Effects of hardening and freezing stress on membrane lipids and CO 2 fixation of Ceratodon purpureus protonemata

Membrane lipids and steady‐state CO 2 fixation rates were studied in moss protonemata in order to evaluate separately the effects of growth temperature, freezing stress and the achievement of frost hardiness. Protonemata of Ceratodon purpureus (Hedw.) Brid, were grown at 20 and 4°C and parts of both materials were then hardened. The low growth temperature increased the content and unsaturation level of membrane lipids significantly. This did not, however, cause a noticeable increase in the frost hardiness. Nor was the achievement of frost hardiness in this material accompanied by further changes in the amount or unsaturtion level of any membrane lipid class. Cytoplasmic membranes were abundant in both unhardened and hardened materials grown at 4°C, which agreed with the high phospholipid content of these protonemata. The only significant difference in membrane lipids between unhardened and hardened materials was a 50% lower level of trans 16:1 fatty acid in the phosphatidylglycerol fraction of hardened protonemata. In hardened protonemata monogalactosyldiacylglycerol (MGDG) was the membrane lipid most liable to decrease during the freeze‐thaw cycle. The loss of MGDG was accompanied by partial inhibition of CO 2 fixation. Provided the content of phospholipids remained unchanged (freeze‐thaw cycle with – 10°C in hardened protonemata), this inhibition was mostly reversible. If loss of the phospholipids also had occurred during the freeze‐thaw cycle, as was the case in unhardened material at or below ‐10°C, CO 2 fixation was severely and nearly irreversibly inhibited after thawing.