The localization of pentosans within the cell wall of aspen (Populus tremuloides Michx.) by high resolution autoradiography.

Radiochemical studies of Populus tremuloides xylem tissue administered L-[1-(3)H]arabinose, D-[1-(3)H]glucose, and D-[6-(3)H]glucose demonstrate that L-[1-(3)H]arabinose is an excellent precursor for pentosan in this tissue. Transverse sections of first-year xylem (from cambial zone to pith) were examined by light and electron microscope autoradiography. Relatively large amounts of labeled pentosan are found in parenchyma cell walls, including the "protective layer" of ray parenchyma. Computeraided analyses of grain distributions in electron micrographs of cell walls of individual fibers localized the labeled wall components after different periods of incubation by comparison to model behavior. These analyses indicate that pentosan is added to the secondary cell wall of developing fibers by an appositional mechanism.