ISHS Acta Horticulturae 463: VIII International Symposium on Plant Bioregulation in Fruit Production
THINNING EFFECTS ON CITRUS YIELD AND FRUIT SIZE |
| Authors: | J.L. Guardiola, A. García-Luis |
| Keywords: | auxins, flower formation, gibberellic acid |
| Abstract:
Thinning is performed in Citrus to increase fruit size and thus to enhance the market value of the crop. Fruit count is reduced and crop yield may be either unchanged or markedly reduced depending on the thinning procedure used. The sink strength of the fruitlets, which determines fruitlet growth rate, is largely determined during the early stages of development. It may be increased reducing flower formation through the application of gibberellic acid during the winter rest period. In some cultivars this reduction in flower number results in an increase in the size of the ovaries and a faster early fruitlet growth rate. Also, the application of a synthetic auxin at flower opening increases selectively in most cultivars the sink strength of some of the ovaries. In both cases, the final number of fruits set is reduced in an indirect way and final fruit size is increased. Crop yield is not affected significantly. A significant proportion of the developing fruitlets is shed within two weeks due to auxin-induced ethylene synthesis when an appropriate auxin is applied before the end of June drop. This fruitlet thinning reduces intersink competition and increases the fruitlet growth rate and final fruit size whenever fruit growth is limited by metabolite supply. This ethylene-induced abscission always results in a reduction in yield. From the direct effect of some auxins on the sink strength of the fruitlets, an increase in fruit size without affecting crop yield may be obtained when the auxin application is timed at the end of June drop when fruitlet shedding has ended and is hardly affected by auxin-induced ethylene synthesis. Although this system of application offers potentially some advantages considerable expertise is needed to time-out precisely these applications which must be performed in a range of a few days. A slightly earlier application may cause a significant thinning, while a delayed application has only a minor effect on fruitlet growth and final fruit size. The effect of these applications on fruitlet growth may be weak in heavily fruit loaded trees. The comparative advantages and pitfalls of the different strategies to increase fruit size in Citrus are briefly discussed. | |
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