Occurrence and implications for postharvest quality of intercellular callus hair growth in the outer cortex of apples of 'Fuji' and 'Fuji' sports

Mature apples of 'Fuji' and 'Fuji' sports sourced from around the world were found to possess clumps of multicellular, branched callus hairs in the outer 17 mm of the outer cortex. The clumps of callus hairs were particularly well developed in samples of 'Fuji Suprema' from Brazil, moderately developed in 'Fuji KIKU' from Italy and in 'Fuji' from South Africa, Chile, New Zealand and the USA, and were consistently least developed in 'Fuji' from China. These previously unreported callus hairs grow in the intercellular air spaces between the parenchyma cells and also in larger air lacunae in the apple flesh. In both of these locations they have the potential to reduce the efficiency of gas exchange into and out of the fruit during storage. Initial observations suggest that the callus hairs within a clump do not continue to develop after harvest. In ripe apples, the cells of the callus hairs contain chlorophyll and starch and under UV light the contents of the vacuoles autofluoresce an intense blue colour. The outer surface of each callus hair cell is covered with characteristic globular protuberances which attach the hairs to each other and to the surrounding parenchyma. After a period of storage, many of the clumps of callus hairs remain packed with starch granules even though starch has been metabolised from the surrounding parenchyma.