Successive Early-season Abscisic Acid and Auxin Applications Consistently Mitigate Bitter Pit of ‘Honeycrisp’ Apples without Increasing Peel Calcium Concentration

Bitter pit is a physiological disorder of pome fruit associated with calcium deficiency that renders fruit unsalable for fresh consumption. Unlike other macronutrients, calcium is exclusively mobile in xylem; however, within the fruit of apple, xylem becomes progressively more dysfunctional with maturity, effectively limiting calcium delivery to fruit tissues. Auxins promote the differentiation of xylem, and abscisic acid (ABA) is linked with calcium mobility. Therefore, we hypothesized that early-season applications of auxins (native and synthetic) or ABA would impart greater longevity and functionality to xylem tissue and/or improve the transport of calcium and mitigate bitter pit incidence. Exogenous indole-acetic acid, naphthaleneacetic acid (NAA), and ABA were applied individually at 30, 45, and 60 days after full bloom in 2021 and 2022 to crop load-adjusted trees. The primary vascular bundle function was three-fold higher for NAA and ABA compared with the control and negatively correlated with bitter pit (−0.87) at harvest in 2021. Treatment differences in the number of vascular bundles were not observed in 2022, when bundle function was higher and fruit size was smaller than those in 2021. Peel calcium was unaffected by any treatments in either year; however, the most efficacious rates of auxins and ABA markedly and significantly reduced the bitter pit incidence of control fruit by 60% to 71% at harvest and by 54% to 65% after postharvest in 2021, and by 32% to 44% at harvest and by 36% to 45% after postharvest in 2022. More research is warranted to elucidate the mechanisms by which these plant growth regulators mitigate the incidence of bitter pit.