SCANNING ELECTRON MICROSCOPY AND ELECTRON MICROPROBE STUDIES OF BITTER PIT IN APPLES

In an extensive physiological and biochemical review of corking disorders of apples, Faust and Shear (1968) stated that the classification and categorization of these disorders had been based on incomplete information. They explained that the various types of corking disorder had been grouped together because of their somewhat similar appearance, but whether they are the same in origin is unknown. One of the most common disorders, bitter pit, is usually regarded as a storage disorder (Smock, 1941). However, fruits may have developed the disorder by harvest time and it seems universally agreed among investigators that the cause lies in the orchard. While the fruit may be free of the disorder when it is placed in storage, its susceptibility to the development of pitting in storage depends upon conditions occurring during the development of the fruit on the tree.

To obtain more knowledge on the interrelationships of these physiological disorders, Simons (1962, 1968) made light microscopy studies of the early formative stages of bitter pit and other corking disorders during fruit development. In a sequential development study on York Imperial apples, Simons, Hewetson and Chu (1971) found abnormalities adjacent to necrotic vascular bundles in the outer cortical region apparent early in the life of the fruit, i.e. by 14 days after full bloom. Changes in cellular structure continued for 65 days after full bloom from the outer cortex to the epidermis and extended to the bundles underlying this area.

In the work described here the light microscopy studies, which have been fully documented, are complemented by comparisons of scanning electron microscopy and electron microprobe analysis. The data concern the development of bitter pit during the formative stages of fruit development and those nearing maturation. Results relate to the variety York Imperial in particular but other varieties were found to follow the same general pattern.