|Author: ||L.J. Edgerton|
Some historical features in the development of chemical thinning sprays, and theories as to their mode of action on apples are discussed.
The thinning action of NAA appears to be through its effect on the developing embryo, and the hormone interactions involved in the continued nourishment of the developing fruit.
NAAm is a milder thinning agent than NAA requiring concentrations 3 to 4 times that of NAA for comparable reduction in fruit set.
Evidence for the action of naphthyl N-methylcarbamate (carbaryl) suggests that it impedes the translocation of hormones from the developing seed and hence the ability of the fruit to mobilize food supplies for continued growth.
Auxins and carbaryl affect thinning over a period extending from 2 to 4 weeks after bloom depending on season and cultivar.
The end of the period of effectiveness is marked by the beginning of cell wall formation in the endosperm or cytokinesis.
The growth regulators NAA, 2, 4, 5-TP and 2, 4, 5-T were used for many years to delay the harvest drop of apples.
With the discovery that the growth retardant SADH will delay harvest drop, it has largely replaced the auxins for this purpose.
The physiological basis for the delay in abscission resulting from foliar application of SADH appears to be its suppression of the biosynthesis of ethylene in the maturing apple tissue.
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