Lipid and fatty acid composition of chloroplast thylakoid membranes was determined in two varieties of wheat (Triticum aestivum L.), the hardy Miranovskaja and the sensitive Penjamo. Plants were grown at room temperature or under frost hardening conditions (1.5°C). Changes in lipid and fatty acid composition of the isolated thylakoids could be related to the temperature dependence of light-stimulated proton uptake. Changes in the thylakoid phospholipids upon hardening of the two varieties did not show any direct relation with low temperature tolerance of light-dependent H' uptake; neither did changes in phospholipid fatty acid chain lengthening to 20 and 22 C-atoms in combination with increased desaturation up to 6 double bonds. Increased low temperature tolerance of light-induced H' uptake by hardening was correlated with the following glycolipid changes: maintained glycolipid level, a proportionally increased digalactosyl diglyceride fraction, a decrease in thylakoid monogalactosyl diglyceride, increased sulfolipid fatty acid chain lengthening (20 and 22 C-atoms), and increased sulfolipid desaturation (4-6 double bonds). We suggest that the above mentioned changes in glycolipids have adaptive value for low temperature tolerance of lightdependent proton uptake.proportionally or remained unaffected in some species/varieties (frost-sensitive wheat variety [17]; pea and spinach [1]), whereas in the thylakoids of other species an increased DGDG/MGDG ratio was observed upon hardening (frost-resistant wheat variety, [17]; pine, [12] Growth Conditions and Hardening Procedure. Seeds of two wheat varieties (Triticum aestivum L.), the frost-sensitive Penjamo and the hardy variety Miranovskaja, sown in soil, were germinated at a day/night temperature regime of 20/12C, 16 h d-length, for 6 d. The hardening program was subsequently started by lowering the temperature gradually during 3 weeks, followed by a continuous period of 4 weeks at 1.50C, with the same light regime. After finishing the hardening, the leaves were harvested for thylakoid isolation.For the isolation of thylakoids the procedure of Lineberger and Steponkus (7) was used. Lipids were extracted as described earlier (4)

The lipid composition of leaves has been investigated in different genotypes of cucumber (Cucumis sativus L.), which differ in temperature requirement for cultivation. In addition the effects of hardening by low but non‐chilling temperature, soil heating and grafting (on the chilling‐resistant C. ficifolia L.) on lipid composition have been studied. Content and composition of phospholipids and sterols were determined as well as phospholipid/sterol ratio, and fatty acid composition of total lipids and the different phospholipids. The effects of genetic differentiation and of the various culture treatments on lipid composition of the leaves were very different. Genetic differentiation was evident as higher levels of Iinolenic acid in several phospholipids in the more cold‐tolerant cultivars. Hardening the plants by low temperature resulted in a higher phospholipid level (especially phosphatidyl choline), more unsaturated phospholipid, and lowering of the sterol/phiospholipid ratio, all properties which may contribute to a higher membrane fluidity and lower growth temperature limit. Soil healing reduced the phospholipid level of the leaves slightly, and a higher content of 3‐trans‐hexadece‐noic acid in phosphatidyl glycerol was observed. Grafting cucumber on the cold‐resistant rootstock of C. ficifolia also raised the level of trans‐hexadecenoic acid in phosphatidyl glycerol. The role of this fatty acid in the functioning of the chloroplast is discussed.

Phospholipid multibilayers, obtained from two cultivars of thermally acclimated wheats of different frost resistances (Triticum aestivum L. cv. Penjamo 62, the sensitive cultivar, and T. aestivum L. cv. Miranovskaja 808, the frost-resistant cultivar), were investigated using electron-spin-resonance and X-ray-diffraction techniques. The former technique revealed two breaks in the motion of the spin-labelled fatty acid 2-(14-carboxyte-tradecyl)-2-ethyl-4,4-dimethyl-3-oxazolidinyloxyl, for both cultivars (+3°C,-17° C and +5° C,-18° C, respectively) when grown at 22° C. The resistant cultivar compensated for exposure to cold (+2° C) by shifting the onset of the apparent phase-separation temperature from +3° C to-16° C. The sensitive cultivar was unable to do so. X-ray diffraction did not reveal fluid-to-gel transitions between +20° C and-10° C in any of the samples. The possible role of the formation of relatively ordered aggregates or clusters of lipid molecules discerned by spin probe within the otherwise freely dispersed liquid-like lipids is discussed in terms of freezing injury of plants.

The contents of free sterols and phospholipids in leaves of wheat, Triticum aestivum L., cultivars of different frost resistances, as well as the physical state of isolated phospholipids in the presence and absence of sterols, were compared before and after hardening. There was an inverse relationship between the sterol/phospholipid ratio and frost tolerance as a consequence of both a decrease in the free sterol, and an increase in the total phospholipid content. Sterol-sterol interactions were investigated using wide angle X-ray diffraction, while the phase behaviour of phospholipid vesicles was studied using the electron-spin-resonance (ESR) technique. No sterol-sterol interactions at-10° C were detected in vesicles obtained from the hardened most cold-tolerant cultivar (Miranovskaja 808), containing sterols in a ratio (0.08) found in the original lipid extracts. In contrast, when the sterol-phospholipid ratio in the vesicles was set to the level (0.39) found in the extracts of the most sensitive cultivar, Penjamo 62, the appearance of sharp reflexion rings at 4.5·10(-1), 4.8·10(-1) and 5.0·10(-1) nm indicated strong sterol-sterol interactions. The temperatures for the onset of phase separation for vesicles of identical sterol/phospholipid ratios found in lipid extracts of hardened Miranovskaja 808 were almost the same as those measured in purified phospholipids (-15 vs.-16° C). In contrast, the temperature for the onset of phase separation of vesicles with a sterol/phospholipid ratio characteristic of hardened Penjamo 62 was shifted upwards (from-6 to-2° C). Phase separation was not completed in the vesicles of Miranovskaja 808 in the temperature range scanned (-30° C) but was shifted from-22 to-18° C in the presence of sterols in the case of Penjamo 62. The results are discussed in terms of the composition and physical state of membranes in relation to survival at freezing temperatures.