THE MORPHOLOGY OF EPICUTICULAR WAX AND ALBEDO CELLS OF ORANGE FRUIT IN RELATION TO ALBEDO BREAKDOWN

Changes in the morphology of albedo cells of 'Leng' navel and 'Valencia' (Citrus sinensis (L.) Osbeck) orange fruit, in relation to fruit development and to the structural basis of albedo breakdown (creasing), were examined by scanning electron microscopy of frozen-hydrated plant material. In albedo tissue sampled from fruit affected by albedo breakdown the fracture paths developed across cell layers in a radial direction, normal to the direction of the stress. The fracturing of albedo tissue was due predominantly to the separation of adjacent cells rather than the cleavage of individual cells, and was, therefore, characteristic of anisotropic tissue. Although separation of albedo cells was also observed in unaffected fruit, it was assumed that the number of cell separations did not exceed a critical threshold required to initiate fracture formation in the tissue. We suggest that changes in the cohesion of adjoining cells at the middle lamella predisposes the fruit to albedo breakdown. The rind of orange fruit was also examined by scanning electron microscopy to identify changes in the ultrastructure of the epicuticular wax of developing and mature fruit. The deposition of crystalline epicuticular wax was observed at an early stage of fruit development when albedo cells were beginning enlargement and differentiation. Although crystalline platelet structure was greater on the shaded side of fruit there were no apparent differences between affected and unaffected fruit in the morphology of the epicuticular wax on the shaded side of the rind.