Longitudinal distribution of applied calcium, and of naturally occurring calcium, magnesium, and potassium, in Merton apple fruits

When 45Ca-labelled calcium chloride solution was applied to the skin of young developing Merton apple fruits, activity in the cortex at maturity was highest in the calyx end region where bitter pit lesions most commonly develop. Fruit age at time of application affected the amount of labelled calcium absorbed but not its longitudinal distribution. Following branch injection of labelled calcium chloride solution 8 weeks before harvest, activity in mature fruits was found mostly in the stem end. Leaves and buds accounted for 95% of the recovered activity, and the fruits for 5%, of which about one-quarter was in the calyx half. The same distribution pattern was found following injection of labelled calcium chloride solution into the fruit stem. The concentration of naturally occurring calcium in the cortex of the mature fruit declined steadily from stem end to calyx end. On the other hand, the magnesium concentration was lowest near the stem end and highest at the calyx end. The potassium concentration remained fairly constant along most of the length of the fruit, with a small increase at each end. The findings are discussed with regard to the longitudinal gradient in bitter pit susceptibility within apple fruits, and to the relative effectiveness of tree sprays of calcium, as compared with soil applications, in the control of the disorder. Results suggest that calcium does not have a trace element role in the control of bitter pit.