Effect of ammonium content in nutrition solution on vitamin C, phenols, and antioxidant capacity of three apple cultivars during cold storage

Pre-harvest nutritional condition is one of the major fruit quality influencers in apple whether directly after harvest or throughout cold storage. Here, the influences of various levels of ammonium to nitrate on vitamin C content, total phenolic compounds, and total antioxidant capacity of three apple cultivars (‘Granny Smith’, ‘Gala’, and ‘Golab Kohans’) throughout cold storage period were studied. Five different ammonium nitrogen levels (0, 0.08, 0.16, 0.19, and 0.27 meq/L) were applied. After harvest, the fruits were transferred to cold storage and stored at 0±1˚C for 45, 90, and 160 days for ‘Golab Kohans’, ‘Gala’, and ‘Granny Smith’, respectively. Vitamin C content, total phenolic compounds, and total antioxidant capacity were determined throughout the storage period. The results showed that all cultivars showed different responses toward ammonium nitrogen application in terms of vitamin C content. ‘Golab Kohans’ recorded an increase in vitamin C content by increasing ammonium in nutrition solutions. On the other hand, minor changes were recorded in ‘Granny Smith’ and ‘Gala’. Total phenolic compounds and total antioxidant capacity were significantly increased under all tested levels of ammonium in comparison to control in all cultivars. Furthermore, all cultivars showed a significant decrease in vitamin C, total phenolic compounds, and total antioxidant capacity throughout cold storage, indicating that these bioactive compounds were subjected to decomposition and metabolic degradation during the storage period. Surface response analysis results showed that more vitamin C degradation was attributed to the increase in ammonium application, while less total phenolic compounds degradation and total antioxidant capacity decrease were observed under similar treatments.