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

Fruit set of apple can be reduced by cloudy weather, short-term shade treatments, or application of photosynthetic inhibitors when the young fruit are approximate to 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 (g(S)) of 'Imperial Gala'/M.7 was reduced by 60% 3 h after application of 250 mg.L-1 ABA or 25 mg.L-1 (+)-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-1 ABA combined with 100 mg.L-1 6-benzyladenine (6-BA) when mean fruit diameter approximate to 10 was mm reduced fruit set of 'Gala'/M.7 but not 'Pink Lady (TM)'/M.7 or 'Morganspur Delicious'/MM.111. Fruit set of 'Pink Lady (TM)'/M.7 was reduced by application of 20 mg.L-1(+)-8'-acetylene ABA + 100 mg.L-1 6-BA at full bloom or 10 mg.L-1 (+)-8'-acetylene ABA + 100 mg.L-1 6-BA at the 10-mm fruit diameter stage. Fruit set of 'Morganspur Delicious'/MM.111 was reduced by application of 25 mg.L-1 (+)-8'-acetylene ABA, either alone or in combination with 75 mg.L-1 6-BA, at the 10-mm fruit diameter stage. ABA and (+)-8'-acetylene ABA triggered leaf abscission at rates above 250 mg.L-1 and 25 mg.L-1, respectively. Fruit set and g(S) 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.