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The effects of removal of young fruit and application of auxin transport inhibitors on endogenous indole- 3-acetic acid (IAA) and abscisic acid (ABA) concentrations were examined in relation to the response of mature 'Valenciaʼ orange (Citrus sinensis (L.) Osb.) fruit to abscission materials. ABA concentrations were increased in the fruit abscission zone and pulp but not in the pedicel, peel, or seed of mature fruit by removal of young fruit during the period of reduced response of mature fruit to abscission materials in early May. However, removal of young fruit slightly decreased IAA concentrations in leaves and the abscission zone and pedicel of mature fruit but had no effect on the IAA concentrations in the peel, pulp, or seed of mature fruit. Young fruit had higher IAA concentrations in the abscission zone and pedicel than mature fruit. Application of 2,3,5-triiodobenzoic acid (TIBA), an IAA transport inhibitor, reduced IAA concentrations in the abscission zone of mature fruit but did not infl uence the IAA concentra- tions in the pedicel and peel when applied directly to an absorbent collar tied around the pedicel 2 cm above the fruit abscission zone during the less responsive period in early May. ABA concentrations were increased drastically in the fruit abscission zone and pedicel but not in peel by TIBA application. Applications of ABA, or IAA transport inhibitors such as naringenin, quercetin, or TIBA comparably increased the response of mature fruit to the abscission material 5-chloro-3-methyl-4-nitro-1 H-pyrazole (CMN-pyrazole) in early May. These data suggest that young fruit reduce the response of mature 'Valenciaʼ oranges to abscission materials through increasing IAA concentrations and decreasing ABA concentrations in the abscission zone of mature 'Valenciaʼ orangees. Strong attachment of mature orange (Citrus sinensis (L.) Osb.) fruit to the tree at harvest is a major factor hampering the effi ciency and economy of commercial mechanical harvesting of oranges. Without the aid of fruit loosening agents, the current mechanical harvesting systems fail to recover all of the fruit from citrus trees (Whitney and Harrell, 1989). However, it has been challenging to fi nd a desirable abscission material to aid mechanical harvesting of the late-season cultivar Valencia, which is harvested from late March through June under Florida conditions. Unlike early and midseason orange cultivars that respond readily to abscission materials, 'Valenciaʼ oranges have an inconsistent response to

journal of the american society for horticultural science

Young Fruit and Auxin Transport Inhibitors Affect the Response of Mature `Valencia' Oranges to Abscission Materials via Changing Endogenous Plant Hormones