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ecodormancy as a result of low winter and spring temperatures. There was,
however, also some evidence of endodormancy in the buds themselves be-
cause the rate of lateral bud outgrowth on decapitated shoots under forcing
conditions still showed a seasonal pattern. It was slowest in late autumn and
throughout the 'deep rest' period up to
December.
Positional and shoot bending effects on winter
dormancy of buds
Crabb´e(
b) showed that on vertically-growing apple shoots the lateral
buds nearest to the apexshowe d their deepest dormancy in December and
at that time were more deeply dormant than those further from the apex.
Subsequently, in February and March, their intensity of dormancy declined.
On shoots which had been arched over in September so that the tip was near
the ground, budbreak from the zone near the tip was achievable fairly quickly
throughout the winter and buds on the basal part of the shoot broke readily in
December but were deeply dormant in February. Thus the maximum depth
of dormancy can be altered and the time at which it occurs can be shifted by
branch training. This again suggests a major influence of correlative inhibition
on the winter dormancy of lateral buds.
Mechanisms involved in seasonal dormancy
This is still very much a 'black box' area of knowledge, with highly heritable
controlling factors that cannot be defined in physiological or biochemical
terms. The following elements appear the most relevant to placing apple and
pear bud winter dormancy in the context of overall theories of bud dormancy
and to its practical management.
There are very large variations in time of entry into endodormancy and in pat-
terns of depth of endodormancy between cultivars, types of buds and buds on
upright and bent-over shoots. These make it difficult to believe that endo-
dormancyistriggeredanddevelopedinresponsetoasingledominantenviron-
mental signal operating directly on the buds. It is possible that the concurrent
development of the deepest dormancy with full leaf senescence (Hauagge and
Cummins,
b) indicates a causal link: the timing of leaf senescence can
vary with all the above factors and leaf removal before senescence can prevent
the onset of endodormancy (Notodimedjo et al. ,
). Some of the variability
could also be explained if the level of endodormancy is the consequence of a
balance of bud growth promoters and inhibitors, i.e. a balance of hormonal
factors.
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