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 'Valenci a' (Citrus sinensis (L.) Osbeck) orange fruit, in relation to fruit de velopment 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 brea kdown the fracture paths developed across cell layers in a radial dire ction, normal to the direction of the stress. The fracturing of albedo tissue was due predominantly to the separation of adjacent cells rath er than the cleavage of individual cells, and was, therefore, characte ristic of anisotropic tissue. Although separation of albedo cells was also observed in unaffected fruit, it was assumed that the number of c ell separations did not exceed a critical threshold required to initia te fracture formation in the tissue. We suggest that changes in the co hesion of adjoining cells at the middle lamella predisposes the fruit to albedo breakdown. The rind of orange fruit was also examined by sca nning electron microscopy to identify changes in the ultrastructure of the epicuticular wax of developing and mature fruit. The deposition o f crystalline epicuticular wax was observed at an early stage of fruit development when albedo cells were beginning enlargement and differen tiation. Although crystalline platelet structure was greater on the sh aded 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.