Agriculture Reference
In-Depth Information
cluster. Their removal leads to an increase in set of the lateral fruitlets. The
king fruitlets have more diffusible IAA than the lateral ones but removal of
the dominant king flowers stimulates diffusible IAA in lateral fruits. Similarly,
early excision of bourse shoot tips results in higher IAA export from nearby
lateral fruits (Gruber and Bangerth,
).
Seedless fruits of many cultivars all shed when seeded fruits are present even
though substantial numbers of seedless fruits can be retained if all pollination
is prevented (Goldwin and Schwabe,
). This suggests that the number
of fruits retained to maturity is not simply a function of available resources
but is also influenced by hormonal signals. This is consistent with the view
that hormones amplify differences between sinks to intensify competition and
intervene to abort the weakest sinks before resources are wasted on them
(Browning,
).
There are also residual effects on fruit set of crop load in the previous year.
Blasco (
) found that flowers on trees that had been induced to crop heavily
in the previous year were smaller and set less well than those on trees that had
borne lighter crops (Table
).
Early harvesting, either by its effect on reserves or effects on fruit bud de-
velopment and 'flower quality', also increases fruit set in the following year
(Williams
et al.
,
.
).
Flower quality and 'tree factor' effects on set
It has long been observed that 'strong' buds and 'bold' blossoms are the ones
most likely to set fruit. Blasberg (
) found a direct linear relationship be-
tween the size (diameter) of spur buds and fruit set even when all the buds
were on vigorous branches.
Factors shown earlier to enhance 'flower quality', in terms of demonstrated
effects on ovule longevity or blossom size, generally increase fruit set. These
include treatment with nitrogenous fertilizer in the previous autumn (Williams,
) and horizontal training of branches (Robbie
et al.
,
). Flower clusters
on one-year-old wood have smaller flowers than those on
- and
-year-old
wood and set less well (Robbie and Atkinson,
). Very young trees also
show much poorer fruit set than mature trees (Robbie and Atkinson,
).
Flowers on old, weak spurs set poorly.
Temperature effects on set
National average yields of 'Cox's Orange Pippin' apple in England are low
in seasons with high February, March and April maximum temperatures and
fruit set of hand-pollinated flowers in such years is also low ( Jackson
et al.
,
; Figure
.
). Fruit set of 'Cox' under controlled conditions is lower when
