A preliminary investigation of the role of auxin and cytokinin in sylleptic branching of three hybrid poplar clones exhibiting contrasting degrees of sylleptic branching

Sylleptic branches grow out from lateral buds during the same growing season in which the buds are formed. This type of branching is present in poplar and in many tropical species. It results in the production of more branches, more leaves and expanded photosynthetic capacity and is thought to assist in increasing the overall growth and biomass of the tree at a young age. However, very little is known about the physiology of sylleptic branching in poplar, which is an extremely important source of fibre and fuel. In the present study of three hybrid poplar clones (11-11, 47-174 and 49-177) of Populus trichocarpa x P. deltoides exhibiting contrasting degrees of sylleptic branching, an analysis was carried out on parent shoot elongation and sylleptic branching, together with a preliminary comparison of the parent shoots' sensitivity to auxin (naphthaleneacetic acid) as a repressor of lateral bud outgrowth, and cytokinin (benzyladenine) as a promoter. Suggestive evidence was found for an inverse correlation between parent shoot sensitivity to auxin and the degree of sylleptic branching, as well as a partially positive correlation with respect to sensitivity to cytokinin. The present data are consistent with the hypothesis that auxin and cytokinin may play repressive and promotive roles, respectively, in the sylleptic branching of hybrid poplar.