Shoot development in Betula papyrifera. III. Long-shoot organogenesis

Abstract

Buds and developing branches of Betula papyrifera were collected weekly from mature trees during three successive growing seasons. Material was prepared to show stages of bud inception, development, flushing, shoot expansion, and male inflorescence inception and early development. Stages of bud development were correlated with axillant leaf and subjacent internodal extension. Long-shoot buds develop in the axils of distal late leaves of the previous season's long shoot. The bud contains one rudimentary leaf with prominent stipules, two or three embryonic foliage leaves which flush as early leaves, and about five smaller primordia which expand in the spring as late leaves. Preformed primordia are produced in mid-May through to early August. A pause in the inception sequence occurs in June, corresponding to the period of rapid late-leaf and internodal expansion of axillant leaves and subjacent internodes. Long-shoot bud determination occurs in mid-July. True late leaves may form during shoot extension. Male inflorescence induction, which occurs before bud break, restricts late leaf production and results in the last-formed leaf primordia maturing as transitional leaves instead of foliage leaves. Male induction limits crown expansion by reducing sites for axillary bud formation. This reduces the photosynthetic potential of the shoot.