We present evidence that plant growth at elevated atmospheric CO2 increases
the high-temperature tolerance of photosynthesis in a wide variety of plan
t species under both greenhouse and field conditions. We grew plants at amb
ient 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 F
ree-Air Carbon Dioxide Enrichment (FACE) facility. Excised leaves from both
the ambient and elevated CO2 treatments were exposed to temperatures rangi
ng 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 high
er temperatures than ambient-grown leaves. This enhanced PSII thermotoleran
ce 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 diff
erences between elevated and ambient CO2-grown leaves persisted for at leas
t 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 maintai
ned a higher maximal rate of net CO2 assimilation following heat shock. Giv
en that photosynthesis is considered to be the physiological process most s
ensitive to high-temperature damage and that rising atmospheric CO2 content
will drive temperature increases in many already stressful environments, t
his CO2-induced increase in plant high-temperature tolerance may have a sub
stantial impact on both the productivity and distribution of many plant spe
cies in the 21st century.