Submitted to: Annual Reviews of Plant Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2004
Publication Date: 6/1/2004
Citation: Long, S.P., Ainsworth, E.A., Rogers, A., Ort, D.R. 2004. Rising atmospheric carbon dioxide: plants face the future. Annual Reviews of Plant Biology. 55:591-628. Interpretive Summary: Atmospheric carbon dioxide atmosphere is on the rise. Since 1900 it has risen 20% and this geologically unprecedented pace will continue resulting in an atmosphere in 2050 containing 50% more CO2 than it did a century ago. Although this enrichment of the atmosphere with CO2 carries certain hazards such as global warming, it also offers the potential for increased crop production as increased carbon dioxide stimulates photosynthesis in many plants. However not in all crops, including corn the third most important food crop globally, is increasing CO2 predicted to stimulate photosynthesis. In all we know too little to allow accurate predictions of future crop production or allow varietal selection for improved productivity in a changing climate. Using technology that permits the atmosphere of 2050 to be mimicked under natural field conditions there is the opportunity to go beyond what has been learned from enclosure studies and discover how plants in field really respond to CO2 fertilization. This information is essential in order to accurately predict how agricultural crops and natural plant communities will actually respond to atmospheric change.
Technical Abstract: Atmospheric CO2 concentration is now higher than at any time in the past 26 million years and is expected to nearly double over this century. C3 land plants respond to an instantaneous increase in CO2 via increased net photosynthesis, due to inhibition of the oxygenation reaction and increased carboxylation at Rubisco, and decreased transpiration, due to decrease in stomatal conductance. In the longer term this increase is often offset by down-regulation of photosynthetic capacity, including loss of Rubisco activity associated with a decline in leaf N content. Understanding of plant response to this increase comes predominantly from chamber and greenhouse (enclosure) studies; this has constrained the species and life-cycle stages that may be examined, while enclosures may suffer a number of potential artifacts that may modify the response. Free-Air Concentration Enrichment (FACE) was developed as a means to grow plants in the field at controlled elevation of CO2 under fully open air conditions. The motivations for developing FACE and its operation are explained. FACE experiments have operated at 12 sites and examined over 40 species from arable crops and weeds to long-lived trees and desert perennials over the past 15 years. The findings of these experiments in 97 peer reviewed publications are quantitatively summarized via meta-analytic statistics, and compared to findings from chamber studies. While trends agree with parallel summaries of enclosure studies, important quantitative differences emerge. Averaged across these studies light-saturated C3 photosynthesis is increased by 34% and production by 20%, somewhat less than expected from enclosure studies. In common with many enclosure studies, Rubisco content was decreased by about 20%, but in contrast there was little change in capacity for Ribulose-1:5 bisphosphate regeneration. The loss of Rubisco appears to account for most of the N decrease per unit leaf area. The large loss of Rubisco had little affect on photosynthesis in elevated [CO2] implying an acclamatory change benefiting nitrogen use efficiency, rather than down-regulation. Also in contrast to chamber studies, there was no significant effect on leaf area index. The findings have important implications both for predicting the future terrestrial biosphere and understanding how crops may need to be adapted to the changed and changing atmosphere.