Late-season changes in allocation of starch and sugar to shoots, coarse roots, and fine roots in two hybrid poplar clones.

Two hybrid Populus clones, Tristis and Eugenei, which had been grown from cuttings in 114-liter pots sunk in the ground, were sequentially harvested during the latter part of the first growing season. Pots were supplied with water and nitrogen at two different rates. Starch and sugar concentrations were determined in samples taken from stems, branches, cuttings, and five diameter classes of roots. Concentrations of starch and sugar in all components of the trees were significantly affected by time of harvest and clone. Water and nitrogen treatments, however, did not significantly affect carbohydrate concentrations. From August to November, as plants developed cold hardiness, starch concentrations declined and sugar concentrations increased in stems and branches, especially in Tristis. In both clones, the large roots (> 1 mm diameter) were major repositories of starch and sugar late in the season. By the end of September, 80% of total-tree nonstructural carbohydrates in Eugenei were located in the roots. Even the fine-root fraction (< 0.5 mm diameter) showed substantial carbohydrate loading in late season, contradicting the hypothesis that starch deposition in fine roots occurs only when they are being formed.