| dc.description.abstract | A nursery and greenhouse study of balsam poplar (Populus
balsamifera L.) were carried out in Thunder Bay, Ontario, to
study the extent and nature of variation in growth and its
relation to the following yield components: phenology,
assimilation rate, leaf morphology, and assimilate distribution.
The studies involved clones of four sources of balsam poplar
selected along an approximate latitudinal gradient: N. Wisconsin
(45-46®N), Thunder Bay (48-49®N), Pickle Lake (51-52®N), and
Severn River (53-54®N). As part of the greenhouse study, the
effects of a reduction in the daily period of photosynthate
production were studied. For this purpose, plants were grown
under either a normal or reduced photosynthetic period of
approximately 16 or 8 hours per day, respectively. The reduced
photosynthetic period was achieved W covering plants with opaque
shade cloth from 4:00 p.m. of one day until 8:00 a.m. of the next
day. Low-level incandescent lighting provided cua 18-hour
photoperiod that prevented height growth cessation under both
photos3mthetic periods.
The nursery study demonstrated significant variation among
clones of the Thunder Bay and Pickle Lake sources, in terms of
date of bud break and growth cessation, initial plant height, and
total shoot elongation. Date of growth cessation differed
significantly between sources, occurring eight days earlier for
the Pickle Lake source than for the Thunder Bay source; source
differences in date of bud breads: were not significant. Total
shoot elongation was moderately correlated with date of growth
cessation, but not with date of bud break.
The greenhouse study entailed assessment of clones from all
four sources. Clonal variation was significant in terms of leaf,
stem, and root dry weight; leaf area and number; shoot length;
and root number. Relative growth rate (RGR) differed
significantly among sources; clonal variation in RGR and in
relative leaf weight growth rate (RLwGR) and relative leaf area
growth rate (RLaGR) was significant for some sources, but not
others. Differences in RGR were closely linked to differences in
unit leaf rate, but not to leaf growth characteristics. Plants
under the reduced photosynthetic period produced less dry weight
and leaf area, fewer leaves, and less shoot growth than those
lander the normal photosynthetic period. Relative growth rate,
unit leaf rate, and the allometric constant relating the relative
rate of shoot growth to that of root growth were also lower under
the reduced photosynthetic period. Leaf area ratio was greater
under the reduced photosynthetic period, largely due to greater
specific leaf area. Marked changes in clonal rankings based on
relative growth rates suggest that differences exist among the
clones studied in their response to the reduced photosynthetic
period. In general, the reduced photosynthetic period affected
the southern sources to a lesser degree than the northern
sources. | |
