Membrane augmentation in freezing tolerance of plant cells

Abstract

THE question of the special physiology and biochemistry of growth and adaptation of plants to low temperature has been raised¹ with reference to the significance of the increased unsaturation of fatty acids observed in such plants. Interest in the phenomenon of increased unsaturation with exposure to low temperatures, first documented by Hilditch², has received impetus from reports^3,4 that membranes and membrane lipids of plants sensitive to chilling change from a liquid–crystalline to a gel structure when exposed to low temperatures, whereas those from plants resistant to chilling undergo no such phase change. The implication was that the less unsaturated membrane lipids of cells or organelles from plants which are sensitive to chilling were less fluid at lower temperatures and consequently lost normal function at such temperatures. Reports of increased fatty acid unsaturation in phospholipids of plant tissues^5,6 which are tolerant of chilling lend credence to these views. The apparently reasonable extrapolation from these findings that has been made^7–11, and that was inferred by Moore¹, is that the resistance of plants to injury from actual freezing may also be derived from changes in unsaturation and fluidity of membrane lipids. This theory warrants closer examination. The two stresses of chilling and freezing are quite different; chilling resistance is an adaptation to low temperatures above freezing and principally involves the interaction between metabolic function and structure, whereas freezing resistance is tolerance to the dehydrative stresses of crystallisation of ice, and seems to be largely structural in nature^12,13. It is reasonable to expect, therefore, that the underlying chemistry of resistance to chilling and freezing would be different. Our findings indicate that this is the case.