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Seasonally Induced Changes in Acyl Lipids and Fatty Acids of Chloroplast Thylakoids of Pinus silvestris¹

A CORRELATION BETWEEN THE LEVEL OF UNSATURATION OF MONOGALACTOSYLDIGLYCERIDE AND THE RATE OF ELECTRON TRANSPORT

Department of Plant Physiology, University of Umeå, S-901 87 Umeå, Sweden

Current-year needles were sampled regularly from an approximately 20-year-old natural stand of Pinus silvestris. Chloroplast thylakoids were isolated. The electron transport capacities of photosystem II + photosystem I, as well as of the partial photoreactions, were measured. The amounts and the fatty acid compositions of monogalactosyldiglyceride, digalactosyldiglyceride, and sulfolipid of the thylakoids were analyzed. The fatty acid composition of the phospholipids (total) was also determined.

There was a 2-fold increase in the content of the galactolipids and the sulfolipid in the thylakoid preparations during the autumn. Except for monogalactosyldiglyceride that decreased in content during early winter, the content of the other two lipids remained high during the winter, until spring when their content fell toward a low summer level. The ratio of monogalactosyldiglyceride to digalactosyldiglyceride was lower in thylakoids isolated from frosthardened (autumn, winter) than in those isolated from unhardened (summer) needles.

Only monogalactosyldiglyceride showed pronounced and significant seasonal variations in the molar ratio of unsaturated to saturated fatty acids. It was low in the winter, and it increased strongly during the spring. The highest ratio was observed in September, prior to the first night frosts, after which a steady decrease toward the low winter value occurred. This decrease is thought to be a reflection of membrane damages (photooxidation of membrane components, including polyunsaturated fatty acids), as earlier seen reflected in chlorophyll destruction, electron transport inhibition, and structural changes.

There was very good correlation between the seasonal variations in the capacity for electron transport from water to NADP and the level of unsaturation of monogalactosyldiglyceride (r = 0.93). It has been shown earlier that the winter inhibition of electron transport preferentially occurs at the lipophilic electron transport carrier plastoquinone and that the free miscibility of plastoquinone and monogalactosyldiglyceride is prevented in saturated monogalactosyldiglyceride. Therefore, it is hypothesized that the decreased level of unsaturation of monogalactosyldiglyceride reduces the mobility of plastoquinone in the thylakoids, resulting in an inhibition of the electron transport by way of plastoquinone.