Impact of postharvest dips with abscisic acid, prohexadione, calcium, or water on bitter pit incidence and apple physiology

Bitter pit causes significant losses to apple producers, packers, and retailers each year. While bitter pit is often associated with calcium deficiency, this postharvest disorder is still not fully understood. Some studies have demonstrated positive effects of preharvest sprays with prohexadione and abscisic acid. To evaluate the effects of these phytohormones as postharvest dips, ‘Granny Smith’ apples were dipped after harvest in prohexadione-Ca, abscisic acid, or CaCl₂. Undipped and water dipped fruit were included as controls. Bitter pit incidence and severity were evaluated on fruit stored at 0 °C and >90 % RH for 68–75 d. Postharvest prohexadione-Ca and abscisic acid treatments did not reduce bitter pit incidence over two years of testing. However, there was an increase in bitter pit incidence in control fruit dipped in water and surfactant compared to undipped control fruit in the first year of testing. This increase (7.5–14 %) was observed again in water dipped control fruit in each of the following 2 years of experimentation. Inclusion of 1 % calcium chloride in the dip solution eliminated this increase in bitter pit incidence. Applying calcium with a surfactant increased the apoplastic calcium concentration and reduced bitter pit development compared to water dipped fruit. Results indicate that the increase in bitter pit induced by water dips may be due to removal of residual calcium on the fruit’s surface from preharvest calcium treatments. Ethylene production was higher in bitter pit fruit compared to healthy fruit. Reduced and total ascorbate were decreased in pitted fruit compared to healthy calcium dipped fruit. Dichlorofluoresceine diacetate fluorescence was higher in tissue adjacent to pits compared to healthy tissue from healthy fruit, indicating accumulation of reactive oxygen species. However, no consistent trend was observed in antioxidant enzyme activity. These results indicate that bitter pit shares some, but not all, of the oxidative metabolic trends observed in other fruit calcium deficiency disorders.