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Title:   Author  item
                                            Ainsworth, Elizabeth

                                                         - *Lisa*item
                                            ROGERS, ALISTAIR - BROOKHAVEN NATIONAL LAB
  Submitted to: Plant Cell and Environment
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 11/28/2006
                                    Publication  Date: 3/1/2007
                                    Citation: Ainsworth, E.A., Rogers, A. 2007. The Response of Photosynthesis and Stomatal Conductance to Rising [CO2]: Molecular Mechanisms and Environmental Interactions. Plant Cell and Environment. 30(3): 258­270.Interpretive Summary: Rising atmospheric carbon dioxide concentration ([CO2]), at levels projected for this century, directly impacts stomatal conductance (gs) and photosynthesis (A).  We first review the molecular and biochemical bases for these direct responses.  Then, we summarize the average response of gs and A to [CO2] levels projected for the middle of this century using results from Free Air CO2 Enrichment (FACE) experiments.  Stomatal conductance decreases on average by 22% at elevated [CO2] (~567 ppm).  This decrease is not caused by a change in stomatal density or independent acclimation of gs to elevated [CO2].  Therefore, the long-term response of gs to elevated [CO2] is likely dominated by the short-term sensitivity of guard cells to [CO2].  In C3 plants, light-saturated photosynthesis (Asat) increases by 31% at elevated [CO2] because ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is not CO2-saturated at current [CO2] (~380 ppm).  Thus, rising [CO2] increases the carboxylation reaction of Rubisco and inhibits the oxygenation reaction.  Environmental conditions impact the magnitude of these general responses.  Drought modifies the response of gs to elevated [CO2], while nitrogen supply and sink capacity modulate the response of A to elevated [CO2].  A greater understanding of both the molecular mechanisms of plant responses to elevated [CO2] and the impact of environmental factors on these mechanisms is important to scientists who are modeling plant and ecosystem productivity and to policy makers who will manage future ecosystems.
                                Technical Abstract: Plants directly sense and respond to elevated atmospheric carbon dioxide concentration ([CO2]) in two ways, decreased stomatal conductance (gs) and increased photosynthesis (A). First, this review summarizes the molecular and biochemical bases for these responses. Second, it examines how downstream processes and environmental constraints modulate these two fundamental responses of plants to elevated [CO2].  The exact signal transduction pathway that causes guard cell closure in response to high [CO2] is not fully understood; however, reduction in gs at elevated [CO2] is a highly conserved response in both C3 and C4 plants.  Plants grown at elevated [CO2] in Free Air CO2 Enrichment (FACE) experiments decreased gs by 22%. The absence of a significant reduction in stomatal density or acclimation of gs to elevated [CO2] suggests that the long-term response of gs to elevated [CO2] is dominated by the short-term sensitivity to CO2.  However, environmental conditions impact the magnitude of reported responses of gs to elevated [CO2].  Even though atmospheric [CO2] has risen 36% in the last 250 years, in C3 plants, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is still substrate limited.  Thus, further increases in atmospheric [CO2] will stimulate the rate of carboxylation.  The continued rise in [CO2] will also inhibit the oxygenation reaction of Rubisco, and decrease the subsequent CO2 loss and energy costs associated with the photorespiratory pathway.  Because of these properties of Rubisco, elevation of [CO2] in FACE experiments, from c. 365 ppm to 567 ppm, stimulated light-saturated photosynthesis (Asat) in C3 plants grown in FACE by an average 31%.  However, the magnitude of the increase in Asat varied with functional group and environment. Functional groups with Rubisco-limited photosynthesis at elevated [CO2] have greater potential for increases in Asat than those where photosynthesis becomes RubP-limited at elevated [CO2].  Both nitrogen supply and sink capacity can modulate the response of A to elevated [CO2] through their impact on acclimation of carboxylation capacity.  Increased understanding of both the molecular mechanisms of plant responses to elevated [CO2], and the impact of environmental factors on these mechanisms will improve our ability to appropriately manage future ecosystems.


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