Agriculture Reference
In-Depth Information
In trees, in general, the mechanisms regulating the time of budburst are
usually highly heritable and finely tuned, e.g. the temperature requirements
for budburst of Douglas-fir (
Pseudotsuga menziesii
) can differ inherently between
natural populations only a few kilometres apart, as discussed in a review by
Cannell (
). In apples and pears the mechanisms of adaptation to winter
conditions may result in very negative effects if the cultivar is grown beyond its
zoneofadaptation.Thusmostcultivarsselectedunderconditionsofprolonged
winters show inability of the buds to break dormancy when grown in climates
withshortmildwinters,eveniftemperaturesareadequateforgrowthandthere
is no correlative inhibition (Samish,
). Conversely, cultivars
selected in short-winter areas emerge from dormancy too soon and suffer frost
damage if grown in less mild regions (Bernardi,
; Saure,
). The rapid increase in
apple and pear production in warm-temperate, subtropical and even tropical
regions, through the use of adapted cultivars and of technologies for dormancy
breaking, has increased the economic importance of understanding the nature
and mechanisms of seasonal dormancy.
Cessation of growth and bud formation
Lateral buds which are dormant because of correlative inhibition remain dor-
mant throughout the winter. They show no visible change of state as shoot
growth slows and ceases and the trees lose their leaves with the onset of winter.
Terminal buds form as shoot growth ceases. This takes place very early in
the summer on short shoots (spurs), Fulford (cf. Abbott,
), noting that the
formation of bud scales under English conditions begins in early May. The
buds so formed are initially held dormant by correlative inhibition; this is
shown by the fact that spur buds can be stimulated into growth by pruning
away the distal part of the branch (Abbott,
). Formation of terminal buds
on long shoots can occur at widely varying times. Vigorous young shoots can
continue to grow until at least mid-October in southern England if amply
supplied with nutrients and water (Hancock and Barlow,
). Commonly,
however, growth ceases and a terminal bud forms much earlier in the season.
Cessation of growth is earlier in trees on dwarfing rootstocks than in those
on invigorating ones, in heavily cropping trees than in vegetative ones and
in water-stressed trees than in those well supplied with water. Webster (
)
showed that extension growth of horizontal branches of apple trees on some
dwarfing rootstocks in England can cease as early as
July. It is obvious,
from the wide range of dates of cessation of growth and formation of buds,
that this event is not triggered by a single clear environmental signal such as
the change in photoperiod which is associated with the formation of resting
buds in other tree species. Instead, the formation of terminal buds in apple
appears to be a response to a number of different factors that check growth
