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We present evidence that plant growth at elevated atmospheric CO2 increases the high-temperature tolerance of photosynthesis in a wide variety of plant species under both greenhouse and field conditions. We grew plants at ambient CO2 (similar to 360 mu mol mol(-1)) and elevated CO2 (550-1000 mu mol mol(-1)) in three separate growth facilities, including the Nevada Desert Free-Air Carbon Dioxide Enrichment (FACE) facility. Excised leaves from both the ambient and elevated CO2 treatments were exposed to temperatures ranging from 28 to 48 degrees C. In more than half the species examined (4 of 7, 3 of 5, and 3 of 5 species in the three facilities), leaves from elevated CO2-grown plants maintained PSII efficiency (F-v/F-m) to significantly higher temperatures than ambient-grown leaves. This enhanced PSII thermotolerance was found in both woody and herbaceous species and in both monocots and dicots, Detailed experiments conducted with Cucumis sativus showed that the greater F-v/F-m in elevated versus ambient CO2-grown leaves following heat stress was due to both a higher F-m and a lower F-o, and that F-v/F-m differences between elevated and ambient CO2-grown leaves persisted for at least 20 h following heat shock. Cucumis sativus leaves from elevated CO2-grown plants had a critical temperature for the rapid rise in F-o that averaged 2.9 degrees C higher than leaves from ambient CO2-grown plants, and maintained a higher maximal rate of net CO2 assimilation following heat shock. Given that photosynthesis is considered to be the physiological process most sensitive to high-temperature damage and that rising atmospheric CO2 content will drive temperature increases in many already stressful environments, this CO2-induced increase in plant high-temperature tolerance may have a substantial impact on both the productivity and distribution of many plant species in the 21st century.