Relationships between fruit nutrients and concentrations of flavonoids and chlorogenic acid in ‘Elstar’ apple skin

The relationships between fruit N, P, K, Mg and Ca concentrations during the season and flavonoid and chlorogenic acid concentrations in skin of 'Elstar' apples at maturity have been studied during three seasons in a nutrition experiment (with the mutant 'Elshof'), and in a separate experiment with standard 'Elstar' using within-tree variation in nutrient concentration due to fruit position in the tree. Negative correlations were frequently found between the concentration of N and Mg and the N/Ca ratio, and occasionally with that of K, in fruit during growth and at maturity, and anthocyanin and total flavonoids concentration at maturity. Calcium concentration showed occasionally positive correlations with anthocyanin and total flavonoids concentration. Chlorogenic acid concentration showed only in the second experiment a correlation with some of the studied nutrients (positive with P and Ca and negative with N, K, Mg and N/Ca). Regression models accounted for up to 40 and 30% of the variance in anthocyanin and total flavonoids concentration of 'Elshof' mutant apples, and up to 70 and 65% of the variance in anthocyanin and total flavonoids concentration of standard 'Elstar' apples, respectively. The most important variable in predictive models for the anthocyanin and total flavonoids concentration was N concentration in the fruit. The results suggest that the concentration of flavonoids in fruit skin could be increased by optimising fertilisation, especially that of N.