Differential compartmentation of o-diphenols and peroxidase activity in the inner sapwood of the Juglans nigra tree

Black walnut (Juglans nigra L.) heartwood is characterised by the appearance of a dark-brown colour in the inner part of the trunk. At the cellular level, phenolic compounds and peroxidases (PODs, EC 1.11.1.7) are most likely involved in the generation of this colour. Ortho-diphenols are revealed by Arnow’s reagent and condensed tannins by vanillin-HCl or p-dimethylaminocinnamaldehyde. These reagents reveal that only a small percentage of the living sapwood parenchyma cells accumulate phenolic compounds in the vacuole. POD activity is revealed either with guaiacol or 3,3’-diaminobenzidine. The simultaneous disclosure of o-diphenols and POD in the same cross-section clearly indicates that POD activity and its phenolic substrates are localised in different ray parenchyma cells of walnut sapwood. None of the cells exhibiting POD activity contain o-diphenols and vice versa. This compartmentation of active PODs and of their potential substrates in different cells within the living woody tissue of walnut is discussed.