The Relationship between Endogenous Hormone Levels in the Plant and Seasonal Aspects of Cambial Activity

Abstract

The auxin levels at different heights in a ring-porous (Ulmus glabra) and a diffuse-porous (Populus trichocarpa) tree were followed in spring. There was shown to be a correlation between the pattern of spread of cambial activity and auxin gradients. The observed continuation of cambial activity after cessation of extension growth in ring-porous species is apparently due to the fact that the mature leaves produce auxin, whereas little auxin is produced by the leaves of diffuse-porous species under these conditions. The spring/summerwood transition in ring-porous species can be related to a fall in auxin level at the time that shoot extension-growth ceases. There is evidence of phloem production after the cessation of xylem production in Ailanthus altissima and Robinia pseudacacia, but not in Betula pubescent and Acer pseudoplatanus. It is suggested that, in Ailan-thus, phloem production continues because high endogenous gibberellin levels are present after the auxin level has been reduced by short-day treatment.