Correlation of pyruvate kinase activity with bitter pit development in apple fruit

The relationship between pyruvate kinase (pyruvate-ATP phosphotransferase, EC 2.7.1.40) activity, fruit cation concentration (Ca²⁺, Mg²⁺ and K⁺) and bitter pit in apple cultivar 'Smoothie Golden Delicious' fruit (Malus domestica Borkh.) was investigated. Fruit was sprayed with 1% (w/v) CaCl₂, 1% (w/v) MgCl₂ or a combination of both chemicals every 10 days, beginning 5 days after full bloom, in a factorial experiment designed to induce a range of post-storage bitter pit symptoms. Pyruvate kinase activity during the early stages of fruit growth was higher in fruit from treatments which developed 100% bitter pit after storage than in fruit from treatments that developed only 3% bitter pit. Fruit with a higher bitter pit incidence had a lower Ca²⁺:Mg²⁺+K⁺ ratio than fruit with a low level of the disorder. There was a positive correlation between pyruvate kinase activity during the early stages of fruit development and bitter pit incidence after storage. Correlation coefficients (r) were highest in June (r=0.96), declined steadily until harvest (r=0.67) and were lowest after storage (r=0.12). Preliminary data indicate that an assay for pyruvate kinase may be valuable for early prediction of post-harvest bitter pit development and cation ratio.