Three-Dimensional visualization of spiral grain and compression wood in pinus radiata imaged by circular polarized light and fluorescence

To visualize the development of spiral grain in young pine trees, a novel technique was developed that is based on tracking the orientation of axial resin canals. Serial transverse sections covering a stem length of more than 5 mm were imaged at high resolution with a professional flatbed scanner using circular-polarized transmitted light. ImageJ was used to fully align the section images and to identify resin canals. Canal locations were used to generate a three-dimensional (3D) view of resin canal organization, and thus spiral grain, using the plug-in 3D viewer in ImageJ. Imaging confirmed the rapid onset of spiral grain, with a near vertical grain adjacent to the pith reorienting to a strong left-handed spiral within the first year of growth. Although vertical trees had a symmetrical grain pattern, trees tilted to generate compression wood showed grain changes on their lower sides. Lignin autofluorescence from these sections was imaged with a stereo-fluorescence microscope, with the combination of blue excitation and the collection of green fluorescence highlighting compression wood. 3D reconstructions were made from overlays of the fluorescence and the processed resin canal images. There were fewer canals in the compression wood, and these appeared to be straighter than the twisted canals found elsewhere. This new method provides new insights to our understanding of the formation of spiral grain and compression wood and a possible link between their occurrences.