No abstract

We studied the mechanism of cold acclimation of Jerusalem artichoke (Helianthus tuberosus L.) tubers with special reference to the role of the cell wall. During the cold-acclimation process from September to January, the freezing tolerance of tubers increased from -2.8 degrees C to -8.4 degrees C (LT50). By contrast, the isolated protoplasts constitutively showed a consistent high level of freezing tolerance (LT50; below -25 degrees C) throughout the period. In tuber tissues, freezing injury was effectively protected by the external addition of isotonic solutions. Cryomicroscopic observations revealed that tissue cells mounted in isotonic solutions plasmolyzed upon freezing; tissue cells mounted in water collapsed with a tight attachment of plasma membrane to the cell wall. Upon freezing of intact tissues in water to temperatures below the critical range, the cytoplasm was irreversibly acidified as revealed by a fluorescence pH-ratiometry, suggesting that occurrence of detrimental cellular events leading to permanent cell injury. The freeze-induced acidification of cytoplasm was also effectively prevented by the external addition of isotonic solutions. These results suggest that the tight attachment of the plasma membrane to the cell wall during freezing may have a harmful effect on cells, in particular on the plasma membrane, possibly due to mechanical or some sort of chemical/ physico-chemical interaction with the cell wall.