Symplasmic, long-distance transport in xylem and cambial regions in branches of Acer pseudoplatanus (Aceraceae) and Populus tremula x P. tremuloides (Salicaceae).

The picture of how long-distance transport proceeds in trees is still far from being complete. Beside the apoplasmic pathway, transport undoubtedly also takes place within the system of living cells in the secondary xylem and cambial region. Because detailed, thorough studies of the symplasmic routes in woody branches, using direct localization with fluorescent tracers, had not been done, here we focused on the main routes of long-distance symplasmic transport in xylem and cambial tissues and analyzed in detail tracer distribution in the rays on the extended cambial surface in branches of Acer pseudoplatanus and Populus tremula ×P. tremuloides. Fluorescent tracers were loaded into branches through the vascular system, then their distribution in xylem and cambial regions was analyzed. Tracer signal was present in the symplast of axial and radial xylem parenchyma cells and in both types of cambial cells. The living cells of xylem parenchyma and of the cambium were symplasmically interconnected via xylem rays. Tracer distribution in rays was uneven on the extended cambial surface; cambial regions with intensively or sparsely dyed rays alternated along the vertical axis of analyzed branches. Symplasmic, long-distance transport is present between the living cells of xylem and the cambial region in woody branches. The uneven distribution of fluorescent tracers in cambial rays along the stems is surprising and suggests the presence of an intrinsic pattern caused by an unknown mechanism.