FROST HARDENING AND PHOTOSYNTHETIC PERFORMANCE OF SCOTS PINE (PINUS-SYLVESTRIS L.) - II - SEASONAL-CHANGES IN THE FLUIDITY OF THYLAKOID MEMBRANES

The fluidity of chloroplast thylakoid membranes of frost-tolerant and frost-sensitive needles of three-to four-year-old Scots pine (Pinus sy lvestris L.) trees, of liposomes produced from the lipids of the thyla koids of these needles, and of liposomes containing varying amounts of light-harvesting complex (LHC) II protein was investigated by means o f electron paramagnetic resonance (EPR) measurements using spin-labell ed fatty acids as probes. Broadening of the EPR-resonance signals of 1 6-doxyl stearic acid in chloroplast membranes of frost-sensitive needl es and changes in the amplitudes of the peaks were observed upon a dec rease in temperature from + 30 degrees C to - 10 degrees C, indicating a drastic loss in rotational mobility. The lipid molecules of the thy lakoid membranes of frost-tolerant needles exhibited greater mobility. Moderate frost resistance could be induced in Scots pine needles by s hort-day treatment (Vogg et al., 1997, Planta, this issue), and growth of the trees under short-day illumination (9 h) resulted in a higher mobility of the chloroplast membrane lipids than did growth under long -day conditions (16 h). The EPR spectrum of thylakoids from frost-tole rant needles at - 10 degrees C was typical of a spin label in highly f luid surroundings. However, an additional peak in the low-field range appeared in the subzero temperature range for the chloroplast membrane s of frost-sensitive needles, which represents spin-label molecules in a motionally restricted surrounding. The EPR spectra of thylakoids an d of liposomes of thylakoid lipids from frost-hardy needles were ident ical at + 30 degrees C and - 10 degrees C. The corresponding spectra f rom frost-sensitive plants revealed an additional peak for the thylako ids, but not for the pure liposomes, Hence, the domains with restricte d mobility could be attributed to protein-lipid interactions in the me mbranes. Broadening of the spectrum and the appearance of an additiona l peak was observed with liposomes of pure distearoyl phosphatidyl gly cerol modified to contain increasing amounts of LHC II. These results are discussed with respect to a loss of chlorophyll and chlorophyll-bi nding proteins in thylakoids of Scots pine needles under winter condit ions.