, and CTAn.

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