NATURAL WOOD COLORING PROCESS IN JUGLANS SP. (J-NIGRA, J-REGIA AND HYBRID J-NIGRA 23 X J-REGIA) DEPENDS ON NATIVE PHENOLIC-COMPOUNDS ACCUMULATED IN THE TRANSITION ZONE BETWEEN SAPWOOD AND HEARTWOOD

Radial distribution of soluble phenolics was investigated at different heights in stems of Juglans nigra, J. regia and hybrids J. nigra 23 x J. regia. Four major phenolic compounds were studied: hydrojuglone gl ucoside (HJG), quercitrin (QUER) and two unknown compounds characteriz ed as two ellagic acid derivatives El and E2. HJG and El content incre ased gradually in the sapwood, peaked in the sapwood-heartwood transit ion zone, and decreased drastically in the heartwood. QUER was accumul ated preferentially around the transition zone, and its content was re latively low in the outer part of the sapwood and in the inner part of the heartwood. E2 content was low in the sapwood and increased in the heartwood. The heartwood formation was marked by the accumulation of new soluble compounds. The relationship between wood extractives and w ood colour were evaluated and discussed. HJG was considered to be a ma jor precursor of heartwood colour providing chromophores through hydro lysis (deglucosylation), oxidation and polymerization processes.