Effects of temperature on the phase behavior and permeability of thylakoid lipid vesicles : relevance to chilling stress.

Large unilamellar vesicles composed of thylakoid glycolipids, phosphatidylglycerol, and varying proportions of dipalmitoylphosphatidylglycerol (DPPG) have been examined for the temperature dependence of their permeability to (86)Rb(+) and for the occurrence of liquid-crystalline to gel (L(alpha)-to-L(beta)) phase separations. In vesicles in which the normal 12 mole percent of moderately unsaturated thylakoid phosphatidylglycerol was partially or completely replaced by DPPG, analysis by differential scanning calorimetry indicated that an L(alpha)-to-L(beta) phase separation did not occur between 0 and 60 degrees C. However, in similar vesicle dispersions that were first subjected to a freeze-thaw cycle, L(alpha)-to-L(beta) phase separations were observed to occur between 17 and 53 degrees C. The temperature and enthalpy of these phase separations were closely related to the proportion of DPPG in the original lipid mixture. In parallel experiments, large unilamellar vesicles were measured for their permeability to (86)Rb(+) between 7 and 30 degrees C. There were no systematic increases in permeability to (86)Rb(+) as a function of DPPG content at the temperatures relevant to chilling stress in higher plants. It is concluded that (a) L(alpha)-to-L(beta) phase separations do not occur in well-defined galactolipid vesicles containing </=12 mole percent DPPG between 0 and 60 degrees C and (b) these vesicles show no alterations in permeability to (86)Rb(+) between 7 and 30 degrees C that are relevant to chilling stress in higher plants.