Photosynthetic down‐regulation over long‐term CO2 enrichment in leaves of sour orange (Citrus aurantium) trees

• Understanding how trees are affected by a long-term increase in atmospheric CO2 is crucial to understanding the future impact of global climate change. Measurements of photosynthetic characteristics were made in sour orange trees (Citrus aurantium) growing under an enhanced CO2 atmosphere and N-replete soil for 14 yr to determine whether photosynthetic down-regulation had occurred. • Photosynthesis, A : Ci gas exchange relationships and Rubisco activity and content were measured throughout the 14th year of the experiment. The CO2-induced enhancement ratio of photosynthesis was calculated and compared with estimates of the enhancement of cumulative wood biomass production. • Content of the large subunit of Rubisco was significantly reduced by CO2 enrichment indicating that down-regulation had occurred. A high correlation between the CO2-induced enhancement of photosynthesis and the enhancement of cumulative wood biomass production suggested that the decline in wood biomass production was closely related to the decline in photosynthesis. • These results indicate that long-term CO2 enrichment can result in photosynthetic down-regulation in leaves of trees, even under nonlimiting N conditions.