The symptoms of water-spot injury, as already described by other investigators, are waterlogging and subsequent cracking of the rind of the ripening Washington navel orange. 2. The epidermis of the rind of the Washington navel, as of other varieties, is structurally weak. Cell division continues during maturity, and discrete, new, thin-wailed ceils are consistently present. In addition, the thinly cutinized accessory cells constitute zones of weakness. 3. That the epidermis and underlying tissues are readily permeable to water is demonstrated by immersion in vital stains. Methylene blue appears in the cell vacuoles, particularly of accessory cells and gland-cover and gland-sheath cells. 4. The sponginess of the rind, especially of the albedo, facilitates water accumulation. 5. The albedo differentiates from tetrakaidekahedral meristematic cells and, when mature, consists of an open meshwork of eight-armed cells interconnected by plasmodesmata. 6. Structural weakness is already apparent in the young fruit. Incipient microscopic cracks, owing to normal growth strains or insect puncture, are, however, healed by concurrent deposition of cutin. 7. In the mature fruit the balance between epidermal extension and increase in volume of underlying tissues is critical and may be upset by excessive water absorption during winter rains. The cracks which result may heal in dry weather by normal cicatrization or, in continued rain, may remain open and allow the entrance of fungal spores. 8. Why the Washington navel orange is more susceptible to water spot than other varieties which ripen at the same time is undetermined. No significant differences in rind structure are evident. 9. The orchard practice of oil-spraying does not visibly affect. rind structure. No anatomical difference is observed between untreated and oil-sprayed, or even oil-dipped, fruits.
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Scott et al. (1947) studied this question.