Arrhenius plots and the involvement of thermotropic phase transitions of the thylakoid membrane in chilling impairment of photosynthesis in thermophilic higher plants

Abstract Breaks and discontinuities in Arrhenius plots of physiological and physical properties of thylakoids are not diagnostic of thermotropic lipid phase transitions of the membrane. Bulk lipid transitions, as first inferred by the membrane phase transition hypothesis, do not occur in any higher plant at chilling temperatures. Solidification of some varying, but always minor, fraction of the total membrane lipid does take place. However, the presence of minor domains of solid thylakoid membrane lipid at chilling temperatures is not unique to chilling sensitive plants but is also found in tolerant species. Minor solidification may in some plants, or groups of plants, be controlled by the specific molecular species of phosphatidylglycerol only recently investigated. In plants containing little, or no, phosphatidylglycerol with this positional distribution of fatty acids, other yet unknown constituents of the membrane must fill a similar function, since DSC thermograms indicate minor solidification also in isolated, unperturbed thylakoids from chilling tolerant species. However, chilling induced phase transitions, or other perturbations, of the thylakoid membrane are not the reason for the chilling lability of net photosynthesis in the intact plant. This conclusion follows from detailed comparison between photosynthetic membranes isolated from prechilled plants and the effects of chilling exposure on CO₂ fixation of the whole plant. Damage at the level of the thylakoid membrane does occur, although not to the extent where it can account for the proportionally much larger damage to CO₂ fixation.