Apical dominance in long shoots of white pine ( Pinus strobus )

Abstract

The roles of auxin and nutrition in determining the expression of dominance within a whorl of current shoots were investigated. It was demonstrated that the removal of shoots from a whorl induces compensatory growth in the remaining shoots. Treatments that block auxin movement (triiodobenzoic acid (TIBA), girdling) or decrease auxin production (defoliation, decapitation) also induce compensatory growth, at the same time inhibiting growth in the treated shoot. On the other hand, capping a decapitated and defoliated shoot with indoleacetic acid (IAA) in lanolin maintains diameter growth in the shoot stump and prevents the occurrence of compensatory growth. It is concluded that the shoots in a whorl compete for dominance by competing for a common supply of nutrients transported from the preceding internode. The quantity of nutrients diverted into a shoot, hence growth, is a function of the shoot's capacity to produce growth substances. Hormone synthesis, as well as shoot length, fascicle number, and number of buds in the current whorl are predetermined in the winter bud.The degree of dominance, expressed as the ratio of the sizes of the terminal and the longest lateral in the whorl, depends on the nutrient supply available during the period of winter bud development as well as during the period of shoot expansion.