ACCLIMATION OF PHOTOSYNTHESIS TO INCREASING ATMOSPHERIC CO2 - THE GAS-EXCHANGE PERSPECTIVE

The nature of photosynthetic acclimation to elevated CO2 is evaluated from the results of over 40 studies focusing on the effect of long-term CO2 enrichment on the short-term response of photosynthesis to intercellular CO2 (the A/C-i response). The effect of CO2 enrichment on the A/C-i response was dependent on growth conditions, with plants grown in small pots (<5 L) or low nutrients usually exhibiting a reduction of A at a given C-i, while plants grown without nutrient deficiency in large pots or in the field tended to exhibit either little reduction or an enhancement of A at a given C-i following a doubling or tripling of atmospheric CO2 during growth. Using theoretical interpretations of A/C-i curves to assess acclimation, it was found that when pot size or nutrient deficiency was not a factor, changes in the shape of A/C-i curves which are indicative of a reallocation of resources within the photosynthetic apparatus typically were not observed. Long-term CO2 enrichment usually had little effect or increased the value of A at all C-i. However, a minority of species grown at elevated CO2 exhibited gas exchange responses indicative of a reduced amount of Rubisco and an enhanced capacity to metabolize photosynthetic products. This type of response was considered beneficial because it enhanced both photosynthetic capacity at high CO2 and reduced resource investment in excessive Rubisco capacity. The ratio of intercellular to ambient CO2 (the C-i/C-a ratio) was used to evaluate stomatal acclimation. Except under water and humidity stress, C-i/C-a, exhibited no consistent change in a variety of C-3 species, indicating no stomatal acclimation. Under drought or humidity stress, C-i/C-a declined in high-CO2 grown plants, indicating stomata will become more conservative during stress episodes in future high CO2 environments.