The problems of relating low fruit calcium concentrations in stored apples to the development of bitter pit lesions are reviewed. They include nomenclature, anomalous fruit analyses, variability in the susceptibility of apples to pitting and in the time and rate of pit development, effect of harvest date, accumulation of calcium in pitted tissue and the apparant translocation of calcium during storage from the core into zones of tissue most at risk. Recent evidence suggests that bitter pit lesions are induced by the removal of calcium from these outer zones to meet intermittent demands by the core tissue.
There is evidence that the incidence of each of three storage disorders of Cox's Orange Pippin apples (senescent breakdown, low‐temperature breakdown and bitter pit) is affected by the mineral composition of the fruit. These effects have been studied by comparing initial average compositions of bulk samples with the storage behaviour of bulk replicates and by experiments with individual apples in which apples were analysed after their storage record was known.
It is suggested that if the Ca level in an apple is less than 3 mg/100 g fresh weight it will be liable to senescent breakdown at an early stage of storage. A P concentration of less than 8 mg/100 g may have the same result even if the Ca level is high.
Low‐temperature breakdown is less likely in apples with high levels of K, P and Mg than in apples with low levels of these elements.
The results confirm that low average Ca concentration is associated with bitter pit. It is suggested that if the average Ca concentration exceeds 5 mg/100 g fresh weight the sample will probably be free from this disorder. The relationship between Ca and bitter pit in apples stored individually was not clearly defined.
The distribution of calcium at harvest, its subsequent redistribution within the fruit during storage in air at 2.8°C and bitter pit development in samples of Cox's Orange Pippin apples picked at intervals during September and October were investigated. The distribution of calcium in the fruit changed on the tree and during storage. The percentages of pitted apples, assessed in January, were poorly related to calcium concentrations in the whole fruit or in any fruit zone at harvest. Redistribution of calcium from the mid and outer cortical tissues to the core zone was followed, at longer intervals over successive picks, by the appearance of bitter pit lesions. The earliest‐picked sample was less affected by bitter pit than samples picked later in September. The least bitter pit occurred in samples picked in October, after the climacteric rise in respiration, and these fruits were apparently less subject to fluctuations in calcium concentrations during storage.
Data for bulked samples of ungraded Cox's Orange Pippin fruit obtained from many sources over a period of 18 years show that there is a general inverse relationship between mean mass per apple and mean calcium concentration. Part of the scatter around the regression curve was accounted for by differences between years but there were larger differences between samples of the same size picked from different orchards in the same year. Even with the most favourable conditions the observed samples with mean mass per apple greater than 150 g had less than 5 mg Ca/100 g when no calcium sprays were applied and in general most samples with mean masses per apple greater than 110 g were likely to have concentrations of calcium below this and hence were liable to develop bitter pit. Fruit calcium level was a more important determinant of liability to bitter pit than was mean mass per apple.Although there was some evidence of the same relationship between calcium concentration and mass when apples from the same tree were analysed individually other factors often negated it. Two-to three-fold differences in calcium concentration were observed in individual apples from the same tree.
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