THE EFFECT OF HORMONES ON GROWTH AND CROPPING OF PEARS

CCC was more effective than SADH in controlling the growth, inducing fruit bud formation and increasing the yield per m² of effective surface cover, although the yield per tree was higher on the SADH than on the CCC sprayed trees.

Compensating growth and reduced yields followed the discontinuation of retardant use.

Tre-hold paint (containing 1% NAA) applied to pruning cuts reduced the growth and increased the yield of Hardy pears.

Single sprays of gibberellin A₃ at 22 ppm increased the yield of Hardy in most years in spite of causing some reduction in blossom. The mixture of gibberellins A₄+₇ applied at various stages of floral development either alone or in combination with auxins or cytokinins rather reduced the yield of Comice. Gibberellins reduced the seed content per fruit and increased the proportion of seedless fruits. They increased the height/diameter ratio while SADH reduced it.

Growth regulators are very useful supplementary tools for regulation of tree growth and cropping but cannot be relied on as sole means of control. At high concentrations they have noxious side effects. High concentrations of retardants may control growth but would leave unacceptable residues on fruit, while high gibberellin concentrations, although effective in setting fruits in the first year, prevent fruit bud differentiation for the following years. Treated trees try to adjust their hormonal balance and react differently in the following years. It is therefore essential to find the lowest concentrations which will be effective, yet have little effect on the natural hormonal balance of the tree and the present programme has been designed around this objective.

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