Electron transport, using succinate as a substrate, was measured polarographically in mitochondria isolated from Phaseolus vulgaris and P. acutifolius plants at 25 degrees C and 32 degrees C. Mitochondria isolated from P. vulgaris plants grown at 32 degrees C had reduced electron transport and were substantially uncoupled. Growth at 32 degrees C had no effect on electron transport or oxidative phosphorylation in P. acutifolius compared to 25 degrees C grown plants. Mitochondria isolated from 25 degrees C grown P. vulgaris plants measured at 42 degrees C were completely uncoupled. Similarly treated P. acutifolius mitochondria remained coupled. The uncoupling of P. vulgaris was due to increased proton permeability of inner mitochondrial membrane. The alternative pathway was more sensitive to heat than the regular cytochrome pathway. At 42 degrees C, no alternative pathway activity was detected. The substantially greater heat tolerance of P. acutifollus compared to P. vulgaris mitochondrial electron transport suggests that mitochondrial sensitivity to elevated temperatures is a major limitation to growth of P. vulgaris at high temperatures and is an important characteristic conveying tolerance in P. acutifolius.