Agriculture Reference
In-Depth Information
relationships, however. Some cultivars have a lower heat requirement than
might have been expected from their chilling classification, and some experi-
ence budbreak either earlier or later than might be expected from their ap-
parent chilling and heat requirements (Spiegel-Roy and Alston,
). These
discrepancies may reflect the weakness of applying standard equations to culti-
vars which differ in their underlying sensitivities to temperature. For example,
Shaltout and Unrath (
◦
C as the base temperature for calcu-
lating growing degree-hours for 'Starkrimson Delicious', but Gianfagna and
Mehlenbacher (
) used
.
) found that the buds of some cultivars, even after their
chilling requirement had been met, did not grow at all at
◦
C. Hauagge
and Cummins (
b) also reported that budbreak of 'Elstar' appeared to be
dependent on accumulated temperatures above
◦
C and that of other
Malus
◦
C.
Chilling of both roots and shoots is essential to achieve maximum budbreak of
dormant apple trees (Young and Werner,
on accumulated temperatures above
,
b; Young,
). Chilling
only the roots or the shoots during rest elicits about
% budbreak while
chilling both gives
%.
With pear, fully chilled 'Bartlett' buds grew less when budded on to inade-
quately chilled host trees than when budded on fully chilled hosts (Westwood
and Chestnut,
).
%
Apple rootstock cultivars have a wide range of chilling and GDH requirements
for budbreak (Young and Werner,
) found that, in
general,themoredwarfingtherootstocktheloweritschillingandheatrequire-
ments, but Young and Werner found 'M.
a). Petropoulou (
a' to have lower chilling and GDH
requirements than more dwarfing as well as more invigorating rootstocks.
In Brazil, in a subtropical climate, Couvillon
et al.
(
) found that 'Rome
Beauty' apple trees on 'MM.
' rootstocks showed classic
symptoms of insufficient chilling whereas those on 'M.
' and 'MM.
' and especially 'M.
'
had a greater degree of lateral vegetative budbreak.
Different pear species have very different chilling requirements, reflecting
their areas of origin.
Pyrus communis
has a fairly high chilling requirement,
P. calleryana
a low chilling requirement. Both are used as rootstocks. Westwood
and Chestnut (
) found that 'Bartlett' (
P. communis
)ona
P. calleryana
rootstock
hadamuchlowerchillingrequirementthanwhenonitsown(
P. communis
)roots.
When trees of 'Bartlett' on
P. communis
and
P. calleryana
rootstocks were each
exposed to
◦
C and buds from them were budded on a fully
chilled host plant, the growth of the buds that had been on
P. calleryana
was
much greater than that of buds that had been on
P. communis.
This indicates that
hours at
.
