Materials
The L. alata was grown in the tree farm of Yihua Lifestyle Technology Co., Ltd. in Gabon. One tree with a straight trunk and ~110 cm in diameter at breast height was sampled for this study. Wood discs about 5 cm thickness was collected at the height of 1.2 m. Then it was stored for more than 6 months to adjust the moisture content to ~12%. Wood samples were collected after air-drying and processed into test samples with different dimensions. Wood powders with ~200 mesh sizes were prepared for determination of diffuse reflection and UV-Vis absorption spectrum. As a comparison of fluorescence observation, a poplar tree (Populus deltoides) was sampled in the Jiaozuo experimental field in Henan, China.
Autofluorescence experiment
Heartwood and sapwood samples of 10 × 5 × 5 mm3 (Longitudinal × Radial × Tangential) were prepared for fluorescence observation, respectively. Meanwhile, wood blacks of the poplar wood with the same size was prepared for comparative observation. The samples were softened until it could be easily sliced off with a small blade [11]. 15-μm-thick slices of cross sections were cut with a sledge microtome (G.S.L.1, Switzerland), and placed between microscope glass slide and coverslip [3]. The properties of wood autofluorescence were observed by fluorescence microscopy (Nikon i55) under the excitation of blue, green and ultraviolet (UV) light, respectively [12].
Diffuse reflection spectra and UV-Vis absorption spectra
The diffuse reflection spectra were recorded on a UV-Vis-NIR spectrometer (Lambda 750, PerkinElmer) within range of 200 to 800 nm by a diffuse reflectance regime. The sample for this test was wood powder of 200 mesh sizes.
The UV-Vis absorption spectra were measured using a UV-Vis spectrophotometer (N5000, PerkinElmer) in the range 200 to 800 nm. 0.1 g and 1 g of air-dried wood powders (200 mesh) of L. alata were extracted in a benzene-ethanol mixture solution for 8 hours, respectively. Then they were placed in a 100 mL hydrothermal reactor by a hydrothermal carbonization treatment at 140°C for 24 hours, respectively [13]. The solution may contain some lignin after hydrothermal treatment, so 0.1 g of commercial lignin solution was prepared as a comparison. The supernatant was taken from the filtrated solution to obtain the UV-Vis absorption spectra.
Macrostructural observation
Wood samples of 50 × 50 × 30 mm3 (L × R × T) were cut for macrostructure observation. A sharp blade was used to trim their plane to ensure the flatness and smooth surface. The macrostructure was observed by a stereo microscope (MZS0745/MZS0745T). The main macroscopic structural features were examined including color, smell, the size and distribution of vessels, rays and grains.
Microstructural and SEM observation
Test samples of 10 × 5 × 5 mm3 (L × R × T) were prepared for microstructural observation. 15-μm-thick slices were cut from softened samples including cross, tangential and radial sections. After stained with 1% safranine O solution and dehydrated, sections were fixed on the slide with neutral quick-drying glue and observed under a biological optical microscope (Nikon ECLIPSE Ci-L). Thirty groups of fibers were randomly selected in cross-section and their double wall thickness and cell lumen diameter were measured using image analysis software (ImageJ 1.50i). Meanwhile, 12 cross sections were observed and photographed to measure the proportion of each major cell types.
Small blocks with dimension of 5 × 5 × 5 mm3 (L × R × T) were trimmed by hand with a new single-edged razor blade for each surface [12]. Before testing, the blocks were coated with a thin layer of gold under vacuum. Then, samples were observed with scanning electron microscope (SEM, ZEISS Sigma 300) under an accelerating voltage of 15 KV.