Agriculture Reference
In-Depth Information
Table
.
The effect of the previous season's crop on fruit set per
blossom clusters of
'Cox's Orange Pippin' apple
Treatments in
Hand
Light
Heavy
pollination
Control
thinning
thinning
Fruits per tree
.
Fruit set
.
From data of Blasco (
). Reproduced with permission.
Apple fruitlets with the growth characteristics of those about to shed have
only
% as much cytokinin activity as those likely to persist, as well as less
gibberellin activity (Fukui
et al
.,
.
-
).
Artificial pollination by hand, followed by fertilization and seed develop-
ment, usually increases fruit set to well above the natural level (e.g. Table
).
Knowledge that this is so has prompted the development of widely used and
successful practices to facilitate pollination in the orchard as a means of in-
creasing yields (Williams and Wilson,
.
).
Parthenocarpy
Apples and pears, as noted above, can develop fruits without fertilization and
seeds. Parthenocarpic fruit set can be vegetative, without pollination or any
other externally applied stimulus. Gorter and Visser (
) tested
apple
and
pear cultivars for their ability to set fruits from flowers whose petals,
stamens and styles had been removed and which were then enclosed in paper
bags. More than
% of the flowers set and matured parthenocarpic fruits
in one apple and four pear cultivars. All of the pear cultivars and
of the
applecultivarssetsomeparthenocarpicfruits.Muchmorecompletevegetative
parthenocarpy is shown by 'Spencer Seedless' apple, the flowers of which lack
petals and stamens and do not attract bees. Although they will set seeded
fruits if hand-pollinated all of their fruits are normally seedless. The apetalous
characteristic and associated parthenocarpy is controlled by a recessive gene
(Tobutt,
). Immature seedless apple fruits contain GA
(Hayashi
et al.
,
).Seedless'Bartlett'pearfruitshaveahighercontentofGA-likesubstances
than seeded ones (Gil
et al.
,
).
Stimulative parthenocarpy can be induced by pollination without fertiliza-
tion, e.g. by irradiated pollen (Marcutti
et al.
,
) or by plant hormones,
primarily gibberellins. Sprays of the latter are widely used to induce pear
crops, either following frost damage or with cultivars which set poorly under
local environmental conditions. 'Conference' and 'Spadona' can be induced to