Sand-culture Studies on the Calcium Nutrition of Young Apple Trees with Particular Reference to Bitter Pit

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Abstract
Calcium stress to the roots, especially during the early stages of apple fruit development, induced bitter pit in the fruits; the pit appeared deep in the fruit cortex of cv Egremont Russet, unlike the sub-cuticular layer pit induced in other apple culti vars. Marginal leaf scorch symptoms in the old leaves were induced by Ca deficiency: the symptoms became acute when combined with Mg deficiency (induced by Ca deficiency and extra K). The development of fine roots and root hairs was also adversely affected by Ca deficiency. Leaf analysis showed that total Ca in leaves does not indicate the functional Ca status of plants or necessarily reflect the Ca supply to the fruit. Extra K decreased leaf Ca and Mg and increased leaf K concentrations. Bitter pit was positively related to fruit size and to K in apple peel, and was negatively related to Mg and Ca in peel and to Ca++ in the juice. Calcium thresholds derived according to “Boundary Line” concepts are proposed for predicting the incidence of bitter pit in apples (cv Egremont Russet): a “Free Limit” (500 ppm Ca in the dry matter), i.e. the concentration of Ca in the peel above which the fruit is completely free from bitter pit, and a “Safe Limit” (450 ppm Ca in the dry matter), the concentration in the peel above which bitter pit in the fruit is unimportant economically.

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