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Jan Bizjak, Nika Weber, Maja Mikulic-Petkovsek, Ana Slatnar, Franci Stampar, Zobayer Alam, Karl Stich, Heidi Halbwirth and Robert Veberic

The influence of two foliar applications of Phostrade Ca, which contains high concentrations of phosphorus and minor amounts of calcium and nitrogen, on color development and selected primary and secondary metabolites was investigated during advanced maturation of ‘Braeburn’ apple. Changes of hydroxycinnamic acids, flavanols, dihydrochalcones, flavonols, and anthocyanins were monitored six times during the advanced ripening until technological maturity of the fruits. Additionally, the changes in the chromatic values a*, h°, and the lightness coefficient L* were recorded weekly. The colorimetric parameters showed a significant difference in the intensity of red coloration between the treated and untreated apples. Spraying with Phostrade Ca also resulted in a significant increase in most individual sugars, total sugars, and concentration of anthocyanins and flavonols. Moreover, the amount of phosphorus (P) in the treated leaves was increased. However, the total phenolic content and accumulation of other classes of flavonoids such as hydroxycinnamic acids, flavonols, and dihydrochalcones were not influenced. Phostrade Ca treatment significantly increased dihydroflavonol 4-reductase (DFR) and slightly flavanone-3-hydroxylase (FHT) activity, which were correlated with anthocyanin synthesis but had no effect on phenylalanine ammonia lyase (PAL) and chalcone synthase/chalcone isomerase (CHS/CHI) activity. The results indicate that two foliar applications of Phostrade Ca late in the growing season represent an effective way to improve the color of ‘Braeburn’ apples at commercial harvest.