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:C_i 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 CO ₂-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.