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Branching out or staying in: it's all a matter of hormones
It is likely that many readers of this journal were introduced to auxin physiology in class experiments on apical dominance. The restoration by auxin of the apical inhibition of axillary bud outgrowth in decapitated Phaseolus seedlings is a simple experiment that 'works' even with large classes of first year students. However, our more thoughtful students may point out that some familiar species do not exhibit strong apical dominance; those of us who grow tomatoes will agree with them. Similar variations in apical dominance occur between tree species, and these variations are particularly apparent in comparing proleptic and sylleptic branching. In proleptic branching, exhibited by many trees in temperate regions, buds remain dormant during their season of formation and grow in the following year. In sylleptic branching, the buds grow out during the season in which they are formed. This is common in tropical trees but also occurs in some temperate genera, including Populus, as described by Cline and Dong-Il (Ohio State University, pp. 417-421). They worked on three clones of hybrid Populus that exhibited very different degrees of sylleptic branching. Saplings of each clone were decapitated and the shoot apices were replaced with auxin applied in lanolin paste. The clone exhibiting the highest degree of sylleptic branching was the least subject to auxin inhibition (and vice versa), leading the authors to suggest that the degree of sylleptic branching may be inversely correlated with auxin sensitivity. Furthermore, there were also indications that clones with the highest degree of syllepsis showed the greatest response (in terms of bud outgrowth) to cytokinin applied to dormant buds. The authors conclude that 'The . . . data are consistent with the hypothesis that auxin and cytokinin . . . play repressive and promotive roles . . . in the sylleptic branching of . . . poplar'. This is very reassuring for those of us who teach plant physiology!
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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