Abstract
Populus spp. show continuous shoot growth on natural long-days from April to August in the open. Pinus densiflora SIEB. et ZUCC., however, never shows continuous shoot growth and ceases its shoot growth even on natural long-days in the open. This is due to the existence of a correlative inhibition between shoot growth (internode elongation) and leaf-primordia differentiation (activity of apical meristems) at shoot apexes. Therefore, if the balance between growth and differentiation can be controlled on a fifty-fifty basis, P. densiflora can show continuous shoot growth, namely, balanced shoot growth. In this pine, growth and differentiation are kept balanced on photoperiods of 18 to 20 hr. In fact, during 28 months, seven plants showed continuous shoot growth without branching, and three plants showed continuous shoot growth with only one growth-stoppage (branching) on 20-hr photoperiods. Unbalanced intensive shoot growth induces the formation of lateral buds and the cessation of shoot growth. The cessation of shoot growth is brought about by the exhaustion of leaf-primordia at shoot apexes. Foxtailing, observed in the tropical zone, also is considered to be induced by balanced shoot growth on photoperiods of 12 to 14 hr, on which growth activity and differentiation activity are balanced on a fifty-fifty basis. Because unbalanced intensive shoot growth brings about branching, constant existence of unexpanded leaf-primordia on shoot apexes induces foxtailing with or without continuous shoot growth.
