| Above freezing chilling temperatures lead to stress and eventual death in plants, mostly of tropical and sub-tropical origin. It is widely held that chilling stress arises in part from a chilling-induced molecular ordering resulting in fluid-to-gel phase change in membrane lipids and, consequently, membrane dysfunction whereas hindrance to cold-induced molecular packing, created by ‘cis’ double bonds in fatty acid acyl chain is a molecular basis from chilling tolerance. This study examine a hypothesis suggesting that cold conditions diminish the hydration of lipids and that lipid unsaturation enhance water binding by lipids and oppose cold-induced water expulsion.; To test these objectives fatty acid methyl esters were hydrated and measurement of water content, using gas chromatography, were used to assess the effect of cold temperature, various molecular features and interaction with solutes on the hydration of fatty acid acyl chain. The results revealed that decrease in temperature from 26° to around 15°C led to a rapid water expulsion and a leveling-off in water content as the temperature decreased further. The results revealed also that cold-induced water expulsion was mitigated by an increase in unsaturation and shortening of fatty acid acyl chain. Addition of ammonia or methylamine, which bind to double bonds through charge interactions, greatly magnified hydration and resistance of unsaturated fatty acids to cold-induced water expulsion. Log plots of water content vs. the reciprocal of absolute temperature revealed also that plot discontinuity, resembling ‘breaks’ in Arrhenius plots observed in other studies, represent an end point of cold-induced water expulsion.; These results support the overall concept that chilling leads to the dehydration of fatty acid hydrocarbon chain and perhaps other hydrophobic matrices and, moreover, that molecular features associated with chilling tolerance enhance water binding by fatty acid. Additional studies are needed to assess whether lipid hydration or dehydration is reflected in corresponding changes in molecular ordering and whether hydration changes in native cellular membranes are a molecular and cellular basis for chilling susceptibility and tolerance. |
