Auxin promotes dormancy callose removal from the phloem ofMagnolia kobus and callose accumulation and earlywood vessel differentiation inQuercus robur.

During winter, the phloem of the diffuse-porous tree magnolia (Magnolia kobus DC.) is dormant and is characterized by heavy deposits of dormancy callose. Application of 1-naphthaleneacetic acid (NAA) to either the top or the lower ends of excised dormant branches before bud break resulted in the removal of the dormancy callose from the sieve tubes. In both intact and auxintreated branches, callose degradation occurred first in the recently formed sieve tubes. There was no new vessel differentiation in magnolia before bud break. In contrast, the sieve tubes of the ring-porous oak (Quercus robur L.), which possess massive dormancy callose deposits during winter, were almost callose-free just before bud break. Application of auxin to the top of excised branches before bud break resulted in callose accumulation on the most recently formed sieve tubes. The first earlywood vessels were evident in oak before bud break, and their numbers were increased by auxin application. The early development of phloem and xylem (before bud break) in ring-porous species is an ecological adaptation which prepares the vascular system of these trees to function immediately at the beginning of their growing season which is relatively short.