Agriculture Reference
In-Depth Information
in Zimbabwe, with fewer than
chilling hours and without any dormancy-
breaking sprays. 'Mollies Delicious' and 'Drakenstein' are also reasonably
adapted to these conditions (Jackson and Bepete,
).
Inbreedingprogrammesithasbeenfoundthatsomecrossesamong'Mollies
Delicious', 'Gala', 'Golden Delicious' and 'Fuji' segregate out for lower chill-
ing requirements than their parents (Denardi
et al.
,
). There are also
'Golden Delicious' mutations, e.g. 'Panorama Golden' and 'Davilla Spur
Golden Delicious' with a lower chilling requirement and some lower chill-
ing requirement mutants of 'Delicious' and 'Fuji' (Hauagge and Cummins,
). A number of
Malus
species with very low chilling requirements are
being evaluated as parents but the greatest chance of producing cultivars
combining high commercial quality with a minimal chilling requirement may
lie in programmes including 'Anna' as a parent. The low-chilling-requirement
character present in 'Anna' is thought to be controlled by at least one ma-
jor dominant gene with minor genes interacting to modulate its effects
(Hauagge and Cummins,
% or more of 'Anna' descen-
dants have a low chilling requirement and fail to develop a deep dormancy
state.
Within
Pyrus
,
P. pashia
,
P. calleryana
and
P. amygdaliformis
have lower chilling
requirements than
P. pyrifolia
and
P. communis
and grow in areas with warmer
winter climates. Westwood and Bjornstad (
c). Usually
) found that the seeds of species
from warm-winter climates required less chilling for germination than those
from colder climates and the former species had higher optimum chilling
temperatures (
◦
C). Temperatures below freez-
ing were relatively ineffective in breaking rest of all species. Seeds within a
species showed considerable variation in their chilling requirement, e.g.
◦
C) than the latter (
-
-
%
of
P. calleryana
seeds germinated after
days of chilling but a few took three
times as long. Trees grown from these latter seeds also had longer than or-
dinary bud chilling requirements. This is in accordance with the conclusion
by Vegis (
) that seeds of many species and varieties in the
Rosaceae
be-
have, in respect of their dormancy, in the same way as their buds. Seeds from
interspecific crosses showed chilling requirements between those of their par-
ents. It seems probable that natural selection for seed chilling requirements
has played a major role in the evolution of highly adapted ecotypes with bud
chilling requirements parallel to those of the seeds. Selection for seed chilling
requirement might also provide a rapid preliminary screen for bud chilling
requirement.
Within a north European environment Spiegel-Roy and Alston (
) found
'Packham's Triumph' and 'Conference' to have much lower chilling and post-
dormant heat requirements than 'Comice' or, especially, 'Williams' '. The
perry pear 'Fleurissant Tard' had very high chilling and heat requirements,
which makes it a useful parent in breeding for avoidance of spring frost.
