The anatomy of »blue ring« in the wood of Pinus nigra

Tree-ring features are widely used to identify specific climate, environment and stress events affecting plant life. Cold temperatures can mainly affect the last phase of cell differentiation by interfering in the process of cell wall lignin deposition. Recently, it was found out that the effect of cold temperature on lignification is visible in wood microsections double stained with astra blue which stains cellulose, and red safranin which stains lignin. This new treering feature was called »blue ring«. In this study, we investigated the anatomical morphology of blue rings in transverse and longitudinal wood microsections of three European black pine trees (Pinus nigra Arnold) growing above the treeline. We aim to understand if the lack of lignification is consistent along the entire tracheid length and whether or not pits, responsible for water transport from one tracheid to another, are lignified. The results show that the lack of lignification extends along the entire tracheids cell wall, including bordered pits. Based on our observations we speculate that blue ring occurrence decreases the safety of water transport and wood mechanical properties.