Differential Effects of Nitrogen Supply On Skin Pigmentation and Flesh Starch Breakdown of ‘Gala' Apple

Apple maturity is often assessed by starch hydrolysis indices, skin color, soluble solids, flesh firmness and the rate of ethylene evolution. In red fruited apple cultivars, the intensity and the extent of coloration is an important factor in determining the time of fruit harvest. Negative relationships have been found between tree N status and fruit skin pigmentation, but how nitrogen affects flesh starch breakdown has not been examined in detail. The objective of this study was to determine how nitrogen supply affects flesh starch breakdown relative to skin color development. Seven-year-old ‘Gala’/M.26 trees were supplied twice weekly with 4 liter of a modified Hoagland’s solution at a nitrogen concentration of 5.0, 15.0, 30.0 or 60.0 mM in Hoagland’s solution. The influences of nitrogen supply on yield, fruit quality, and fruit maturation were evaluated. At harvest, fruit in the lowest N supply treatment was significantly smaller, and had lower soluble solids, but higher starch, better color, and higher firmness than those grown at higher N supplies. Increasing N supply decreased both chlorophyll degradation and anthocyanin synthesis in fruit skin, but increased amylase activity in flesh. These results indicate that increasing N supply delays skin red color development but enhances flesh starch degradation in ‘Gala’ apples. These differential effects of nitrogen supply should be taken into account when assessing fruit maturity for optimizing harvesting time.