Photosynthetic down-regulation over long-term CO enrichment in leaves of sour orange (Citrus aurantium) trees.

•  Understanding how trees are affected by a long-term increase in atmospheric CO 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 CO atmosphere and N-replete soil for 14 yr to determine whether photosynthetic down-regulation had occurred. •  Photosynthesis, A : C gas exchange relationships and Rubisco activity and content were measured throughout the 14th year of the experiment. The CO -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 CO enrichment indicating that down-regulation had occurred. A high correlation between the CO -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 CO enrichment can result in photosynthetic down-regulation in leaves of trees, even under nonlimiting N conditions.