Foliar nutrition and wood growth in red pine: the distribution of radiocarbon photoassimilated by individual branches of young trees

Abstract

The distribution of photoassimilated carbon-14 in young plantation trees was studied 6 or 10 days after supplying 14CO2 for a day to a single branch in the second, third, or fourth whorl. Both apical and cambial growth occurred during the interval, and apical growth throughout the trees was measured. Elongating terminals and products of cambial growth in the fed branch were highly labeled. In all trees some 14C was exported to the adjacent side of the tree. Movement in the trunk was bidirectional, but the position of the donor branch determined the direction of major transport. Only from whorl 2 was it upward; from whorl 3 or 4 it was downward. In both directions activity decreased with distance from the base of the donor branch, and the leader did not accumulate more, per unit weight, than the intervening internodes. Some 14C entered branches arising in the path of transport.Radioactivity was concentrated only in regions of growth, whether apical or cambial. Most of the 14C was in ethanol-insoluble compounds, largely in cell wall constituents. Autoradiographs of stem sections confirmed that 14C was deposited in currently developing tracheids of secondary xylem during most of the 10-day growth period. The ratio of activity in lignin to that in cellulose was inversely related to the total 14C in the cell wall constituents.