Mechanism of chilling injury in sweet potato VII. Changes in mitochondrial structure during chilling storage1

Mitochondrial protein-N amounts per the contents of heme a, cytochrome b and cytochrome c, the tightly bound components in the mitochondrial inner membrane, were not changed during 14 days of chilling stored sweet potatoes. However, the lipid-P amount per amount of protein-N in mitochondria decreased by about 20% during the 14 day chilling period. Via electron microscopy, two types of mitochondria were found in the mitochondrial fraction prepared from healthy sweet potatoes. One, which had reticulately-developed cristae and extremely-electron dense matrix spaces, was called Form A. The other, which had cristae which were not clearly distinguished from the matrix spaces, was called Form B. In the fraction prepared from 14 day chill-stored sweet potatoes, a third type of mitochondria was found besides Forms A and B. This one, which had an extremely swollen form, was called Form C. Form C is thought to occur through the degradation of Form A or B, concomitant with the release of phospholipid from both the inner and outer membranes during chill-storage. It is quite likely that Form C occurs during physiological deterioration in sweet potato root tissues, which proceeds irreversibly during chill-storage.