Execution of the auxin replacement apical dominance experiment in temperate woody species

The classic Thimann-Skoog or auxin replacement apical dominance test of exo genous auxin repression of lateral bud outgrowth was successfully executed in both seedlings and older trees of white ash, green ash, and red oak unde r the following conditions: (1) decapitation of a twig apex and auxin repla cement were carried out during spring flush, (2) the decapitation was in th e previous season's overwintered wood, and (3) the point of decapitation wa s below the upper large irrepressible lateral buds but above the lower repr essible lateral buds. Although it has been suggested that neither auxin, th e terminal bud, nor apical dominance have control over the outgrowth of the irrepressible buds during spring hush, there is evidence in the present st udy that indicates that such control over the repressible buds exists. Ln s eedlings, second-order branching, which resulted from decapitation of elong ating current shoots, was also inhibited by exogenous auxin in the three sp ecies. Hence, the auxin replacement experiments did work on year-old prolep tic buds (of branches of older trees) that would have entered the bud bank and also on current buds of seedlings. Cytokinin treatments were ineffectua l in promoting bud growth.