Phase properties of membranes after freezing injury in winter wheat

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Abstract
The structural and compositional changes which occur in cellular membranes after freezing stress were examined in a microsomal membrane fraction from the crown tissue of 7-day-old, nonacclimated Triticum aestivum L. cv. Fredrick seedlings, which were frozen to a lethal temperature of −12 °C. The freezing treatment induced a lipid phase separation and increased the gel to liquid – crystalline phase transition temperature of the isolated membrane fraction from 25 to 65 °C as detected by wide-angle X-ray diffraction. This structural change was not accompanied by significant changes in the fatty acid saturation of the total lipid extract or by changes in the free sterol components, but a 13-fold increase in the free fatty acid – phospholipid ratio was observed in the membranes from the freeze-stressed tissue. It is proposed that the accumulations of free fatty acids and other phospholipid degradation products in the membrane are factors relating to the formation of extensive gel-phase domains and that this structural change contributes to the changes in membrane properties, such as loss of semipermeability and osmotic responsiveness, which are typically observed after freezing stress. Possible mechanisms leading to the accumulation of free fatty acids are discussed.