FORMATION OF HEARTWOOD SUBSTANCES IN THE STEMWOOD OF ROBINIA-PSEUDOACACIA L .2. DISTRIBUTION OF NONSTRUCTURAL CARBOHYDRATES AND WOOD EXTRACTIVES ACROSS THE TRUNK

The distributions of reserve carbohydrates and of three dominant heart wood extractives were determined in the trunkwood of Robinia pseudoaca cia L. The trees were cut at different times of the year (September, N ovember, January, and April). With the exception of the tree felled in January, all trunks exhibited highest contents of nonstructural stora ge carbohydrates (glucose, fructose, sucrose, and starch) in the young est, outermost sapwood zone. With increasing depth of the trunk, the l evels of carbohydrates decreased. At the sapwood-heartwood transition zone, only trace amounts of nonstructural carbohydrates were present. The heartwood itself contained no storage material. The wood zones of different ages of the trees cut in September, November, and January ex hibited glucose/fructose ratios of approximately 1. In April, however, there was a shift to glucose. In the youngest sapwood the amounts of soluble sugars were higher in the early- than in the latewood. Older z ones of the sapwood and the sapwood-heartwood transition zone showed t he opposite behaviour. Three main wood extractives of Robinia were cha racterized and quantified: the flavanonol dihydrorobinetin (DHR), the flavonol robinetin (ROB) and a hydroxycinnamic acid derivative (HCA). Only DHR was present - in very low amounts - in the younger sapwood of all trunks investigated. Higher amounts (>1 mumol/g dry weight) of th is compound and the HCA were present in the sapwood-heartwood transiti on zone. DHR augmented within the heartwood up to a more or less const ant level. HCA increased towards the heartwood and decreased again in the inner heartwood parts. ROB appeared in the innermost parts of the sapwood-heartwood transition zone and reached maximum values in older parts of the heartwood. The results indicate that starch is hydrolyzed at the sapwood-heartwood boundary and thus represents a primary major source of hydroxycinnamic acid and flavonoid synthesis.