3D VISUALISATION OF SPIRAL GRAIN AND COMPRESSION WOOD IN PINUS RADIATA WITH FLUORESCENCE AND CIRCULAR POLARISED LIGHT IMAGING

Abstract

To visualise 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. 60 µm-thick complete serial transverse sections covering a stem length of nearly 10 mm were imaged at high resolution with a professional flatbed scanner using circular-polarised transmitted light. Lignin autofluorescence from compression wood in these sections was also imaged with a stereo-fluorescence microscope, using blue excitation and green fluorescence. Images were aligned and canals detected with ImageJ macros. A series of image processing steps were applied to the image stack and only resin canals were identified as black dots in the resultant image stack which was used to generate a 3D view of spiral grain using a plug-in ‘3D Viewer’ in ImageJ. The 3D visualisation showed the organisation of resin canals and spiral grain inside the wood. 3D reconstruction to show the compression wood was made from overlays of the fluorescence and the scanner images. Imaging confirmed the rapid onset of spiral grain, with a near vertical adjacent to the pith reorienting to a strong left-handed spiral within the first year of growth. There were fewer canals in the compression wood which appeared to be straighter than the twisted canals found elsewhere. This new method provides new insights in to our understanding on the formation of spiral grain and compression wood and a possible link between their occurrences.