ISHS Acta Horticulturae 463: VIII International Symposium on Plant Bioregulation in Fruit Production
FRUITLET AGE AND INFLORESCENCE CHARACTERISTICS AFFECT THE THINNING AND THE INCREASE IN FRUITLET GROWTH RATE INDUCED BY AUXIN APPLICATIONS IN CITRUS |
| Authors: | A.G. Ortolà, C. Monerri, J.L. Guardiola |
| Keywords: | Citrus unshiu Marc, naphthalene acetic acid, abscission, ethylene inhibitors, fruit size |
| Abstract:
The application of naphthalene acetic acid (NAA) delays abscission at the base of peduncle and enhances abscission at the calyx. This increase in abscission is maximal when the application is performed from a few days after petal fall until mid June drop. Beyond that moment the fruit becomes largely insensitive and hardly can be thinned. Fruitlet abscission is caused by NAA-induced ethylene synthesis, which is accompanied by the accumulation of ACC and MACC. Cobaltous ion and AVG inhibit the NAA-induced ethylene production, but only cobaltous ion inhibit the basal ethylene production. The phenoxy auxins 2,4-D and 2,4,5-T, which are erratic thinners, have only a marginal effect on ethylene synthesis by fruit. Both abscission and ethylene synthesis are accelerated detaching the inflorescences and incubating them in the laboratory. Under these conditions, both abscission and ethylene production are higher in leafless inflorescences that in leafy inflorescences. Further, the presence of the leaves in the inflorescence reduces the NAA-induced ethylene synthesis. The application of NAA increases fruitlet growth rate in a direct way unrelated to fruitlet thinning and intersink competition. This effect of NAA on fruitlet growth may be partially separated from the effect on thinning through the timing of the NAA application. Early applications (up to mid June drop) causes a significant thinning, while later applications (at the end of June drop) hardly affects fruit count. The effect of NAA on the sink strength of the fruitlets changes with fruitlet age. In early NAA applications this effect is higher for the smallest surviving fruitlets. | |
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