The round blue cells are the normal apple cells. The brown mass round and between them is callus hair growth. The callus hairs remain full of starch even though the normal cells have lost their starch during storage. This is a light microscopy image.
Scientists in the United Kingdom have discovered clumps of unusual cells in the flesh of Fuji apples that they think might affect storage of the fruit.
Dr. Mary Parker, a researcher at the Institute of Food Research in Norwich, England, notes that in some years, Fuji apples—particularly large, late-harvested fruit—are vulnerable to internal browning during controlled-atmosphere storage. Unrestricted gas flow through the fruit is vital for successful long-term storage, particularly in CA.
While studying the structure of the fruit of several apple varieties, Parker and her colleague Walter Guerra noticed hairlike growth in air pockets between the parenchyma (functional) cells in the outer flesh of organic Fuji apples. The hairs resemble the white velvety tufts that sometimes grow around the seeds of some apple varieties, and are often mistaken for fungal growth. No one had previously documented the growth of such hairs in the flesh of apples.
She believes that the hairs, which she calls callus hairs, might be of great relevance to postharvest quality, as she found them growing in the main gas diffusion pathway in the spaces between cells. Parker thinks that the presence of callus hair growth, with its own oxygen requirement and carbon dioxide output, may reduce the efficiency of gas transport. It’s been shown that the continuity of the network between cells is important for gas transportation.
To find out if callus hairs are found in all mature Fuji apples, Parker examined imported Fuji and Fuji sport apples that she bought in U.K. supermarkets in 2004 to 2006. As Fuji apples are not grown commercially in the United Kingdom, she obtained domestic Fuji apples from the U.K. National Apple Collection.
She found callus hair growth in all the Fuji apples that she examined. However, they were most developed in Fuji Suprema apples from Brazil, and moderately developed in Fuji Kiku from Italy and Fuji from South Africa, Chile, New Zealand, and the United States. They were consistently the least developed in Fuji apples from China.
Most of the callus hair clumps were in the outer 10 mm (half inch) of the flesh, though some were as deep as 17 mm (two-thirds of an inch) below the skin. The cells of the callus hairs are usually green because they contain chlorophyll. They also frequently contain small starch granules, which remain even after starch is metabolized from most of the apple.
It is not yet known what triggers callus hair growth. Parker has studied an unrelated apple cultivar called Lord Derby, which also has callus hairs in the flesh, and reports that the initial stages of hair growth are visible when the apple measures 25 mm (one inch) in diameter. The clumps of hairs continue to grow until harvest, but do not seem to grow further during storage. This means that Fuji apples that are picked early for long-term storage may have less callous growth than more mature fruit.
Callus cells are small, compared to the other fruit flesh cells. Parker believes the reason they have not been observed before is because the full extent of their growth can only be appreciated in three dimensions. She used light microscopy and scanning electron microscopy in her research.
With the new knowledge generated by her research, breeders could pick parent varieties with the positive traits of Fuji apples but with less developed callus hairs, Parker suggests. The presence or absence of callus hairs could also be used to test the authenticity of dried apples labeled as Fuji.
Future research needs to focus on the role of the callus hairs in storage disorders as well as phytonutrient and starch content, she said.
Dr. Jim Mattheis, a postharvest physiologist with the U.S. Department of Agriculture in Wenatchee, Washington, said Parker’s findings were another interesting example of the genetic diversity in apple. However, he thinks that the significance of the callus hairs in relation to storage performance and fruit quality remains to be demonstrated.
A detailed report on the research entitled "Occurrence and implications for postharvest quality of intercellular callus hair growth in the outer cortex of apples of Fuji and Fuji sports" is published this month in ScienceDirect – Postharvest Biology and Technology (www.science direct.com).
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