High Susceptibility of "Honeycrisp" Apple to Bitter Pit Is Associated with Low Fruit Calcium Level and Its Imbalance with Other Nutrients

‘Honeycrisp’ is highly susceptible to bitter pit, but its underlying cause is not well understood.  In this study, we compared fruits with and without bitter pit of ‘Honeycrisp’ and also with ‘Gala’, a variety not susceptible to bitter pit, in terms of fruit Ca level and its balance with other nutrients.  Two types of tissues were sampled for ‘Honeycrisp’ fruit with bitterpit: pitted tissue and apparently healthy outer cortex tissue at the calyx end.  The apparently healthy cortex tissue of ‘Honeycrisp’ fruit with bitterpit had a lower Ca level, but the pitted tissue had a higher Ca level than that of fruit without bitter pit (CK). There were comparable levels of K, Mg and P between the apparently healthy cortex tissue and the CK, but the pitted cortex tissue accumulated higher levels of K, Mg and P than the CK.  The apparently healthy peel of fruit with bitter pit had significantly lower Ca level than the CK, with the pitted peel having the lowest Ca concentration.  In contrast, the apparently healthy peel of fruit with bitter pit had significantly higher levels of K and P than the CK, with the pitted peel having the highest concentrations of K and P.  As a result, the apparently healthy peel and the pitted peel have higher values of K/Ca, (K+Mg)/Ca and P/Ca.  Measurements of nutrients in the apoplastic fluid indicate that the apparently healthy cortex tissue and pitted tissue of fruit with bitter pit had similar concentrations of apoplastic Ca, but higher concentrations of apoplastic K and P than fruit without bitter pit.  Compared with ‘Gala’, ‘Honeycrisp’ fruit without bitter pit had much lower concentrations of Ca in both cortex tissue and peel, but comparable levels of K, Mg and P in cortex tissue and significantly higher levels of K and P in peel, resulting in higher values of K/Ca, Mg/Ca, (K+Mg)/Ca and P/Ca in both cortex and peel.  In addition, ‘Honeycrisp’ leaves had a higher level of Ca, but lower levels of P, K, and Mg than in ‘Gala’. These results suggest that 1) imbalance of Ca with other nutrients is closely associated with bitter pit development in ‘Honeycrisp’ and 2) ‘Honeycrisp’ has a more acute partitioning of calcium between leaves and fruit than ‘Gala’, leading to a much lower Ca level and higher ratios of K/Ca, (K+Mg)/Ca and P/Ca in fruit, which may make the fruit more susceptible to bitter pit.