Vegetative bud development and cone differentiation in Abies amabilis

Abstract

In the trees studied, vegetative buds began development in early April, bud burst occurred in early June and shoot elongation was completed by late July. Vegetative buds initiated bud scales from mid-April until mid-July and then initiated leaf primordia until the vegetative buds became dormant in Nov. All axillary buds were initiated in mid-May and their bud scales were initiated until early July. During bud-scale initiation, distal vegetative lateral apices were more conical but had a mitotic frequency similar to other lateral apices. Near the end of bud-scale initiation, vegetative apices accumulated more phenolic and ergastic compounds in future pith cells than did potential seed-cone or pollen-cone apices. Bud differentiation occurred in mid-July at the end of lateral shoot elongation. During bud differentiation the mitotic frequency of pollen-cone and seed-cone apices increased much more than that of distal vegetative apices. This resulted in a marked increase in apical size and a change in apical shape and zonation that made reproductive apices easily distinguishable from vegetative apices. Bracts began to be initiated in mid-July, and ovuliferous scales, in mid-Aug. Both continued to be initiated until seed-cone buds became dormant in Nov. A single megaspore mother cell formed in each ovule before dormancy. Microsporophylls were initiated from mid-July until early Sept. Microsporangia began to differentiate in Sept. and contained microspore mother cells when pollen cones became dormant in mid-Oct. Meiosis did not begin before dormancy. A few potential vegetative and many potential seed-cone and potential pollen-cone apices became latent during bud-scale initiation. Some potential seed-cone apices became vegetative buds. The number of cone buds formed was determined primarily by the proportion of apices that developed fully and the pathway along which they developed.