Agriculture Reference
In-Depth Information
1.0
0.5
0.0
0.5
−
1.0
−
−
1.5
−
2.0
−
5
0
5
10
15
20
25
Temperature (
°
C)
Figure 6.8
The relative efficiencies of different temperatures in
meeting the chilling requirements of 'Starkrimson Delicious' apple.
From Shaltout and Unrath (1983). Reproduced with permission.
◦
N, the Asian pears 'Tsu-li'
Crocker (
) found that in Florida, at latitude
and 'Ya-li' had chilling requirements of
-
hours whereas in Oregon
◦
N 'Tsu-li' needs at least
at
h and 'Ya-li'
-
h. Hauagge and
Cummins (
a) noted that some apple cultivars which show good adaptation
and a short chilling requirement under subtropical conditions had a longer
chilling requirement, much more similar to that of conventional, high-chilling
requirement cultivars, under the conditions of Geneva, NY.
As has been known for many years, the depth of dormancy begins to de-
cline after exposure to a period of cold weather. Initially the most relevant
measurement of chilling for deciduous tree fruits was taken as hours below
◦
F(
◦
C). Further studies showed that temperatures below freezing did
not contribute, that some temperatures within the chilling range were more
effective than others, and that high temperatures within the chilling period
could have a negative effect. 'Starkrimson Delicious' apple trees showed a
maximum chilling response at
.
◦
C, with declining responses as tempera-
.
◦
C and fell to
◦
C and progressively more negative
tures increased to
.
−
.
◦
Cto
◦
C (Figure
effects of temperatures from
). This response curve
is used to calculate 'chilling units' (CU). However, Thompson
et al.
(
.
.
) found
that budbreak of 'Jonathan' was stimulated to a greater extent by chilling at
◦
C than at
◦
C and exposure to
◦
C had a greater negative (de-chilling)
.
◦
C when the chilling periods were interrupted by
treatment at these higher temperatures. A chilling temperature of
effect than exposure to
.
◦
C was
