Agriculture Reference
In-Depth Information
120
80
40
(a)
(b)
0
160
120
(c)
(d)
80
40
0
−
1400
−
700
0
700
1400
Chilling units
Figure 6.7
Seasonal changes in the intensity of bud dormancy in
'low-chilling-requirement' and 'high-chilling-requirement apple
cultivars. Days to 50% budbreak under 'forcing' conditions of
(
a
) 'Dorsett Golden',(
b
) 'Anna',(
c
) 'McIntosh' and (
d
) 'Delicious'.
Negative chilling units have no physiological meaning but are used
as a measure of time. Sampling on 14 July and 1 Sept corresponded
to
−
1276 CU and
238 CU. Data from Hauagge and Cummins
(1991b). Reproduced with permission.
−
colour and advanced leaf abscission. A third group of cultivars are interme-
diate in their behaviour. In general the cultivars which develop only shallow
dormancy reach their peak dormancy earlier in the season than those which
develop deep dormancy. Under subtropical conditions, however, 'Gala' may
attain much deeper dormancy than 'Golden Delicious' and 'Fuji', even though
it enters into dormancy much earlier (Herter
et al.
,
). Mauget and Rageau
(
) found terminal buds of orchard trees of 'Golden Delicious' in central
France to reach deepest dormancy by
September, well before winter.
) concluded that at low latitudes the cues for endodormancy
induction are weak and some cultivars do not attain full endodormancy. They
do not, therefore, require as much chilling to break dormancy. Sherman and
Westwood (
