Gibberellic acid and fruit set in sweet orange

A single GA₃-spray at petal-fall to the entire tree enhanced initial set in the ‘Navelate’ sweet orange (Citrus sinensis L. Osbeck), but this effect was transient and in most cases final yield was not increased. When followed by girdling, final yield was increased even in the most productive orchards, this effect being due to an increase in fruit number while fruit size was unaffected. The response was markedly dependent on the GA₃ concentration, 5 mg l^?1 giving the best results. Higher concentrations up to 20 mg l^?1 even reduced yield compared to the untreated control trees. Chlormequat reduced the number of fruits finally cropped without affecting their size, an effect which may be explained through a reduction in the endogenous gibberellin levels.