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

Submitted by admin on Sat, 10/04/2008 - 17:16.

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

In the past 250 years the amount of atmospheric CO₂ has increased by over 35% (from 280 to 383 ppm), due mainly to human activities such as deforestation and the burning of fossil fuels. 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? And 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?

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

Recent evidence, however, suggests that this may not be the case. It's clear that predicting the outcomes of elevated levels of CO₂ on crop plants is complex, and 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.

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 temperature on photosynthesis.

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

The process of photosynthesis is particularly temperature sensitive. For example, investigators have recently reported that a 4 degree C (about 8.5^o F) increase in temperature above background led to decreased carbon absorption by a simulated grassland.

The bottom line: 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.