EVIDENCE FOR THE CELL-WALL INVOLVEMENT IN TEMPORAL CHANGES IN FREEZING TOLERANCE OF JERUSALEM-ARTICHOKE (HELIANTHUS-TUBEROSUS L.) TUBERS DURING COLD-ACCLIMATION

We studied the mechanism of cold acclimation of Jerusalem artichoke (H elianthus tuberosus L.) tubers with special reference to the role of t he cell wall. During the cold-acclimation process from September to Ja nuary, the freezing tolerance of tubers increased from -2.8 degrees C to -8.4 degrees C (LT50). By contrast, the isolated protoplasts consti tutively showed a consistent high level of freezing tolerance (LT50; b elow -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 mou nted in isotonic solutions plasmolyzed upon freezing; tissue cells mou nted in water collapsed with a tight attachment of plasma membrane to the cell wall. Upon freezing of intact tissues in water to temperature s below the critical range, the cytoplasm was irreversibly acidified a s 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 prevent ed by the external addition of isotonic solutions. These results sugge st that the tight attachment of the plasma membrane to the cell wall d uring freezing may have a harmful effect on cells, in particular on th e plasma membrane, possibly due to mechanical or some sort of chemical /physico-chemical interaction with the cell wall.