Reduced late-season leaf potassium and phosphorus levels influence decreases in sugar contents of bagged apple fruit

Abstract

Fruit bagging has been widely used in the fruit production industry; however, in apples, it is known to cause a significant decrease in fruit sugar contents. To address this issue and identify the changes in leaf mineral contents associated with fruit sugar levels in bagged apples, aqueous solutions of 10 g L−1 CaCl2, 5 g L−1 KH2PO4, or 2 g L−1 Na2B4O7·10H2O were foliar sprayed during four fruit developmental stages. The late-season leaf phosphorus (P) and potassium (K) contents after the rapid fruit growth period and the soluble sugar contents in ripening fruit were significantly lower in bagged fruit than in non-bagged fruit (11.1–15.09 %). The decreases in leaf P and K contents caused by bagging were almost completely compensated for by foliage applications of CaCl2, KH2PO4, or Na2B4O7 during the fruit set period. Therefore, the fruit soluble sugar contents were significantly higher in bagged ripening fruit with foliar spray than in bagged fruit without foliar sprays, reaching the levels of non-bagged apples. The decrease in the sugar contents of bagged apples was closely associated with the decrease in late-season leaf P and K levels caused by fruit bagging.