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'Honeycrisp' is among the most widely grown apple cultivars in the United States and 'WA 38' is a new apple cultivar released in Washington State. 'Honeycrisp' is highly susceptible to bitter pit and other physiological disorders; however, 'WA 38' is not susceptible to bitter pit but little is known about its susceptibility to other disorders. Bitter pit is a calcium-related disorder that has been associated with localized calcium deficiencies in fruit in addition to the proportions of calcium relative to the presence of other nutrients like potassium and magnesium. The objective of this study was to compare physiological differences and fruit quality between 'Honeycrisp' and 'WA 38' to determine how these differences might correspond to differences in mineral nutrient composition and bitter pit susceptibility. Here, 'Honeycrisp' and 'WA 38' elemental composition in leaves, fruit, and xylem sap was measured every 20 days starting 30 days after full bloom and compared with leaf gas exchange and stem water potential. 'Honeycrisp' had greater foliar transpiration rates that corresponded with greater calcium in the leaves and lower leaf K+Mg/Ca ratio, when compared with 'WA 38'. In contrast, fruit calcium concentrations were higher for 'WA 38' with lower K+Mg/Ca ratios. Xylem conductance was higher during late summer in 'WA 38' compared with 'Honeycrisp'. 'WA 38' fruit was denser than 'Honeycrisp' and more research is needed to determine whether differences in fruit structure may affect susceptibility to bitter pit in apple. [ABSTRACT FROM AUTHOR]

Calcium (Ca) sprays are commonly used to control Ca-related disorders such as bitter pit in apple. Increases in the frequency and the amount of Ca applied directly to the fruit have increased fruit Ca levels and are associated with a reduction in bitter pit incidence. However, the absorption efficiency at different fruit developmental stages is poorly understood. Here, the absorption efficiency was measured using 44Ca stable isotope applied to 30 individual fruit at five different times every 2 weeks between June drop and 2 weeks before harvest in a medium-density 'Honeycrisp' orchard. Fruit size, spray adhesion, and Ca and potassium (K) content were monitored weekly for 12 weeks between 26 May and 13 Aug. 2015. At harvest, the 44Ca-labeled fruit was picked and separated into peel and inner fruit for mass balance analysis of 44Ca absorption to regions of the fruit that are important to prevent Ca-related disorders. As expected, 44Ca was greater in the peel than the interior of the fruit. However, there was a significant amount of 44Ca present in the inner fruit at harvest for all five applications applied during the growing season. Using a stable isotope tracer approach, we present evidence that Ca is absorbed throughout fruit development. These findings support current recommendations for frequent Ca applications in low concentrations throughout fruit development to increase fruit Ca levels and reduce the incidence of bitter pit in 'Honeycrisp' apple. [ABSTRACT FROM AUTHOR]