Mechanism of Protection by Nonpenetrating and Nontoxic Solutes against Freezing Injury to Plant Cells.

Both the existence and the cause of protection, by nonpenetrating solutes such as sugars, against freezing injury to plant cells have been alternatively and repeatedly proclaimed and questioned for some time. Since theories of the mechanism of frost injury have been both rejected (8) and proposed (2, 3) on the basis of this protection, it is of fundamental importance to understand the reasons for the disagreements and to find out whether the protective effect is real or an artifact. The question may conceivably be answered on the basis of the SH = SS hypothesis of frost resistance (5). According to this concept, injury is due to intermolecular protein SS bonds that arise during freezing, and resistance is due to the prevention of such bond formation. It could be suggested that surface proteins are involved and that the sugar protects their SH bonds. If this is true, the effect of such solutions should be duplicated by substances that prevent SS formation, and opposed by those that induce SS formation. The purpose of the following investigation was to test this hypothesis.