CHANGES IN XYLEM FUNCTIONALITY DURING APPLE FRUIT DEVELOPMENT: IMPLICATIONS ON CALCIUM CONCENTRATION AND INCIDENCE OF BITTER PIT

Apple xylem becomes dysfunctional as the fruit develops. This change in xylem functionality has been related to a nutritional imbalance and, consequently, to the increased development of fruit disorders after harvest. The aim of this study was to assess the relationship between xylem failure in developing apples and fruit Ca concentrations in cultivars with different susceptibilities to bitter pit (BP). ‘Fuji’ and ‘Catarina’ apple fruits (considered with low and high susceptibility to BP, respectively) were harvested from an orchard in São Joaquim, southern Brazil, in 2009-2010. Developmental changes in xylem functionality of the fruit were investigated by using a dye infusion technique (1% w/w aqueous acid fuchsin) to stain the vasculature along the path of dye movement. The vascular bundles (cortical and carpellary) with presence of dye were assessed visually in fruits sectioned equatorially at regular intervals throughout the season. The concentrations of Ca, Mg, K and N at the distal end of the fruits were assessed in apples harvested at 40, 68, 96, 131, 173 and 188 days after full bloom (DAFB). As the season progressed, an increasing proportion of cortical (primary and secondary) vascular bundles failed to show any staining. The loss of xylem functionality takes place earlier in the season in ‘Catarina’ (~40 DAFB) than in ‘Fuji’ (~100 DAFB) apple fruit. This event is associated with lower Ca concentration and higher (K+Mg+N)/Ca ration at the distal section of the fruit in ‘Catarina’ compared with ‘Fuji’, and it might be a major reason for the different susceptibility to BP between the two cultivars.