Acclimation of photosynthesis to increasing atmospheric COz: The gas exchange perspective

The nature of photosynthetic acclimation to elevated CO 2 is evaluated from the results of over 40 studies focusing on the effect of long-term CO 2 enrichment on the short-term response of photosynthesis to intercellular CO 2 (the A/C i response). The effect of CO 2 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 CO 2 during growth. Using theoretical interpretations of A/Ci 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 CO 2 enrichment usually had little effect or increased the value of A at all C i. However, a minority of species grown at elevated CO 2 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 CO 2 and reduced resource investment in excessive Rubisco capacity. The ratio of intercellular to ambient CO2 (the Ci/C a ratio)was used to evaluate stomatal acclimation. Except under water and humidity stress, C~/C a exhibited no consistent change in a variety of C 3 species, indicating no stomatal acclimation. Under drought or humidity stress, Ci/C a declined in high-CO 2 grown plants, indicating stomata will become more conservative during stress episodes in future high CO 2 environments.