Cell organelles and fluorescence of parenchyma cells in Eucalyptus bosistoana sapwood and heartwood investigated by microscopy

Background: Eucalyptus bosistoana is currently investigated in New Zealand for its potential to produce naturally durable timber in short-rotation plantations. Little is known of heartwood formation in young trees. The objective of this study was to identify conventional and confocal microscopy methods which allow the observation of cell organelles and the chemical composition in the E. bosistoana parenchyma cells before and after heartwood formation. Results: Nuclei, microtubules and peroxisomes in parenchyma cells of 2-year-old E. bosistoana stems were visualised by confocal microscopy combined with optimised immunolabelling protocols. Sequential staining of the tissue with toluidine blue and iodine/potassium iodide identified different cell organelles in parenchyma cells of sapwood. iodine/potassium iodide stained starch (amyloplasts), while amido black stained proteins in sapwood. Fluorescence emission spectra confirmed the presence of chloroplasts in parenchyma of 2-year-old E. bosistoana. Fluorescence emission spectral (lambda) scans showed differences between parenchyma and fibre cells as well as sapwood and heartwood. Conclusions: Physiological changes between sapwood and heartwood were visualised in parenchyma cells. Labelling of cell organelles was challenging due to unspecific binding and high background signals. Understanding heartwood formation is critical for the success of a plantation forest industry aiming to produce ground-durable timber, as heartwood formation is variable.