Watercore - A physiological disorder of apple related to imbalance carbohydrate metabolism in fruit tissues

Watercore is a physiological disorder of apple fruits resulted from the deviation of normal carbohydrate metabolism and accumulation of alcohol sugar, sorbitol in the intercellular spaces of the flesh tissues. The disorder is difficult to detect externally, however, in extremely severe conditions, the water-soaked areas can even extend out to the surface of the fruit and visible from outside the fruit. Transverse cutting of affected apple fruit showed a glassy appearance with translucent-water-soaked tissue in the fruit flesh around the vascular bundles and/or fruit core when the intercellular spaces are filled with a Sorbitol-rich fluid. Depending upon spatial distribution and pattern of water-soaked area on the flesh, watercore may be of block or radial type. The former a rectangular block of translucent tissue covering entire core area and extends into the flesh beyond the core area while as later affects carpellary bundles around the core-line and flesh that is adjacent to the affected vascular bundles. Block watercore generally occurs on immature apples during summer while ripening apple fruits mostly are affected by radial type of watercore. The disorder is associated with adverse pre- and/or post-harvest environment, especially temperature or to a nutritional deficiency during growth and development. Present understanding suggests that disorder is related to the unloading of sorbitol-rich liquid from the SE-CC of the vascular tissue to the apple fruit. During normal metabolism sorbitol must be converted to fructose by the apple fruit. Due to one or other reasons normal sorbitol metabolism is impaired, but that of sucrose is not. In affected tissues, the sorbitol is somehow inhibited from being metabolized by the cortex cells, leading to the accumulation of the ‘sorbitol-sap’ unloaded from the phloem cells and a reduction in reducing sugars (fructose and glucose) in the fruit cortex resulting in glassy appearance. However presence of watercore is difficult to recognize on the basis of external appearance. Various non-destructive techniques put forward for detection of watercore in apple including mass density, X-Ray CT, MRI, NIR, NMR and thermography have been discussed.