Partitioning of respiration between the cytochrome pathway (CP) and the alternative pathway (AP) may play an important role in plant adaptation to extreme environments. We examined changes in partitioning between CP and AP, and viability of roots associated with plant exposure to high soil temperature for two Agrostis species: Agrostis scabra Willd., a species adapted to high-temperature soils in geothermal areas in Yellowstone National Park, and Agrostis stolonifera L. (cv. Penncross) a heat-sensitive grass widely used in cool-climate regions. Roots of A. scabra and A. stolonifera were exposed to soil temperature of 37 or 20 degrees C, while shoots were exposed to 20 degrees C for 28 days. Root viability decreased, and total root respiration increased for both species at 37 degrees C. The decline in root viability and the increase in respiration rates were less pronounced for A. scabra than for A. stolonifera. A larger proportion of total root respiration was attributed to the AP in A. scabra compared with that in A. stolonifera when both species were exposed to 37 degrees C. At 7 and 14 days at 37 degrees C, the relative proportion of respiration passing through AP increased by 12 and 10%, respectively, in A. scabra, whereas in A. stolonifera, AP increased by 4 and 1%, respectively. Our results suggest thatmaintaining a higher proportion of AP at a high soil temperature may contribute to root thermo-tolerance in A. scabra in comparison with A. stolonifera, and alternative respiration may play an important role in plant adaptation to high soil temperature.