As Carbon Emissions Reach Record Levels, How Will Increased CO2 Affect Photosynthesis in Plants?

In the past 250 years the amount of atmospheric CO2 has increased by over 40% (from 270 to 387 ppm), due mainly to human activities such as deforestation and the burning of fossil fuels.

Carbon dioxide (CO2) emissions are rising faster than the worst-case scenario postulated by the Intergovernmental Panel on Climate Change ( IPCC ). And the rate of CO2 emissions is currently accelerating.

Green plants use CO2 as their primary source of carbon to make carbohydrates (sugars) via photosynthesis.

Have these elevated levels of atmospheric CO2 affected photosynthesis?

How will even higher levels of CO2 (500 to 750 ppm), projected to occur this century, affect plants and plant communities upon which we all ultimately depend for our survival?

What Does Scientific Research Say?

Early research regarding this question supported the idea that higher levels of atmospheric CO2 would increase rates of photosynthesis, thus producing more biomass.

Recent evidence, however, suggests that this may not be the case.

What’s becoming clear is that accurately predicting the outcomes of elevated levels of CO2 on crop plants is very complex. And it’s even more problematic when considering native plant populations.

This is a big old subject, with thousands of pertinent reports published within the past 30 years. But, having been involved in related research over the past decade, I’ll take a stab at presenting “the big picture” in this and subsequent posts.

Gas Valves and Heat

Two important factors confounding attempts to accurately predict the effects of increased CO2 on land plants include (1) the effects of CO2 on stomates and (2) the effects of high temperatures on photosynthesis.

Stomata are the tiny valves, composed of two “guard cells”, usually located mainly on the underside of leaves. The stomates have a tough job. They need to open enough during the day to allow CO2 gas into the leaf for photosynthesis, while at the same time trying to minimize the opening in order to reduce evaporative water loss.

Elevated levels of CO2 tend to close the stomates of most plants. This is nice for the plant, since it can make more biomass per unit of water lost via the stomates, and thus use water more efficiently. (Indeed, a recent report provides evidence that this may help some grasslands cope with global climate change.)

But this CO2-induced stomatal closing may also limit the potential photosynthetic benefit the plant may obtain by the higher levels of atmospheric CO2.

In addition to CO2 effects, the cellular processes of photosynthesis and respiration are both affected by high temperatures. For example, in a previous post, we’ve seen why photosynthesis is so heat-sensitive. But elevated temperatures both during daytime and nighttime can also lead to increased respiration, thus lowering net photosynthetic productivity. (Scientists have reported that a 4 degree C, that’s about 8.5o F, increase in temperature above background led to decreased carbon absorption by a simulated grassland.)

So, the effects of CO2 on stomates coupled with possible effects of elevated temperatures due to global warming on net photosynthesis complicate studies aimed at predicting the effects of increased atmospheric CO2 on plants.

Despite these complications, extensive “Free Air CO2 Enrichment” (FACE) studies on crop plants, such as soybeans, have provided crop plant developers important data regarding what factors to focus on in order to alleviate the effects of climate change on food production in the future. (e.g., see reference below)

1. Leakey, A.D.B., et al. (2009) “Elevated CO2 effects on plant carbon, nitrogen, and water relations: six important lessons from FACE.” Journal of Experimental Botany, Vol. 60, pp. 2859-2876. (Full Text) © 2008-2011 All Rights Reserved.

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