Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2003
Publication Date: 5/15/2003
Citation: Singsaas, E.L., Ort, D.R., Delucia, E.H. 2003. Elevated CO2 effects on mesophyll conductance and its consequences for interpreting photosynthetic physiology. Plant Cell and Environment. 27:41-50. Interpretive Summary: Carbon dioxide in the atmosphere is rising globally by 0.4 percent per year. A rate of increase that is expected to accelerate. By 2050, concentrations will be 50 percent higher than today. Because C3 photosynthesis is limited by the current low level of CO2, this increase is widely anticipated to stimulate photosynthesis, plant, and biomass production. However, many crop and native species show only a transient stimulation to carbon dioxide fertilization thereafter reverting back to pre-fertilization growth and production performance. This reversion is often referred to as carbon dioxide acclimation and there is broad interest to understand its biochemical and genetic bases. In this study we explored the effect that elevated growth CO2 has on leaf morphology as a potential contributor to CO2 acclimation. Working in the Free Air Carbon Enrichment facility in the Duke forest we measured the effect of CO2 enrichment on the movement of this gas within leaves of several different woody species. We discovered the effects of CO2 enrichment on conductance within leaves was large, the size and even the direction of the effect was species dependent. We also discovered that internal CO2 conductance always co-varied with photosynthetic capacity in a fashion that was not species dependent. These results will be of interest to agricultural scientists and environmentalists interested in growth at elevated CO2 and provides an important data base for extending these measurements into crop species.
Technical Abstract: Photosynthesis models predict that plants grown at elevated pCO2 will show changes in the relationship between RuBP carboxylation and regeneration. Lack of evidence for this may result from reduced conductance of CO2 from the intercellular airspaces to the site of carboxylation (mesophyll conductance; gm). The fundamental importance of gm to this issue led us to investigate the relationship between growth pCO2 and gm. No theory allows the prediction of mesophyll conductance. Thus, we measured gm in five species grown in CO2 elevation experiments. Mesophyll conductance increased linearly with photosynthetic capacity, and there was no CO2 effect on gm separate from the effects on photosynthesis. Including the effects of mesophyll conductance in gas-exchange changed the relationship between RuBP carboxylation and regeneration capacities, and improved the agreement between gas-exchange and biochemical measurements of carboxylation activities. Because this change was proportional to photosynthetic acclimation, changes in diffusion did not affect the relationship between pCO2 in the leaf airspaces and at the site of carboxylation. Thus changes in gm cannot affect the balance of photosynthetic processes as suggested by the models. Thus, considering gm in photosynthesis models improves the accuracy of gas-exchange measurements, but cannot help explain the differences between modeled and measured photosynthesis responses to elevated pCO2.