S-Abscisic Acid (S-ABA): A New Strategy for Controlling Bitter Pit in Apple

Calcium (Ca) uptake into fruit and leaves is highly dependent on xylemic water movement, hence presumably driven by foliage transpiration and stomatal activity and shoot growth. Due to high transpiration rate of leaves compared with that of fruits, most of the xylemic water and Ca flow towards the leaves and away from fruit. Increasing the ratio of fruit/leaf transpiration will be a potential solution to improve Ca inflow into fruit and reduce Ca-related disorders in fruit, such as bitter pit in apples. S-Abscisic acid (S-ABA) is known to control stomatal conductance and, therefore, transpiration. The objective of this study was to evaluate the efficacy of exogenous S-ABA applications during the growing season on bitter pit control in apples. Two-year field studies were conducted in two different regions: 1) In 2012, foliar applications of 350 mg L^-1 ABA were applied to ‘Honeycrisp’ and ‘Melrose’ apple trees in Wooster OH, US. 2) In 2015, foliar applications of 400 mg L^-1 ABA were applied to ‘Honeycrisp’ trees in Geneva NY, US. Fruit were harvested at maturity and evaluated for yield, quality and nutrient concentrations in order to assess effects of S-ABA. Incidence (%) and severity (spots/fruit) of bitter pit were evaluated on 40 apples per tree. Shoot growth was visually assessed according to four different length classes. Foliar applications of S-ABA significantly reduced both bitter pit incidence and severity of apples in both regions. Whereas 8% in 2012 and 23% in 2015 of untreated ‘Honeycrisp’ fruit showed bitter pit, S-ABA reduced the incidence to approximately 2% in both years. Great reduction in bitter pit was also observed in ‘Melrose’ apples.  No negative effects of ABA on yield, crop load, fruit size or other quality aspects were observed.  Neither thinning effect nor phytotoxicity was observed at S-ABA rates used. Calcium uptake into the fruit tissue increased with S-ABA foliar spray treatments; nutrient analysis showed an increased Ca concentration in the fruit in both years and cultivars. Moreover, X-ray microanalysis (EDS) showed a larger allocation of Ca in the epidermins and cortex of Honeycrisp ABA-treated apple tissues. Shoot length in the ABA-treated trees was reduced compared to the untreated control trees. This study concluded that S-ABA is a promising tool for bitter pit control in sensitive apple varieties, such as ‘Honeycrisp’.