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nodes in spur buds. Zhu et al. (
) found a critical node number of
or
for spur buds. As was discussed earlier, shoot orientation plays an important
role in the induction of flowering on long shoots: fruiting is less dominant in its
effects than on spurs where the presence of fruit on the individual spur is highly
inhibitory. Very heavy cropping on the tree as a whole can, however, reduce
fruit bud initiation on shoots. This is not just because of the reduction in shoot
length, which is not necessarily great in the season of a heavy crop. In the year
following a heavy crop all the axillary buds may prove to be vegetative (Blasco,
for terminal or lateral buds of 'Summerred' apple compared with about
). Flowering on long shoots is much more important on some cultivars, e.g.
'Elstar' and 'Golden Delicious', than others and has become more important
with the adoption of minimal pruning in the early years, branch tying-down
and, probably, the shift of production to areas with longer growing seasons.
In general, old apple spurs are less likely to initiate flowers than young ones
and pruning systems are usually designed to give a preponderance of spurs
on young fruiting wood. In general pruning that stimulates growth depresses
flowering. This is especially important in young trees. Increasing severity of
pruning usually decreases flowering and cropping (Forshey and Elfving,
),
and 'heading back' methods of pruning may convert potential fruiting spurs
into shoots. Inadequate pruning may, however, result in excessive within-tree
shade and inhibition of flowering (Feucht,
).
) considered bud development on extension shoots in relation
to apical dominance and concluded that pinching out of the shoot tip could
determine the development of sub-terminal buds. If done too early, too close
to growth cessation, the buds below the pinching point break out and sprout
as leafy shoots. If carried out too late, some weeks after growth cessation, the
buds remain inactive and vegetative to the next season. If at the right time,
they develop as flower buds. In practice, attempts to achieve this have met with
variable success. Pruning somewhat later in the summer by partial removal of
the current season's shoots gives variable effects on flowering depending on the
type of dormant season pruning on which it is superimposed (Wagenmakers,
Crabb´e(
).
In Japanese pear ( P. serotina ) most flower buds are produced on young wood,
especially in cultivars which commonly produce flowers from lateral buds, and
pruning systems are designed to maximize the proportion of this (Klinac et al. ,
).
Cultivar differences in flowering
Apple and pear cultivars differ greatly in the proportion of their buds which
become floral. This has been studied in most detail in relation to biennial
bearing caused by the inhibition of flower initiation by fruits. This is generally
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