LOW-TEMPERATURE TREATMENTS INDUCE AN INCREASE IN THE RELATIVE CONTENTOF BOTH LINOLENIC AND DELTA(3)-TRANS-HEXADECENOIC ACIDS IN THYLAKOID MEMBRANE PHOSPHATIDYLGLYCEROL OF SQUASH COTYLEDONS

The effect of low temperatures on the fatty acid composition of phosph atidylglycerol (PG) in thylakoid membranes, in particular on the ratio s of nmol% 16:1(3t) (mg fresh weight)(-1) of cotyledons and nmol 16:1( 3t) (mg chlorophyll)(-1) were measured during squash seedling growth. Plants were germinated and grown for one day at 30 degrees C, then wer e either kept at 30 degrees C (control plants) or transferred to low t emperatures (18, 14 or 10 degrees C). When plants were transferred fro m 30 degrees C to low temperatures, the increase in fresh weight was g radually limited. The lower the temperature, the smaller was the fresh weight. In contrast, the relative content of 16:1(3t) and 18:3, as we ll as the ratios of nmol 16:1(3t) (mg chlorophyll)(-1) and mol% 16:1(3 t) (mg cotyledon fresh weight)(-1) increased indicating that the incre ase of fresh weight and chlorophyll was more sensitive to low temperat ure than PG desaturation in thylakoid membranes. Furthermore, low temp eratures induced an increase in 16:1(3t) and 18:3 (the final products of PG 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 1 8 degrees C, the extent of these changes (nmol% of 18:3 or 16:1(3t) pe r day) was gradually limited by lower temperatures, We therefore propo se that low temperatures inhibit both fatty acid synthesis and desatur ation activities. However, at low temperatures the fatty acid synthesi s is likely to be more strongly inhibited than the desaturation activi ties, thus explaining the observed increase in the relative content of PG-18:3 and PG-16:1(3t). Results are discussed in terms of the mechan ism which could be involved in the metabolism of PG in squash cotyledo ns.