Mechanisms regulating apple cultivar susceptibility to bitter pit

• Loosely cell wall bound or non-cell wall bound Ca2+ may help preventing BP.
• Fruit tissue with BP has high N/Ca2+ and [K+ + Mg2+]/Ca2+ ratios.
• Fruit with high malic acid content have high BP incidence.

Fruit susceptibility to Ca2+ deficiency disorders has been associated cellular localization of Ca2+ and Ca2+ interactions with other nutrients. This study aimed to better understand the mechanisms regulating bitter pit (BP) development by comparing nutrient content and cell wall Ca2+ binding capacity in ‘Fuji’ and ‘Granny Smith’ (GS) apples. After two months of storage at 0 °C, ‘Fuji’ apples had no BP, while GS apples had 52.5% BP incidence. Non-pitted GS fruit had higher flesh and skin Ca2+ concentrations than pitted GS fruit, the latter having similar flesh and skin Ca2+ concentrations to ‘Fuji’ fruit. Although ‘Fuji’ apples had the highest cell wall Ca2+ concentration, most of the Ca2+ shifted from the water-insoluble pectin into the water-soluble pectin fraction during storage. In GS, there was a large increase in the Ca2+ concentration in water-insoluble pectin during storage and a small increase in Ca2+ content in the water-soluble pectin. Pitted GS fruit had higher malic acid content, as well as higher [(K+ + Mg2+)]/Ca2+ and N/Ca2+ ratios in flesh and skin tissues than non-pitted GS and ‘Fuji’ fruit. The results indicate that apple susceptibility to BP can be enhanced by increasing water-insoluble pectin Ca2+, as well as [(K+ + Mg2+)]/Ca2+ and N/Ca2+ ratios in the fruit.