Abstract
The temperature boundary for phase separation of membrane lipids extracted from Nerium oleander leaves was determined by analysis of spin label motion using electron spin resonance spectroscopy and by analysis of polarization of fluorescence from the probe, trans-parinaric acid. A discontinuity of the temperature coefficient for spin label motion, and for trans-parinaric acid fluorescence was detected at 7 degrees C and -3 degrees C with membrane lipids from plants grown at 45 degrees C/32 degrees C (day/night) and 20 degrees C/15 degrees C, respectively. This change was associated with a sharp increase in the polarization of fluorescence from trans-parinaric acid indicating that significant domains of solid lipid form below 7 degrees C or -3 degrees C in these preparations but not above these temperatures. In addition, spin label motion indicated that the lipids of plants grown at low temperatures are more fluid than those of plants grown at higher temperatures.A change in the molecular ordering of lipids was also detected by analysis of the separation of the hyperfine extrema of electron spin resonance spectra. This occurred at 2 degrees C and 33 degrees C with lipids from the high and low temperature grown plants, respectively. According to previous interpretation of spin label data the change at 29 degrees C (or 33 degrees C) would have indicated the temperature for the initiation of the phase separation process, and the change at 7 degrees C (or -3 degrees C) its completion. Because of the present results, however, this interpretation needs to be modified.Differences in the physical properties of membrane lipids of plants grown at the hot or cool temperatures correlate with differences in the physiological characteristics of plants and with changes in the fatty acid composition of the corresponding membrane lipids. Environmentally induced modification of membrane lipids could thus account, in part, for the apparently beneficial adjustments of physiological properties of this plant when grown in these regimes.