The Ontogeny and Cytogenesis of Cork Spot in ‘York Imperial’ Apple Fruit1

Abstract

Abstract Histogenetic studies of the young fruit revealed that incipient cork spot in ‘York Imperial’ apples (Malus domestica Borkh.) may appear as early as 3 weeks after fruit set, and that it initially becomes evident as a sporadic gradual necrosis of several parenchymatous cells of the outer cortex. About 1 month later, healthy enlarging cortical cells contiguous to the necrotic region undergo a redifferentiation via direct nuclear or amitotic divisions that result in a promiscuous intracellular proliferation of daughter cells. These remain within the confines of the original mother cell wall until it ruptures. The amitotic nuclei assume a variety of configurations and divide by cleavage. The method by which cell walls form between the intracellular proliferations is undetermined, and there is no evidence of cell plate formation. Once initiated, cork spot proliferation occurs only within the parenchymatous cells of the fruit cortex and is continuous throughout the growing season. There is no evidence of an internal cork cambium per se. Cork spot is not invariably associated with the vascular bundles, and there is no distinct evidence of the senescence of vascular bundles resulting principally from the necrosis. The necrotic condition becomes visible externally as a slightly sunken epidermal discoloration about 3 months after fruit set.