Agriculture Reference
In-Depth Information
The continued growth of the fruitlet or fruit then depends on its ability
to compete with other fruits and shoots for metabolites. The concentration of
suchmetabolitesinfluencestheconsequencesofthiscompetitionandifthefruit
growth is checked, even for as little as two days (Fukui
et al.
,
a), it will shed.
The process of abscission is usually considered within the scheme proposed
by Reid (
). In this it is considered that shed is prevented by keeping the
abscission zone in a non-sensitive state as a result of a gradient of auxin from
the potentially shedding organ to the plant axis: this gradient being maintained
by factors which inhibit senescence such as auxins, cytokinins, light and good
nutrition. Reduction or reversal of the auxin gradient by application of auxin
proximal to the abscission zone, by shading or by poor nutrition hastening
senescence, allows the abscission zone to become sensitive to ethylene. Once
sensitized the cells of the abscission zone respond to low concentrations of
ethylene by secretion of hydrolytic enzymes and the organ is shed. Recent
work suggests that carbohydrate supply is especially important in preventing
the abscission process being triggered in the abscission zone.
Either de-fruiting but leaving the pedicel with its abscission zone intact at-
tached to the tree, or steam-girdling the pedicel between the fruit and the
abscission zone, results in abscission whenever carried out (Beruter and Droz,
). Steam-girdling the pedicel between the tree and the abscission zone
induces fruit drop if carried out at
days after full bloom, but leads to a
cessation of growth without abscission if done at
or
days after full bloom. This
shows that up to the end of the June drop abscission can be induced by treat-
ments which reduce or block nutrient supply from the leaves to the abscission
zone. After that period the growing fruit has become a storage organ which
is resistant to abscission. The fact that seed removal after June drop does not
stimulate activation of the abscission zone in the pedicel, but fruit removal or
phloem ringing between the fruit and the pedicel does, leads to the conclusion
that substances produced or stored in the fruit flesh control post-June-drop
separation at the abscission layer. Beruter and Droz (
) concluded that the
glucose concentration in the pedicel is a key factor controlling abscission. In
citrus, fruitlet abscission induced by carbon shortage appears to be regulated
by ABA (abscisic acid) and ACC, an ethylene precursor (Gomez-Cadenas
et al.
,
). ABA may act as a sensor of the intensity of the carbohydrate shortage
that modulates the levels of ACC and ethylene. There is, as yet, no valida-
tion of this hypothesis for pome fruits. Effects of shade on fruit abscission are
discussed on pp.
-
and of chemical thinning agents on p.
.
Effects of seeds on fruit shed
Following fertilization the presence of immature, developing seeds prevents
fruitlet shed and enables continued growth. Abbott (
) found that if seeds
