Shading affects flesh calcium uptake and concentration, bitter pit incidence and other fruit traits in “Greensleeves” apple

• Shading favors BP development in fruit tissue.
• Shading increases total fruit tissue calcium content.
• High cell wall calcium binding capacity increases BP incidence.

Bitter pit (BP) is a physiological disorder in apple (Malus × domestica Borkh) believed to be triggered by low Ca2+ concentrations in the fruit, and which may be influenced by environmental conditions. The objectives of this study were to explore the effects of tree shading on total and cell wall bound fruit Ca2+ content and fruit susceptibility to BP. ‘Greensleeves’ (GS) apple trees were cultivated under shaded and non-shaded conditions. The shading condition was applied 70 days after full bloom by reducing about 50% of the sunlight reaching the tree canopies. Shading increased stem water potential and leaf stomatal conductance. Bitter pit was observed only in shaded fruit. Cortical Ca2+ was most abundant in shaded fruit without BP symptoms, intermediary in shaded fruit with BP, and lowest in non-shaded fruit. The cell wall Ca2+ concentration was higher in shaded than non-shaded fruit, but shaded fruit with and without BP had similar cell wall Ca2+ concentrations. The degree of pectin deesterification and the expression of two pectin methylesterases (PME1 and PME2) were higher in shaded fruit than in non-shaded fruit. The percentage of total cortical Ca2+ bound to the cell wall was highest in shaded fruit with BP. Shaded fruit without BP had lower Mg2+ content and Mg2+/Ca2+ ratio in cortical tissue than shaded fruit with BP. The results indicate that although shade increases fruit Ca2+ uptake, it also enhances fruit susceptibility to BP by increasing Mg2+ uptake and Ca2+ binding to the cell wall in fruit cortical tissue.