The role of glycinebetaine in the protection of spinach thylakoids against freezing stress.

The quaternary ammonium compound glycinebetaine has been tested for cryoprotective properties, using isolated spinach thylakoids as a model membrane system. The effect of a 3-h,-20°C freezing regime on different photosynthetic parameters was measured. These parameters were the light-stimulated ΔpH formation and dark ΔpH decay, light-stimulated proton uptake, electron flow through photosystem II, photosystem I and total linear electron flow, and pyocyanine-mediated cyclic photophosphorylation. It was shown that below 100 mM glycinebetaine was superior as a cryoprotectant to sucrose on a molar, a molal and an activity basis. At higher concentrations, glycinebetaine was less efficient in preventing inactivation of thylakoids during freezing than sucrose. These observations are discussed in relation to the permeability of biomembranes to glycinebetaine and the colligative theory of cryoprotection. It is concluded that colligative protection is modified by direct interaction between cryoprotectant and membranes.