A study of DNA and mitotic activity in the vegetative apex of Douglas fir during the annual growth cycle

Abstract

Vegetative apices of Pseudotsuga menziesii (Mirb.) Franco were studied throughout the annual growth cycle. When observations based on anatomy, histochemistry, and external morphology are combined, the growth cycle of the vegetative apex should be subdivided into five stages: (1) dormancy, (2) early bud-scale initiation, (3) late bud-scale initiation and rapid apical enlargement, (4) early, rapid leaf initiation, and (5) late, slow leaf initiation. The same cytohistological zonation pattern is present in vegetative apices throughout the growth cycle and usually consists of apical, peripheral, and rib meristem zones. During dormancy, early bud-scale initiation, and early leaf initiation, the apical zone is separated into apical initials and central mother cells based on nuclear characteristics and mitotic activity. Cytohistological zonation is supported by constant differences in nuclear volume, mitotic activity, and DNA content between zones. The peripheral zone is mitotically more active than the apical zone; however, the apical zone is not quiescent. Zones vary in size, prominence, and mitotic activity, which often relates to a particular developmental stage of the apex. The dormant apex has no mitotic activity, and cytohistological zonation is present but not distinct. Zonation increases throughout the first half of the growing season, reaches a maximum during late bud-scale and early leaf initiation, and decreases as dormancy approaches. In general, increases in nuclear volume, percentage of nuclei at 4C, and average DNA content per nucleus correlate with increases in the prominence of zonation. Zonation did not result from different zones being "held" at certain C levels of DNA. Although nuclear volume was used in calculating the DNA content, the DNA level often varied independently of volume. Mitotic activity and dormancy appear to be related to carbohydrate levels within the bud and subtending shoot. The period of most prominent zonation is also the period of most active primordial initiation, largest apical size, and the time when new axillary shoots become determined in their pathway of development.