Protective agents: regulation of synthesis.
The exogenous cues to overwintering adaptations vary not just between components of hardening but between species. One species, P. brevicornis, initiates glycerol synthesis in response to 0 degree C exposures while a second species, E. solidaginis, increases glycerol levels not in response to temperature but in apparent association with changes in total body mass. This species maintains a constant annual percentage of water while occupying a hibernaculum that dries considerably. During overwintering, E. solidaginis losses approximately 50% of its total body mass. In addition to the changes described, this species (northern populations) increases the amount of water bound to both protein and low-molecular-weight compounds during hardening. The increase in binding exceeds threefold between 25 and -30 degrees C (0.193 to 0.633 g/g dry wt) (29). These data do not unequivocally demonstrate the existence of a hydration trigger to glycerol synthesis but are adequate to put forth such a hypothesis. A decrease in total bulk water levels due to both wet weight loss and increases in bound water may provide conditions of functionally reduced intracellular metabolic water. Since polyol production necessitates the disruption of carbon flow between glucose-6-phosphate and pyruvate, one or more enzymes may be sensitive to water reductions. Pyruvate kinase is sensitive to available water levels. Inhibition of this enzyme would likely cause a shunting of carbon metabolism to glycerol production. This hypothesis becomes attractive in light of the observation that in a variety of species, glycerol accumulations have been correlated with dehydration and hyperosmotic conditions. A common adaptative mechanism may exist in response to apparently different environmental perturbations.