Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/8/2003
Publication Date: 8/15/2003
Citation: BERNACCHI, C.J., CALFAPIETRA, C., DAVEY, P.A., WITTIG, V.E., SCARASCLE-MUGNOZZA, G.E., RAINES, C.A., LONG, S.P. 2003. Photosynthesis and stomatal conductance responses of poplars to free-air CO2 enrichment (PopFACE) during the first growth cycle and immediately following coppice. New Phytologist.
Interpretive Summary: We measured the rates of carbon sequestration through the process of photosynthesis and water loss through the leaf for three species of poplar trees over a period of three years. Half of the trees measured were exposed to concentrations of atmospheric carbon dioxide expected for fifty years from present. We found that these trees exhibited much higher rates of carbon assimilation with growth in elevated CO2, suggesting that future environments will lead to higher rates of growth for these tree species. However, we also noticed that two of the three species showed lower activities of the enzymes used to drive photosynthesis, suggesting that these trees will not take full advantage of the higher concentrations of carbon dioxide expected in the future. In addition to the full three year experiment, photosynthesis was measured during regrowth after the trees were harvested for biomass energy. We found that rates of photosynthesis were much lower during regrowth than was observed during the initial growth phase such that no differences between the control and elevated carbon dioxide treatments were noticed. Elevated CO2 did not appear to influence the amount of water lost from the leaves at any point of the experiment.
Technical Abstract: Photosynthesis and stomatal conductance were measured on Populus alba L.,Populus nigra L. and Populus x euramericana Dode (Guinier) grown underelevated atmospheric CO2 concentrations ([CO2]) of 550 umol mol-1 for onecomplete coppice cycle (3 years from planting to harvest) and for re-growth immediately after the first coppice at the Poplar Free Air CO2 Enrichment(PopFACE) facility. Diurnal measurements of photosynthesis were made in situ and in parallel in vivo maximum capacity for carboxylation (Vc,max) and maximum rates of electron transport (Jmax) were determined by gas exchange measurement. Elevated [CO2] increased light saturated photosynthesis (Asat)at 25°C by 37%, 40%, and 31% for P. alba, P. nigra and P. x euramericana, respectively. Over the diurnal course daily integrated photosynthesis (A')increased, on average, 56% across all three species. Elevated [CO2]decreased Vc,max and Jmax for P. nigra and Jmax for P. x euramericana. There was no effect of elevated [CO2] on stomatal conductance in any of the species throughout the first growth cycle. During post-coppice re-growth, elevated [CO2] did not increase Asat in P. nigra and P. x euramericana due to large decreases in Vc,max and Jmax. A 50% increase in [CO2] under these open air field conditions resulted in a large and sustained increase in Asat. Although there were some differences between the species, these had little effect on the photosynthetic rates at the growth [CO2]. Nevertheless the results show that even fast growing trees grown without rooting volume restriction in the open may still show some down-regulation of photosynthetic potential at elevated [CO2].