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Much discussion has been had about the affects of climate change on plantlife, but how will rising carbon dioxide concentrations affect the photosynthetic process itself? Since CO2 is a reagent in photosynthesis, would we expect higher CO2 to mean an increased rate of photosynthesis in a real-world context? Has there been any research on this?

I am thinking more of large-scale field tests rather than lab plants.

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I presume you've already seen this and this, and need a bit more? –  user132 Jan 16 '12 at 14:10
    
Its a good starter, although adding co2 to a plant in a box may not be particularly illuminating. Horticulturalists have added co2 to tomatoes in greenhouses for a long time. I was thinking of something more sophisticated. I will edit my question. –  Poshpaws Jan 16 '12 at 14:44

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There are several key ways in which rising atmospheric CO2 concentrations will affect photosynthesis, and these are related to the different types of photosynthesis. In order to properly answer your question, I'll provide some background about photosynthesis itself.

Photosynthesis evolved in a high-CO2 atmosphere, before the oxygen-enrichment of the atmosphere (which actually happened as a result of photosynthesis). Most plant species operate C3 photosynthesis. In these plants, carbon dioxide diffuses into the cell where it is fixed by Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) into a 3-carbon molecule (hence C3), which is then polymerised to make sugars. A crucial fact about RuBisCO is that it has both carboxylase (carbon-fixing) activity and oxygenase (oxygen-fixing) activity. This means that oxygen and carbon dioxide compete for the active site on the enzyme complex, leading to RuBisCO being quite inefficient and slow at fixing carbon in higher oxygen concentrations. That didn't matter in the high-CO2 atmosphere of the early Earth, but in todays atmosphere O2 concentrations are high enough that they severely limit the productivity of C3 plants.

However, plants haven't just been growing slowly all that time - several mechanisms for increasing photosynthetic efficiency have evolved. The most influential systems involve concentrating carbon dioxide in a particular area, excluding oxygen, and concentrating RuBisCO in that same area. This avoids the oxygen competition for the active site and allows RuBisCO to operate more efficiently. The key adaptation here is C4 photosynthesis - the system which is present in most grasses and many of the most productive plants on Earth (e.g. maize, sugarcane, Miscanthus). It has evolved at least 62 times independently. It works by having RuBisCO concentrated within 'bundle sheath' cells which are surrounded by a layer of suberin wax. This layer prevents CO2 escaping and O2 from getting in. CO2 from the atmosphere is then fixed in different cells - 'mesophyll cells' - by another enzyme - Phosphoenolpyruvate carboxylase (PEPC), resulting in a four-carbon molecule (hence C4). This 4-carbon acid, (malate or oxaloacetate depending on the system) is then shuttled into the bundle sheath cells. There, the CO2 is released again by a variety of enzymes depending on the system, creating a high CO2 concentration in the cell where RuBisCO can then work efficiently.

In general, C4 plants are much (about 50%) more efficient than their C3 counterparts, and they are particularly well adapted to high temperatures and moist environments. So, to answer your first question: as atmospheric CO2 levels continue to rise, C3 plants will gradually be able to photosynthesise more efficiently. Interestingly though, C4 plants are predicted to also benefit from increased atmospheric CO2. If global temperatures rise as predicted, both C3 and C4 plants will be able to operate more efficiently than they currently do, up to a maximum temperature beyond which enzymes will begin to denature faster and efficiency will drop. One consideration is that the difference in efficiency between C3 and C4 systems will decrease, which may significantly alter the makeup of plant communities around the world.

This is a vast oversimplification, but it is accurate for the predicted overall effects. Localised effects (i.e. productivity changes in a particular region or for a particular crop) will depend on habitat, physiology, etc.

Some key papers to launch you into the literature:

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Thanks for your answer @Richard Smith. Very interesting. I was also wondering whether many plants are nutrient limited or not - i.e., even though CO2 increases, their growth may be deteremined by the availability of N, P, K etc...? –  Poshpaws Feb 1 '12 at 9:07
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@Poshpaws Good question. It's quite a fundamental one, and important, so it deserves to be posted as a separate question - I'll ask and answer it and post the link back here. –  Richard Smith Feb 1 '12 at 17:07
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In temperate and cold climate C3 plants do better than C4, becouse they do not have to invest in additional enzymes when photorespiration effect is neglible. So temperature increase would promote C4 plants. –  Marta Cz-C Feb 1 '12 at 20:10
    
@MartaCz-C yes that's true, good point. –  Richard Smith Feb 6 '12 at 21:06
    
Could you also accout for cell breath in your answer? That is how oxygen deficit affects plants? Also I wonder why researches into plant survival in Martian-type atmosphere show that photosynthesis is more efficient at low pressure CO2-only atmosphere rather than at normal pressure. –  Anixx Mar 17 '12 at 18:20

I wanted to add a little more to the excellent answer above, especially since the OP asks about research into this question in a "real-world context".

There is a substantial body of evidence on exactly this question that comes from experiments at "Free Air CO2 Enrichment" (FACE) sites. FACE is an experimental method/technology in which standing ecosystems undergo CO2 enrichment without (much) disturbance to the ecosystem. It has been known that higher CO2 increases plant growth since the 1960s or so, but the motivation for FACE was to understand what the long-term and ecosystem scale effects of rising CO2 in the atmosphere would be. Many different ecosystem types (forests, crops, shrublands, etc) have been studied with this technique to date, some for fairly long periods of time. I think that many of these sites are now being shut down.

Some key findings:

  • Net primary production (Plant C assimilation - Respiration) generally increased for plant species, but increases in productivity varied a lot between ecosystem types.
  • This increase in productivity diminished over time, an effect that was largely mediated by changes in plant N availability.
  • Different plant functional types show different responses. For example, herbaceous species saw less enhancement of assimilation (due to a decrease in leaf N) compared to woody plants at some sites.
  • At the ecosystem level, primary production is often limited by factors other than available CO2 - nitrogen or water for example.

There are a couple of excellent reviews available:

  • Nowak, R. S., Ellsworth, D. S. and Smith, S. D. (2004), Functional responses of plants to elevated atmospheric CO2– do photosynthetic and productivity data from FACE experiments support early predictions?. New Phytologist, 162: 253–280. doi: 10.1111/j.1469-8137.2004.01033.x pdf

  • Norby, R. J. and Zak, D. R. (2011), Ecological Lessons from Free-Air CO2 Enrichment (FACE) Experiments. Annual Review of Ecology, Evolution, and Systematics, 42: 181-203. doi: 10.1146/annurev-ecolsys-102209-144647 link.

This article discusses the importance of nitrogen in limiting forest productivity under high CO2:

  • Norby, R.J., Warren, J. M., Iversen, C. M., Medlyn, B. E., and McMurtrie, R. E. (2010), CO2 enhancement of forest productivity constrained by limited nitrogen availability. PNAS, 107 (45) 19368-19373. doi:10.1073/pnas.1006463107 link
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+1 the network of FACE sites answers this exact question comprehensively. Still, providing referenced in std format rather than links would make the answer more useful. –  David Apr 8 '12 at 2:16

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