Articles
FACTORS AFFECTING HORMONE-ASSISTED SETTING OF COX’S APPLE
Approximately ¾ of the spray deposited on flowers was found to reside on the petals.
When labelled gibberellic acid (GA3) was applied to petals only, an insignificant amount of radioactivity was found to pass to the rest of the flower in the first 71 hr after treatment.
In contrast, radioactivity was detected in the flower pedicel as soon as "Wye mixture" containing labelled GA3 had dried following application to the calyx "well" of decapitated flowers.
After 9 days, radioactivity in the flower had declined to about 1/3 of that applied.
It was concluded that differences in the setting response with time of application cannot be due to differences in the surface area available for spray deposition since negligible movement from petals occurs.
In addition, since movement into the flower is so rapid and spray is still available to the flower many days after treatment, it is unlikely that flowers fail to set through lack of hormonal stimulus due either to inadequate penetration or too rapid dispersion.
The role of between-fruit competition as a factor determining parthenocarpic fruit set was also investigated.
When present in the same cluster, parthenocarpic and seeded fruitlets were both able to set initially but between-fruit competition preceding "June drop" was so intense that all the parthenocarpic fruit abscinded despite being larger initially than seeded fruit.
In 1981, the initial sets of parthenocarpic and seeded fruits were so similar as to suggest that the number of flowers of "good quality" within the cluster was the major factor determining initial set.
A model was proposed which showed that this was a conceivable hypothesis.
It was concluded that competition between seeded and parthenocarpic fruits is so great that the induction of parthenocarpy or setting of pollinated but seedless fruit is unlikely in commercial Cox orchards treated with setting sprays except in years where cross-pollination is minimal.