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and the surplus of supply over proximate demand influences export to more
distant sinks. Teng
et al.
(
) found that spurs of all ages exported assimilated
Cat
weeks after full bloom, the amount exported increasing with spur age
and the associated increase in leaf area. By
weeks after full bloom,
when fruit demand was higher, no assimilated
C left spurs of any age. Corelli-
Grappadelli
et al.
(
and
) found that shading eliminated export of assimilated
Cfromextensionshootleavestofruitsat
weeksafterfullbloomandreduced
exporttofruitat
weeksfromfullbloom.UsingringedbranchsectionsHansen
(
) found a linear relationship between fruit growth from early July to mid-
September and leaf area per fruit. The distance between leaves and fruits
had no effect on this relationship. Palmer
et al.
(
) found that when apple
flower clusters were removed at full bloom, either uniformly throughout the
tree canopy, on alternate branches or on whole sides of trees, mean fruit
weight at harvest was linearly dependent on leaf area per fruit and on tree
light interception per fruit (Figure
.
). This is compatible with earlier work by
Parry (
), who found blossom removal on a half-tree basis to be as effective
as individual cluster thinning throughout the tree volume in increasing fruit
size. The clear implication is that, at least with dwarf trees, assimilate is mobile
within the tree and individual fruit growth is greatly influenced by potential
tree photosynthesis and the number of fruits competing for the assimilates.
The fruits on a tree compete for available resources and competition between
fruits is a major factor controlling their growth and size. Denne (
) showed
that, at harvest, fruits from trees of 'Miller's Seedling' apple thinned at the
pink-bud stage of blossom were four times as heavy as those of control trees.
The rate of cell division on thinned trees was higher from
weeks after
full bloom. The number of cells per fruit in thinned trees continued to increase
up to
or
weeks in control
trees. At harvest there were about three times as many cortical cells in the
fruits of thinned as of unthinned trees and the individual cells were about
weeks from full bloom but remained constant from
%
wider, implying that cell volume was about doubled.
In extreme cases, when the thinning is carried out early, e.g. pre-bloom, the
yield of thinned trees may be very similar to that of unthinned trees, i.e. the
increase in individual fruit weight fully compensates for the lower numbers
(data of Parry,
). Any delay in fruit thinning reduces the effect on the
growth of the remaining fruits because the opportunity for much increase in
cell division is lost. Commercial thinning is often delayed until the level of fruit
set is known, so the increase in fruit size does not fully compensate for the loss
in numbers.
Although the effect of fruit thinning by reducing competition for assimilates
operates essentially at a whole-tree level, selective thinning so as to retain the
