JXB Advance Access originally published online on October 29, 2003
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Journal of Experimental Botany, Vol. 54, No. 393, pp. 2769-2774,
December 1, 2003
© 2003 Oxford University Press
Variation in acclimation of photosynthesis in Trifolium repens after eight years of exposure to Free Air CO2 Enrichment (FACE)
Received 10 July 2003; Accepted 30 July 2003
1 Departments of Crop Sciences and of Plant Biology, University of Illinois, Urbana-Champaign, 379 Edward R. Madigan Laboratory, 1201 W. Gregory Drive, Urbana, IL 61801, USA
2 Department of Environmental Sciences, Brookhaven National Laboratory, Upton, NY 11973-5000, USA
3 Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zürich, Switerland
* To whom correspondence should be addressed. Fax: +1 217 244 7563. E-mail: stevel{at}life.uiuc.edu
Abbreviations: Asat, light-saturated rate of leaf CO2 assimilation at growth [CO2]; ca, atmospheric CO2 concentration; ci, intercellular CO2 concentration; FACE, Free Air CO2 Enrichment; gs, stomatal conductance to water vapour; Jmax, in vivo maximum rate of electron transport; Vc,max, in vivo maximum rate of carboxylation.
The initial stimulation of photosynthesis observed on elevation of [CO2] in grasslands has been predicted to be a transient phenomenon constrained by the loss of photosynthetic capacity due to other limitations, notably nutrients and sinks for carbohydrates. Legumes might be expected partially to escape these feedbacks through symbiotic N2 fixation. The Free-Air Carbon dioxide Enrichment (FACE) experiment at Eschikon, Switzerland, has been the longest running investigation of the effects of open-air elevation of [CO2] on vegetation. The prediction of a long-term loss of photosynthetic capacity was tested by analysing photosynthesis in Trifolium repens L. (cv. Milkanova) in the spring and autumn of the eighth, ninth and tenth years of treatment. A high and low N treatment also allowed a test of the significance of exogenous N-supply in maintaining a stimulation of photosynthetic capacity in the long-term. Prior work in this Free Air CO2 Enrichment (FACE) experiment has revealed that elevated [CO2] increased both vegetative and reproductive growth of T. repens independent of N treatment. It is shown here that the photosynthetic response of T. repens was also independent of N fertilization under both current ambient and elevated (600 µmol mol–1) [CO2]. There was a strong effect of season on photosynthesis, with light-saturated rates (Asat) 37% higher in spring than in autumn. Higher Asat in the spring was supported by higher maximum Rubisco carboxylation rates (Vc,max) and maximum rates of electron transport (Jmax) contributing to RuBP regeneration. Elevated [CO2] increased Asat by 37% when averaged across all measurement periods and both N fertilization levels, and decreased stomatal conductance by 25%. In spring, there was no effect of elevated [CO2] on photosynthetic capacity of leaves, but in autumn both Vc,max and Jmax were reduced by approximately 20% in elevated [CO2]. The results show that acclimation of photosynthetic capacity can occur in a nitrogen-fixing species, in the field where there are no artificial restrictions on sink capacity. However, even with acclimation there was a highly significant increase in photosynthesis at elevated [CO2].
Key words: Acclimation, gas exchange, global atmospheric change, global climate change, grassland, nitrogen fixation, photosynthetic electron transport, Rubisco, white clover.