Objectives The suitable application amount and method of calcium fertilizer were studied to find an effective way to reduce the occurrence of bitter pit in apple production.

Methods A field experiment was conducted using apple cultivar of 'Nagafu 2' apple trees (15-year-old) as materials and nitrate calcium as Ca fertilizer. We designed three nitrate calcium rates of 0.48, 0.72 and 0.96 kg/plant (Ca_0.48, Ca_0.72, Ca_0.96), and all the three rates were applied in three methods, as: one application before flowering (1), two applications before flowering and after flower falling (2), three applications before flowering, after flower falling and at fruit expanding stage(3), and no calcium application (Ca₀) was used as control. At maturing stage, fruit samples were collected for the measurement of antioxidant enzyme activities, MDA content, nutrient element contents, quality and bitter pit incidence.

Results All the Ca application treatments increased the antioxidant enzyme activities significantly compared with Ca₀. The activities of SOD, POD and CAT in Ca_0.72-3 were significantly higher than those in all the others; those in Ca_0.96-3, Ca_0.96-2, Ca_0.96-1, Ca_0.72-2, Ca_0.72-1, Ca_0.48-2 and Ca_0.48-3 were generally not significantly different with each other, but all significantly higher than those in Ca_0.48-1. The N, P, K, Ca and Mg contents of fruit in all the Ca treatments were significantly higher than those in the control. The top two N contents were in Ca_0.72-2 and Ca_0.96-1, which were significantly higher than those in all the other treatments, except Ca_0.48-1. No obvious order was found in P and K contents among treatments; both the highest Ca and Mg contents and the lowest P content were in Ca_0.72-3. In treatment Ca_0.72-3, the fruit weight, transverse diameter and longitudinal diameter were all significantly higher than those in the others, and the soluble solids and total sugar contents were the highest, the titratable acid content was the lowest (0.29%), with the sugar to acid ratio of 38.81, the incidence of bitter pit was the lowest (19%).

Conclusions The application of calcium fertilizer could significantly increase the antioxidant enzyme activities and nutrient contents, especially the K, Ca and Mg contents, which are very important for fruit resistance to bitter pit and quality, and the fruit commercial quality. Under the experimental condition, the most satisfactory effect could be obtained by applying calcium nitrate three times in a total amount of Ca 0.72 kg/plant.