Imaging spiral grain in Pinus radiata with X-ray microtomography

A new method was developed to visualise spiral grain in 1-year-old Pinus radiata trees by tracking the orientation of resin canals which follow the grain. Complete serial transverse sections were imaged at high resolution (2400 dpi) with a professional flatbed scanner using circular polarised transmitted light. Circular polarised light was created by arranging linear polariser sheets and quarter wave-retarder plates at specific angles. These caused the resin canals to appear as black dots against the bright background of birefringent tracheids. ImageJ macros were used to align the images, and a series of image processing steps were applied to detect and map the location of the canals. Only resin canals were identified, and when shown as white dots in the resultant image stack, they could be used to generate a 3D view of spiral grain using the ImageJ '3D Viewer' plug-in. These 3D visualisations showed the organisation of resin canals and confirmed the rapid onset of spiral grain, with the near-vertical grain adjacent to the pith generally reorienting to a strong left-handed spiral within the first year of growth. Using the SkyScan 1172 X-ray microtomography system, tomograms of the remaining portions of the wood specimens were collected with resolutions of 2-3 μm per pixel and converted to transverse section images. Processed images and 3D visualisations showed a similar view of resin canal orientation and spiral grain as compared to the scanner method. These methods provided new insights into our understanding on the formation of spiral grain.