Growth in elevated CO2 protects photosynthesis against high-temperature damage
We present evidence that plant growth at elevated atmospheric CO 2 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 CO 2 (- 360 pmol mol -1 ) and elevated CO 2 (550-1000 μmol 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 CO 2 treatments were exposed to temperatures ranging from 28 to 48 °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 CO 2 -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 CO 2 -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 CO 2 -grown leaves persisted for at least 20 h following heat shock. Cucumis sativus leaves from elevated CO 2 -grown plants had a critical temperature for the rapid rise in F o that averaged 2.9 °C higher than leaves from ambient CO 2 -grown plants, and maintained a higher maximal rate of net CO 2 assimilation following heat shock. Given that photosynthesis is considered to be the physiological process most sensitive to high-temperature damage and that rising atmospheric CO 2 content will drive temperature increases in many already stressful environments, this CO 2 -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.