Phase transitions in liposomes formed from the polar lipids of mitochondria from chilling-sensitive plants

The thermal response of mitochondrial polar lipids from a variety of chilling-sensitive and chilling-insensitive plants was determined by differential scanning calorimetry. A phase transition was observed at 15°C for mitochondria from soybeam (Glycine max. cv Davis) hypocotyl, at 16°C for tomato (Lycopersicon esculentum cv Flora-Dade and cv Grosse Lisse) fruit, at 15°C for cucumber (Cucumus sativus L.) fruit, at 14°C for mung bean (Vigna radiata var Berken) hypocotyl, and at 15°C for sweet potato (Ipomea batatas L.) roots. The transition temperature was not significantly altered by the scan rate and was reversible. Changes in the temperature coefficient of motion for a spin label, intercalated with the polar lipids, occurred at a temperature slightly below that of the phase transition, indicating that the polar lipids phase separate below the transition. No phase transition was observed for mitochondrial polar lipids from barley (Hordeum vulgare) roots, wheat (Triticum aestivum L. cv Falcon) roots, and Jerusalem artichoke (Helianthus tuberosus L.) tubers. The results show that a phase change occurs in the membrane lipids of mitochondria a few degrees above the temperature below which chilling injury is evident in the sensitive species. Thus they are consistent with the hypothesis that sensitivity to chilling injury is related to a temperature-induced alteration in the structure of cell membranes.