The unusual tolerance to heat stress of STR7, an atrazine-resistant mutant isolated from photosynthetic cell-suspension cultures of soybean (Glycine max L. Merr. cv. Corsoy) and characterized previously [M. Alfonso et al. (1996) Plant Physiol 112:1499-1508] has been studied. The STR7 mutant maintained normal growth and fluorescence parameters at higher temperatures than the wild type (WT). The temperature for 50% inactivation of the oxygen-evolving activity of STR7 thylakoids was 13 degrees C higher than in the WT. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analysis with specific antibodies revealed that the integrity of photosystem II in the STR7 mutant was maintained at higher temperatures than in the WT. This unusual intrinsic tolerance to high temperatures contrasted with the higher sensitivity to heat stress reported as a feature linked to the triazine-resistance trait. The chloroplast membrane of STR7 accumulated an unusually high content of saturated C16:0 and reduced levels of C16:1 and C18:3 unsaturated fatty acids compared with the WT. Among all the lipid classes, chloroplastic lipids synthesized via the prokaryotic pathway (mono-galactosyl-diacyl-glycerol, phosphatidylglycerol and di-galactosyl-diacyl-glycerol), which represented more than 75% of the total lipid classes, showed the most substantial differences in C16:0 and C18:3 levels. In addition, changes in the physicochemical properties of the thylakoid membrane and chloroplast ultrastructure were also detected.