Regulation of tillering by apical dominance: Chronology, interpretive value, and current perspectives

Abstract

The range science profession has traditionally relied upon the concept of apical dominance to explain tiller initiation in perennial grasses. The physiological mechanism of apical dominance is assumed to follow the direct hypothesis of auxin action, which was originally proposed during the 1930's. This hypothesis indicates that the plant hormone auxin (IAA), produced in the apical meristem and young leaves, directly inhibits axillary bud growth. The direct hypothesis was, and continues to be, the sole interpretation of the physiological mechanism of apical dominance since the concept was initially adopted by the range science profession. However, the direct hypothesis was abandoned by plant physiologists during the 1950's because of experimental and interpretive inconsistencies and the demonstrated involvement of a second hormone, cytokinin, in apical dominance. The cytokinin deficiency hypothesis has replaced the direct hypothesis as the current hormonally based interpretation of apical dominance. This hypothesis indicates that IAA produced in the apical meristem blocks the synthesis or utilization of cytokinin within axillary buds inhibiting their growth. Despite wide acceptance, numerous issues remain unresolved concerning this hypothesis, suggesting that it may also be an incomplete interpretation of the physiological mechanism of apical dominance.