Cross-sectional images and lignin content analysis of cambial scraping samples used in gene expression analyses. (a) Representative microscopic images of thin transversal cross-sections of cambial scrapings stained with phloroglucinol. The orientation of the differentiating tracheids in the scraping relative to the cambium and developed xylem is indicated at the bottom of the pictures. Blue arrows mark typical intercellular spaces in compression wood, and black arrows mark cells with characteristic rounded shape in compression wood or straight sides in opposite wood. Scale bar at the bottom left represents 0.1 mm. (b) Lignin content of cambial scrapings from compression (black bars) and opposite (grey bars) sides of the eight different maritime pine trees analyzed, sampled in May 2005 (T1-05, T2-05, T3-05, T4-05) and May 2008 (T1-08, T2-08, T3-08, T4-08) after two months of artificial mechanical bending. Lines on the top of every bar show standard error of the mean for four independent replicates. Statistical significance of differences in lignin content between cambial scrapings from compression and opposite sides from the same tree was determined with a paired Student´s t-test (p < 0.05). All differences were statistically significant except for tree T4-08.