Seasonal variations in the polar-transport pathways and retention sites of [(3)H]indole-3-acetic acid in young branches ofFagus sylvatica L.

Branches were cut from young beeches (Fagus sylvatica L.) at various stages of the annual cycle and [(3)H]indole-3-acetic acid (0.35 nmol) was applied to the whole surface of the apical section of each branch, just below the apical bud. The labelled pulse (moving auxin) and the following weakly radioactive zone (auxin and metabolites retained by the tissues) were localized by counting: microautoradiographss were made using cross sections from these two regions. During the second fortnight of April, auxin was transported by nearly all the cells of the young primary shoot, but the label was more concentrated in the vascular bundles. Auxin transport became the more localized: the cortical parenchyma appeared to lose its ability to transport the hormone (end of April), followed in turn by the pith parenchyma (May). Polar auxin movement at that time was limited to the outer part of the bundle (cambial zone and phloem) and to the inner part (protoxylem parenchyma). Later protoxylem parenchyma ceased to carry auxin. During the whole period of cambial activity, auxin was transported and retained mainly by the cambial zone and its recent derivatives. In September, before the onset of dormancy, and in February, at the end of the resting period, the transport pathways and retention sites for auxin were mainly in the phloem, where sieve tubes often completely lacked radiolabel. When cambial reactivation occurred in the one-year shoot, auxin was mainly carried and retained again in the cambial zone and differentiating derivatives.