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Fruit set of apple can be reduced by cloudy weather, short-term shade treatments, or application of photosynthetic inhibitors when the young fruit are ’8 to 15 mm in diameter, indicating that fruit are sensitive to a transient carbohydrate stress during this period. We investigated the potential for S-abscisic acid (ABA) and an ABA analog [(+)-8#-acetylene ABA] to chemically thin apple fruit by causing a stomatal limitation of photosynthesis. Stomatal conductance (gS) of ‘Imperial Gala’/M.7 was reduced by 60% 3 h after application of 250 mg·L ABA or 25 mg·L (+)-8#-acetylene ABA. Stomatal conductance began to recover 4 days after application but did not return to control levels until 19 days after treatment. Application of 250 mg·L ABA combined with 100 mg·L 6-benzyladenine (6-BA) when mean fruit diameter was ’10 mm reduced fruit set of ‘Gala’/M.7 but not ‘Pink Lady’/M.7 or ‘Morganspur Delicious’/MM.111. Fruit set of ‘Pink Lady’/M.7 was reduced by application of 20 mg·L (+)-8#-acetylene ABA + 100 mg·L 6-BA at full bloom or 10 mg·L (+)-8#-acetylene ABA + 100 mg·L 6-BA at the 10-mm fruit diameter stage. Fruit set of ‘Morganspur Delicious’/MM.111 was reduced by application of 25 mg·L (+)-8#-acetylene ABA, either alone or in combination with 75 mg·L 6-BA, at the 10-mm fruit diameter stage. ABA and (+)-8#-acetylene ABA triggered leaf abscission at rates above 250 mg·L and 25 mg·L, respectively. Fruit set and gS data from the present studies indicate the biological activity of (+)-8#-acetylene ABA is 10-fold higher than ABA. These results suggest that ABA and (+)-8#-acetylene ABA reduced fruit set by causing a stomatal limitation in photosynthesis that resulted in a transient carbohydrate stress. Thinning responses to ABA and (+)-8#-acetylene ABA at the concentrations used in these experiments were reduced compared with standard concentrations of currently available chemical thinning agents. However, increasing the concentration of ABA or (+)-8#-acetylene ABA to levels that would achieve comparable thinning are also likely to result in unacceptable leaf abscission. Developing apple (Malus ·domestica) fruit represent a relatively small carbohydrate sink between budbreak and bloom. Fruit are insensitive to an environmentally induced carbohydrate stress during this period and the carbohydrates required for growth are derived from reserves (Lakso, 2011). However, as the supply of carbohydrates from reserves is depleted and the fertilized fruit begin to grow rapidly, they become increasingly dependent on current assimilates. Shoot growth has priority over fruit growth for carbohydrate partitioning when light levels during the first 40 d after bloom are limiting (Bepete and Lakso, 1998). The negative effect of postbloom shade treatments (Byers et al., 1985, 1990, 1991; McArtney et al., 2004; Zibordi et al., 2009) or photosynthetic inhibitors (Basak, 2011; Byers et al., 1985, 1990; Clever, 2007; Deckers et al., 2010; Dorigoni and Lexxer, 2007; Lafer, 2010; McArtney and Obermiller, 2012a, 2012b) on fruit set provides evidence that a transient limitation in carbohydrate supply can act as a strong trigger for abscission of apple fruit. Exogenously applied ABA during early fruit development stimulates fruit abscission in apples (Edgerton, 1971; Greene et al., 2011), European pears (Pyrus communis) (Greene, 2012), and cherries (Prunus avium) (Zucconi et al., 1969) at concentrations ranging from 150 to 1000 mg·L. The exact mode of action of ABA in apple fruitlet abscission is unknown but is probably related to a combination of ethylene induction (Edgerton, 1971) or the creation of a transient carbohydrate stress resulting from stomatal limitation of photosynthesis. Lakso (1979) reported a close linear correlation between gS and net photosynthesis in apple trees growing under a wide range of water stress conditions, indicating that photosynthesis is primarily limited by gS under nonlimiting light and temperature conditions. More recently, Massonnet et al. (2007) reported that the relationship between gS and photosynthesis was different in ‘Braeburn’ and ‘Fuji’ apples, although net CO2 assimilation rate in both cultivars was positively related to gS. ABA is rapidly catabolized in plants, principally through oxidation of the 8#-methyl group to 8#-hydroxy ABA, which is itself further cyclized to biologically inactive phasic acid (Balsevich et al., 1994). This reaction is catalyzed by ABA 8# hydroxylase, a cytochrome P450 monoxygenase (Krochko et al., 1998). It has been suggested that ABA levels in plants could be regulated by controlling ABA catabolizing enzymes, resulting in increased stress tolerance or plants with altered seed germination and composition traits (Rose et al., 1997). This possibility was recently demonstrated by Kondo et al. (2012) who reported prompt stomatal closure and increased dehydration tolerance in apple seedlings after treatment with a specific inhibitor of ABA 8# hydroxylase [Abscinazole-F1, an analog of uniconazole with no growth retardant activity (Todoroki et al., 2009)]. An analog of ABA, (+)-8#-acetylene ABA (Fig. 1), was shown to weakly inhibit ABA 8#-hydroxylase (Rose et al., 1997). (+)-8#-acetylene ABA is a poor substrate for ABA 8# hydroxylase, resulting in greater persistence and stronger hormonal activity than ABA itself (Cutler et al., 2000). The objectives of the present study were to confirm the thinning activity of ABA and to compare the effects of ABA and (+)-8#-acetylene ABA on fruit set and gS in apple. Materials and Methods All trees used in this series of experiments were growing at the North Carolina State University Mountain Horticultural Crops Research and Extension Center in Mills River, NC, and exhibited uniform heavy bloom in the year of treatment. The trees were in mature orchards planted at 6.0 m between rows and 3.0 m between trees in the row (556 trees/ha), trained to a central leader, and managed according to accepted commercial practices as described in the Integrated Orchard Management Guide for Commercial Received for publication 16 Dec. 2013. Accepted for publication 24 Feb. 2014. Technical assistance of J.D. Obermiller is grate-

Effects of S-Abscisic Acid and (+)-8′-Acetylene Abscisic Acid on Fruit Set and Stomatal Conductance in Apple