Relationship between xylem functionality, calcium content and the incidence of bitter pit in apple fruit

•Illustrates the relationship between xylem functionality and the incidence of bitter pit in apple fruit.•Elucidates how the losses of the xylem functionality occur in the fruit.•Emphasizes how the losses of xylem function reduce Ca flow to the fruits.

The objective of this study was to evaluate the relationship between xylem functionality, calcium (Ca) deficiency and the incidence of bitter pit (BP) in 'Fuji' and 'Catarina' apples (low and high susceptibility to BP, respectively). Fruits were assessed for fresh weight, xylem functionality (of primary and secondary cortical vascular bundles) and mineral content (Ca, Mg, K, and N) during development (40-188 days after full bloom DAFB), as well as for the incidence (%) and severity of BP at commercial harvest (188 DAFB). During fruit development, 'Catarina' apples demonstrated an earlier loss of xylem functionality, lower Ca content, higher K content, and higher K/Ca, (K + Mg)/Ca and (K + Mg + N)/Ca ratios compared to 'Fuji' apples. The large loss in xylem functionality in 'Catarina' apples which led to a higher (K + Mg + N)/Ca ratio in the fruit seems to explain the higher susceptibility to BP as compared to 'Fuji'. Showing with this, the xylem functionality may be a key physiological to infer about this disorder.