Thinning ‘Washington’ Navel and ‘Late Valencia’ Sweet Orange Fruits with Photosynthetic Inhibitors1

Abstract

Abstract Compounds found to reduce photosynthesis of sweet orange leaves were tested for fruit thinning activity. Two applications of phenylmercuric acetate (PMA) at 7 × 10−4m, or N-(3,4-dichlorophenyl)-NN-dimethylurea (DCMU) at 0.25% w/v during fruit-setting significantly reduced fruit-set in ‘Washington’ navel sweet orange (Citrus sinensis (L.) Osbeck). Two applications of a plastic film-forming emulsion (Acropol) reduced fruit-set, but not significantly. Acropol had less effect on photosynthesis than PMA or DCMU. Residue problems would exclude the practical use of these substances so monomethyl n-decenyl succinic acid (MDSA) was screened and tested in the field. MDSA caused stomatal closure at 660 ppm for at least 4 days, and reduced the yield of ‘Late Valencia’ sweet orange the following year when applied at 700 ppm 6 weeks after full bloom. PMA and MDSA stimulated ethylene production from fruitlets up to 4 days after application. DCMU did not stimulate ethylene production but did reduce photosynthesis and thinned fruitlets, although significantly less effectively than PMA. This suggests that fruit setting in sweet orange may be determined by competitive inhibition, and that a reduction in photosynthesis may cause fruit abscission.