Influence of 1-Aminocyclopropane-1-Carboxylic Acid (ACC) on Abscission and Fruit Quality of ‘Mcintosh' Apples

The search is ongoing to identify compounds that can effectively thin apples.  Emphasis is frequently placed on naturally occurring compounds because they may be easier and less expensive to register and consumers are demanding more naturally occurring products be used in the production of fruit and vegetables they purchase.  ACC is a naturally occurring compound that is the immediate precursor in the biosynthetic pathway of ethylene.  ACC was evaluated over the past two years at rates between 200 and 400 mg/L and at two different times of application 10 mm and 20 mm fruit size.  Unlike most thinners, ACC appears to have the greatest thinning activity when applied at the 20 mm rather than the 10 mm fruit size.  The 10 mm stage it caused some leaf yellowing and abscission, whereas the later time of application caused little or none.  Fruit treated with ACC had high amounts of ethylene production the day after application.  Elevated levels of ethylene were detected for several days after.  More ethylene was given off by fruit at the 10 mm stage than at the 20 mm stage.  Fruit from trees treated with A CC were generally larger and had higher flesh firmness.  These fruit also had a lower starch index suggesting that ripening was delayed.  ACC appears to be a promising new apple thinner, especially since there are so few options available to thin apples at larger fruit sizes such as at the 20 mm stage.