Effects of Various Rates of Freezing on the Metabolism of a Drought-tolerant Plant, the Moss Tortula ruralis

The response of the drought-tolerant moss Tortula ruralis ([Hedw.] Gaertn., Meyer, Scherb.) to freezing and thawing at controlled rates has been studied. Slow freezing (at 3 C per hour to -30 C) of hydrated T. ruralis leads to only temporary, reversible changes in metabolism. These changes can be considered to result from desiccation due to extracellular ice formation. In contrast, rapid freezing in liquid N(2) and thawing in 20 C water leads to deterioration in all aspects of metabolism studied: ribosome, protein, and ATP levels decrease, and in vivo and in vitro protein synthetic activity is lost rapidly. Such changes probably result from intracellular ice formation. Following freezing and thawing at an intermediate rate (60 C per hour), only ATP levels and in vivo protein synthesis are reduced. The protein-synthesizing apparatus (the polyribosomes) remains intact and active in an in vitro protein-synthesizing system even 24 hours after one 60 C per hour freeze-thaw cycle. These metabolic responses are discussed in terms of the two-factor hypothesis of Mazur et al. (1972 Exp. Cell Res. 71: 345-355).