STRUCTURAL CHANGES IN POPULUS DELTOIDES TERMINAL BUDS AND IN THE VASCULAR TRANSITION ZONE OF THE STEMS DURING DORMANCY INDUCTION

Morphological and anatomical changes in shoots of vigorously growing cottonwood plants (Populus deltoides Bartr.) were studied during dormancy induction in 8-hr short days (SD) and in control plants grown in 18-hr long days (LD). Pronounced structural changes occurred in terminal buds after 4 wk and full dormancy was achieved in 7 wk of SD. Leaf expansion ceased after 5 wk of SD as foliage leaves matured to the terminal bud base at leaf plastochron index 0 (LPI 0). Within the bud, total leaf length (lamina + petiole) decreased and stipule length increased progressively each week; thus, the ratio total leaf length/stipule length decreased rapidly, especially at the position of incipient bud-scale leaves LPI -1 and LPI -2. These budscale leaves were fully developed by wk 6 and were derived from enlarged stipules and aborted laminae. The full complement of primordia within the bud at the start of SD eventually matured as foliage leaves and the first bud-scale leaf (LPI - 1) was initiated immediately following transfer to SD. Acropetal advance of the primary-secondary vascular transition zone (TZ) was associated with leaf maturation. However, it did not advance throughout the entire vascular cylinder as in LD, but only in those leaf traces serving mature leaves beneath the terminal bud. In both LD and SD treatments the same linear relationship was maintained between LPI of the TZ and LPI of the most recently matured leaf; both parameters simultaneously increased in LD and decreased in SD. Thus, the relationship between leaf maturation and advance of the TZ was maintained irrespective of environment.