Polar lipids and net photosynthesis potential of subarctic Diapensia lapponica

The overall net photosynthesis potential of Diapensia lapponica was very low. It was highest at the end of August (11.1 micromoles CO2 (micromoles chlorophyll)-1 hr-1) and decreased during the autumn and winter, reaching a minimum in February (0.6 micromoles CO2 (micromoles chlorophyll)-1 hr-1). In March, with increased light in the subarticum, the potential rose temporarily, and after another minimum at the beginning of May, the potential slowly rose to the summer level (2.3-6.5 micromoles CO2 (micromoles chlorophyll)-1 hr-1). The seasonal fluctuation pattern of the potential of net photosynthesis was the same when calculated on a dry weight basis. Seasonal changes also occurred in the chlorophyll content and in the contents of polar lipids, particularly DGDG (digalactosyl diacylglycerols) and PC (phosphatidyl choline) and less clearly in MGDG (monogalactosyl diacylglycerols), PE (phosphatidyl ethanolamine) and PG (phosphatidyl glycerols). The contents of chlorophylls, DGDG and PC increased during autumn and early winter during the hardening process and decreasing light and temperature. Their contents decreased in late winter and spring in response to dehardening and increased light and temperature in the subarcticum. Thus, the molar ratios of MGDG:DGDG and PE:PC varied throughout the year, being lowest in winter. In addition, fatty acids of individual lipid classes showed seasonal fluctuation. In both MGDG and DGDG, the proportion of linolenic acid was higher in summer than in winter and that of linoleic acid was vice versa. In both PC and PK, the proportion of palmitic acid was highest in summer and lowest in winter and, particularly in PC, this variation was compensated for by changes in linoleic acid and less clearly in linolenic acid, and, in PE, by the long-chain behenic acid. In PG, the proportions of trans-16:1 and oleic acid were higher in summer than in winter, whereas the proportions of palmitic and linoleic acids were higher in winter than in summer. Thus, there was not a clear increase in the degree of unsaturation of fatty acids during winter-time.