Mode of Action and Efficacy of Abscisic Acid (S-ABA) As a Postbloom Thinner in Apples, Poster Board #202

Abscisic acid (S-ABA) is a naturally occurring plant hormone that regulates a number of physiological processes of plants. The most characteristic role of abscisic acid is in the abscission of leaves, flowers and young developing fruits. Therefore, external application of this plant growth regulator is advantageous where abscission of such plant organs is desired such as by fruit thinning. In the last few years, efforts have been made to evaluate abscisic acid as a posbloom thinner in apples. This study reports research results obtained in Golden Delicious cultivar and over a range of S-ABA concentrations (alone or in combination with 6-benzyladenine) from 50 to 1,000 ppm applied at 10–12 mm king fruitlet size. The most remarkable physiological effect observed was a significant decrease in net photosynthesis of both leaves and fruitlets. Decrease in photosynthesis was concentration-dependent, and leaves and fruitlets recovered from this photosynthetic depression within 10 days. Significant increase in ethylene evolution of apple fruitlets was observed after S-ABA application, generally in a concentration-dependent manner. At 1,000 ppm concentration S-ABA caused leaf yellowing and chlorophyll loss evidenced by the reduction of SPAD values, but not at lower concentrations. Such yellowing and chlorophyll loss were not observed in combined applications of S-ABA with 6-benzyladenine. Final fruit set was generally concentration-dependent, however fruit size was not always closely related to crop load. In summary, external application of S-ABA at 10–12 mm fruitlet size seems to be an effective chemical thinner in apples; it is a photosynthesis inhibitor that enhances ethylene evolution of apple fruitlets, and causes their abscission.