Cesium in Liriodendron and Other Woody Species: Organic Bonding Sites

In trees labeled with cesium-137, the isotope in the sap streams is primarily in the free ionic form. Much ionic Cs$^{137}$ also appears to occur in intracellular fluids other than the sap streams. However, a small quantity of Cs$^{137}$ forms ionic bonds with organic compounds (probably as salts of carboxylic acids). Even though most of the activity appears in the phloem tissue, some activity is dispersed in much of the sapwood and even in the dead xylem tissue of the heartwood. Some of this Cs$^{137}$ is retained, at least temporarily, by the numerous carboxylic groups of the natural plant compounds of cell walls, cytoplasm of living cells, and cell debris of heartwood.