Possible roles of calcium and ammonium in the development of bitter pit in apple

Abstract

Apple trees ( Malus pumila Mill . var. domestica Fuji/ Malus prunifolia rootstock) showed a high susceptibility to bitter pit when supplyed with ammonium salt instead of nitrate (control) in the nutrient solution. When apple fruit was affected by bitter pit, a lower calcium as well as a higher nitrogen and ammonium‐nitrogen contents was observed in the fruit flesh near the calyx end. The activity of the mitochondrial Ca 2+ ‐uptake of the fruit flesh near the calyx end was higher when the tree was grown with ammonium salt than when grown with nitrate. Both the activities of succinate: cytochrome c oxidoreductase and the mitochondrial Ca 2+ ‐uptake per g of tissue were higher in affected fruit than in healthy fruit. Each of chlorpromazine, N‐(6‐aminohexyl)‐5‐chloro‐l‐napthalenesulfonamide (W‐7) and N‐(6‐aminohexyl)‐l‐naphthalenesulfonamide (W‐5), calmodulin antagonists, was infiltrated into the fruit for 20 min under reduced pressure (about 1 × 10 4 Pa). Few days later, numerous bitter pit‐like spots were observed in both fruit treated with W‐7 and chlorpromazine, while only a few spots were observed after the infiltration with W‐5, a less potent calmodulin antagonist. A possible mechanism for the occurrence of bitter pit is discussed.