One-sided responses to local temperature changes
Which factor plays the major role in bringing perennials out
of dormancy in the spring? Data obtained by Gričar et al. (Ljubljana, Slovenia and Hamburg, Germany,
pp. 943951) address this challenging question with respect to Picea
abies (Norway spruce). About
30 days before the normal spring reactivation of cambial activity, they applied
a heating coil to a length of mature stem, raising its surface temperature to
2325 ēC. Within 10 days, the width of the cambium had increased from 45
layers of cells to 78 layers of cells, caused by divisions on the phloem side.
There was also some differentiation of new phloem sieve cells. Within 20 days,
new cambial divisions were also visible on the xylem side with some
differentiation of early-wood tracheids. There was no evidence of vertical
transmission of any signal; cambium above and below the heated zone remained
dormant. The asymmetry of the cambial reactivation in the heated tree raises
interesting questions about how it is brought about. Was this differential development
across the cambium caused by the temperature gradient generated by the heating
coil or was some other factor involved? At present, the answer remains
uncertain. However, what is clear is that temperature is a major factor in this
species in bringing about renewal of cambial growth overall. But
what about the cessation of growth later in the year? The authors
applied a cooling coil (911 ēC) to another tree at a time of high cambial
activity in the summer. No effects of this were seen for 30 days. By this time,
cambial activity was reduced and there were only five layers of cambial cells.
Late-wood started to form, although there were fewer layers of this than would
normally develop in control trees. Interestingly, in the 30 days of the
experiment, cooling had no effect on the development of phloem, again
indicating subtly different effects on the differentiation of xylem and phloem
from the cambium.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
