Agriculture Reference
In-Depth Information
give substantial crops of parthenocarpic fruits with GA (Modlibowska,
a;
Herrero,
). Set of 'Abbe Fetel', 'Doyenne du Comice' and 'Williams'' is
achieved by use of a commercial product containing GA ,GA
and benzyl-
adenine (Calzoni and Speranza,
). GA + is also effective on the triploid
'Bramley's Seedling' apple (Modlibowska,
b) and, to a lesser extent, on
'Cox's Orange Pippin' (Goldwin,
). For the latter cultivar a three-hormone
mixture of a gibberellin, an auxin and a cytokinin appears most effective
(Kotob and Schwabe
; Goldwin,
). Applications at blossom time,
within
weeks of this, or multiple applications, are most effective. Those culti-
vars with a strong tendency to natural parthenocarpy are the most responsive
to hormone sprays.
GA effects on seeded fruit set
Application of GA + at any time between one and
days after full bloom
does not influence the initial fruit set of 'Cox's Orange Pippin' apples but
greatly reduces the June drop of fruitlets from pollinated flowers (Wertheim,
). Application in July after June drop has started does not reduce this.
In a number of other trials on apple gibberellins applied alone or in com-
bination with other hormones have resulted in more, rather then less, fruit
shed.
Auxin and cytokinin effects on fruit set
When applied as an overall spray at blossom time or later, NAA usually in-
creases fruit drop, so is widely used as a chemical thinning agent (Wertheim,
). This may result from its side effects on assimilate availability and is also
in keeping with the concept that it is the auxin gradient across the abscission
zone that is important in controlling shed. Application of
-TP either in
the previous autumn or the spring may, however, increase fruit set on some
pears (Westwood et al. ,
,
,
).
Cytokinin sprays may also increase rather than reduce fruit shed (Wertheim,
; van Zyl and Strydom,
). This may be a consequence of stimulating other growing points.
Ethylene and fruit set
There is a general association between ethylene and fruit shedding but less cer-
tainty as to whether increases in ethylene production pre-date the first stages of
shedding and are a direct cause of this. Kondo and Takahashi (
) found that
'Starking Delicious' apple fruitlets, which had a high rate of ethylene output,
shedheavilywithupto
%Junedrop.Ethyleneoutputfrom'Fuji'waslowand
there was little or no fruitlet drop. 'Tsugaru' was intermediate in both respects.
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