Agriculture Reference
In-Depth Information
800
'Cox' apple
700
600
500
'Conference' pear
400
0
20
Number of days after 1 Oct <3
°
C
40
60
80
Figure 6.10
The inverse relationship between thermal time to full
bloom of 'Cox' apple and 'Conference' pear and the accumulated
number of 'chill days' in winter at East Malling,Kent,England. From
Cannell (1989),from
Manipulation of Fruiting
by C.J. Wright.
Reprinted by permission of Elsevier Science Ltd.
has accumulated (Figure
). Statistical studies on the factors controlling the
date of budburst over many years and sites of 'Cox' apples and 'Conference'
pears in England showed that the greater the number of days from
.
October
◦
C, the fewer day-degrees above
in which temperatures were lower than
◦
C after
December were needed to attain full bloom, i.e.
% budburst
(Figure
). There have also been numerous reports that chilling over and
above the accepted chilling requirement advances budbreak. This is explained
by the concept that at any time during dormancy a bud (vegetative or mixed)
has the potential to respond to warm temperatures with morphological devel-
opment, extension growth or a change in physiological activity. The effect of
chillingistochangetheresponseofthebudtotemperature,i.e.increasethepo-
tential rate of bud development (Vegis,
.
; Campbell,
; Cannell,
).
Young (
) showed that chilling dormant apple trees
increases the effect of temperature on subsequent shoot respiration (the Q
).
In general those cultivars with little requirement for winter chilling also
have low heat requirements to induce budbreak. This is true for both apple
(Petropoulou,
) and Young
et al.
(
; Hauagge and Cummins,
b) and pear (Spiegel-Roy
and Alston,
). Both chilling requirement and heat requirement are
closely correlated with the time of budburst. There are exceptions to these
