Figure 3. A hypothesis for the interaction of temperature and photoperiod
during autumnal bud development. The change in photoperiod
from day 235 to 275 of the year is considered for three arbitrarily
chosen latitudes (40°N, 50°N and 60°N), as in Figure 1. (a) Concept
of actual CDL. Growth cessation typically occurs when photoperiod
drops below a CDL (filled asterisk). The actual CDL at which growth
cessation (green tree) occurs depends on a genotype-specific temperature
range. Sensing of CDL is centered on an optimal temperature
(Topt). Higher as well as lower temperatures delay growth cessation to
a shorter CDL (T ≠ Topt). (b) Temperature influence on bud formation.
Higher temperature accelerates bud formation (T > Topt), whereas
lower temperature (T < Topt) results in slower bud formation. However,
temperature conditions may change in more dynamic ways. (c) A
hypothesis for latitudinal transfers integrating (a) and (b). The timing
of bud set (bare tree) is influenced by both the timing of growth cessation
and the duration of bud formation. If a transfer north is associated
with colder than optimal temperature, growth cessation will be
delayed and bud formation needs a relatively longer time and higher
total TS. If a transfer south is associated with warmer than optimal
temperature, growth cessation will be delayed and bud formation will
proceed more quickly. Other scenarios are possible depending on
dynamic changes in temperature throughout the process.