Interactions of Hormones in Shoots

In some species, apical dominance may involve only the presence or absence of auxin; in others, there is an interplay of two or three hormones. Active roots synthesize cytokinins that arc transported to the shoot and stimulate axillary buds. Whether buds become active or remain dormant depends on the relative amounts of the two hormones. If a plant is growing vigorously, its roots are active and cytokinin levels are high; many buds at a distance from a shoot apical meristem have a low auxin/high cytokinin ratio and become active. Such a mechanism is adaptive because if a plant is growing well, activating me dormant buds increases the rate of new leaf production. The role of ABA in apical dominance is uncertain. It is present in quiescent buds but does not decrease either just before or as buds are becoming active and growing out.

Apical dominance in prickly pear cacti (Opuntia polyacantha) is more elaborate. Is spine clusters are short shoots and the spines are highly modified leaves. If the spine cluster is excised and placed in tissue culture with cytokinin, the dormant short shoot apical meristem grows out as a long shoot—a new "pad" similar to a normal branch. If the culture medium contains gibberellins instead of cytokinin, the short shoot apical meristem produces more spines—it acts as a rejuvenated short shoot.

A second interaction of hormones occurs in the vascular cambium. Auxin alone activates the cambium and elicits differentiation of xylem. But gibberellin is also present in a healthy stem and causes some of the new cells to differentiate as phloem. Without the interaction of both auxin and gibberellin, a normal, functional vascular system would not develop.