Low Temperature Treatments Induce an Increase in the Relative Content of Both Linolenic and 3-Hexadecenoic Acids in Thylakoid Membrane Phosphatidylglycerol of Squash Cotyledons

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Abstract
The effect of low temperatures on the fatty acid composition of phosphatidylglycerol (PG) in thylakoid membranes, in particular on the ratios of nmol% 16:1(3t) (mg fresh weight)−1 of cotyledons and nmol 16:1(3t) (mg chlo rophyll)−1 were measured during squash seedling growth. Plants were germinated and grown for one day at 30°C then were either kept at 30°C (control plants) or trans ferred to low temperatures (18, 14 or 10°C). When plant were transferred from 30°C to low temperatures, the increase in fresh weight was gradually limited. The lowe the temperature, the smaller was the fresh weight. In contrast, the relative content of 16:1(3t) and 18:3, as well a the ratios of nmol 16:1(3t) (mg chlorophyll)−1 and mol% 16:1(3t) (mg cotyledon fresh weight)−1 increased indicating that the increase of fresh weight and chlorophyll was mor sensitive to low temperature than PG desaturation in thyla-koid membranes. Furthermore, low temperatures inducei an increase in 16:1(3t) and 18:3 (the final products of PC synthesis) at the expense of 16:0 and 18:1 (the initial products of PG synthesis). However, within a range of temperature from 10 to 18°C, the extent of these changes (nmol% of 18:3 or 16:1(3t) per day) was gradually limited by lower temperatures. We therefore propose that low temperature inhibit both fatty acid synthesis and desaturation activities. However, at low temperatures the fatty acid synthesis is likely to be more strongly inhibited than the desaturation activities, thus explaining the observed increase in the relative content of PG-18:3 and PG-16:l(3t). Results an discussed in terms of the mechanism which could be in volved in the metabolism of PG in squash cotyledons.

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