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To successfully use abscission agents for 'Valencia' sweet orange mechanical harvesting throughout the harvest season, unwanted flower, fruitlet, and leaf drop must be assessed and minimized. Ethephon (400 mgL -1 ), 1-methylcyclopropene (1-MCP; 5m M), ethephon + 1-MCP, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP; 200 mgL -1 ), and a kinetic adjuvant control (0.15% (v/v)) were applied to 'Valencia' branches at various times from full bloom in Mar. 2006 to the end of full bloom in Mar. 2008. Effects of these treatments on fruit detachment force (FDF) and abscission of developing and mature fruit, flowers, and leaves were recorded. Three separate response periods to abscission agent applications were observed: the first spanned the first 100 days after bloom (DAB) and was characterized by high initial response followed by decreasing sensitivity; the second spanned between 100 and 225 DAB and was characterized by little to no response; and the third spanned from 225 DAB to harvest and was characterized by a gain in sensitivity. Young fruitlets in the first response period were highly sensitive to ethephon but were less sensitive to CMNP or ethephon + 1-MCP. Mature fruit in the third response period were highly sensitive to CMNP and less sensitive to ethephon or ethephon + 1-MCP. The application of ethephon resulted in high leaf abscission and showed no clear sensitivity pattern throughout both cropping years. CMNP or ethephon + 1-MCP application caused minimal leaf abscission. The same abscission agent treatments were applied on whole tree canopies 6 and 28 DAB in Mar. 2007. Application date had no significant effect on the measured parameters. Although ethephon application induced high initial leaf drop, leaf area indices determined 7 months after any compound application were not significantly different. However, subsequent 2008 yield in trees sprayed with ethephon in 2007 was significantly less, whereas 2008 flower number was higher. The results indicate a complex interaction of fruitlet abscission and leaf loss during the first response period contributed to yield reduction and increased flower number in ethephon-treated trees. The use of the abscission agent 5-chloro- 3-methyl-4-nitro-1H-pyrazole (CMNP) in combination with mechanical harvesting increases mature sweet orange fruit removal without causing phytotoxicity to leaves and young developing fruit through most of the harvesting season (Burns et al., 2005; Li et al., 2008). Work to register this product for Florida mature citrus fruit abscission is ongoing. In the event that CMNP registra- tion fails or is delayed, other abscission agent options such as ethephon (2-chloro-2- ethyl-phosphonic acid), methyl jasmonate, and coronatine (Burns, 2002; Burns et al., 2003; Hartmond et al., 2000) remain as viable candidates. Ethephon is an attractive second choice because it is currently registered for use on several food crops and is relatively inexpensive to apply. Previous work demon- strated that unacceptably high defoliation associated with ethephon applications, espe- cially at high temperatures (Bukovac et al., 1969; Yuan and Burns, 2004), could be abated during the majority of the harvest season by coapplication of the ethylene binding inhibitor 1-methylcyclopropene (1- MCP; Pozo and Burns, 2000; Pozo et al., 2004). To be fully adopted by the citrus industry, an abscission agent must not impact tree health and yield. The sweet orange cultivar Valencia (Citrus sinensis (L.) Osbeck) is a high-quality citrus juice orange in Florida that usually commands a premium price. 'Valencia' is considered a 13- to 16-month crop in Florida, meaning that mature fruit and flowers and/or fruitlets are usually present at the same time. This poses unique challenges for selective mature fruit loosening at and after bloom when abscission agent applica- tions will be required for mechanical harvest- ing. During bloom and shortly thereafter, fruitlets are in an active cell division stage (Bain, 1958) and highly responsive to growth regulator applications as a result of rapidly changing hormonal and carbohydrate balan-

Organ Loss and Yield Impacts of ‘Valencia’ Sweet Orange in Response to Fruit Abscission Agents