Photosynthesis, Epicuticular Wax and Lipid Changes In Cowpea Cultivars Grown Under Hyperthermic Conditions

Abstract

The cellular mechanisms of long-term thermoadaptation and acquisition of thermotolerance in plants have not yet been explored. Characteristic changes in membrane lipid are associated with exposure to elevated temperatures, such as reduced C_18:3, and increased C_18:2 and C_16:0, i.e. an overall reduction in unsaturation (Pearcy, 1979; Raison et al., 1982). Using mutants of Arabidopsis thaliana (a “C_16:3” plant) deficient in various aspects of lipid metabolism, Somerville and his associates have associated specific shifts in lipid composition with thermotolerance (Somerville and Browse, 1991). With a mutant deficient in n-6 desaturase activity, which accumulated C_18:1 and C_16:1 in acyl lipids, Hughly et al. (1989) showed that lipid unsaturation directly affects the thermal stability of photosynthetic membranes. Enhanced thermotolerance was also exhibited in a mutant deficient in C_16:0 unsaturation (Kunst et al., 1989b). Results of studies where the membrane lipids were catalytically hydrogenated led Thomas et al. (1986) to suggest that decreased unsaturation raises the temperature at which nonbilayer forming lipids (MGDG) phase separate to non-bilayer structures which disrupt the thylakoid structure. Mutants deficient in plastid glycerol-3-phosphate acyl-transferase activity show a 15–20% reduction in phosphatidyl glycerol (PG) and minor alterations in other complex lipids (Kunst et al., 1988), and exhibit “C_18:3” lipid metabolism, grew slightly more rapidly at elevated temperatures (Kunst et al., 1989c).