Agriculture Reference
In-Depth Information
Spurs
Terminal shoot segments
Decapitated shoot segments
100
80
60
40
20
0
8/30
9/27
10/25
11/24
12/20
1/17
2/14
Collection date
Figure 6.9 Seasonal changes in depth of endodormancy as
measured by budbreak after 28 days of forcing of spur buds,shoot
terminal buds and uppermost lateral buds on decapitated shoot
segments. After Latimer and Robitaille (1981). Reproduced with
permission.
C for 'Gala' and 'Fuji' (Petri and Stuker,
also found to be more effective than
).
Latimer and Robitaille (
) showed that spur terminal buds enter into rest
later, as measured by budbreak under forcing conditions, and emerge earlier
than shoot terminal buds (Figure
.
).
Temperature effects on emergence from dormancy
Two aspects of temperature determine the time at which buds emerge from
dormancy. These are low temperatures, to meet the chilling requirement as
described above, and accumulated temperatures above the threshold for bud
growth calculated as growing-degree-hours (GDH).
These effects were originally considered to be sequential and additive; for
example, Shaltout and Unrath (
) found that 'Starkrimson Delicious' apple
trees in North Carolina attained budbreak after about
CU followed by
GDH.
There is, however, increasing evidence of inter-relationships between
chilling and the thermal time (GDH) requirement. Increased chilling can be
accompanied by a decrease in the number of days at 'forcing' temperatures
needed to achieve budbreak for high-chilling-requirement cultivars. This
effect is also shown for low-chilling-requirement cultivars but is much less
pronounced and ceases to be evident once a fairly modest amount of chilling
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