POLAR LIPIDS AND NET PHOTOSYNTHESIS POTENTIAL OF SUB-ARCTIC DIAPENSIA-LAPPONICA

The overall net photosynthesis potential of Diapensia lapponica was ve ry low. It was highest at the end of August (11.1 mu mol CO2 (mu mol c hlorophyll)(-1) hr(-1)) and decreased during the autumn and winter, re aching a minimum in February (0.6 mu mol CO2 (mu mol chlorophyll)(-1) hr(-1)). In March, with increased light in the subarticum, the potenti al rose temporarily, and after another minimum at the beginning of May , the potential slowly rose to the summer level (2.3-6.5 mu mol CO2 (m u mol chlorophyll)(-1) hr(-1)). The seasonal fluctuation pattern of th e potential of net photosynthesis was the same when calculated on a dr y weight basis. Seasonal changes also occurred in the chlorophyll cont ent and in the contents of polar lipids, particularly DGDG (digalactos yl diacylglycerols) and PC Ophosphatidyl choline) and less clearly in MGDG (monogalactosyl diacylglycerols), PE (phosphatidyl ethanolamine) and PG (phosphatidyl glycerols). The contents of chlorophylls, DGDG an d PC increased during autumn and early winter during the hardening pro cess 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:DG DG and PE:PC varied throughout the year, being lowest in winter. In ad dition, fatty acids of individual lipid classes showed seasonal fluctu ation. In both MGDG and DGDG, the proportion of linolenic acid was hig her in summer than in winter and that of linoleic acid was vice versa. In both PC and PE, the proportion of palmitic acid was highest in sum mer and lowest in winter and, particularly in PC, this variation was c ompensated for by changes in linoleic acid and less clearly in linolen ic acid, and, in PE, by the long-chain behenic acid. In PG, the propor tions of trans-16:1 and oleic acid were higher in summer than in winte r, 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. (C) 1997 Pub lished by Elsevier Science Ltd.