Atmosphere And Weather

Why a Rise in Carbon Dioxide Levels would help Agriculture

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Plants are able to make biomass (leaves, roots,…) from a gas found in the atmosphere, namely carbon dioxide (CO2). Carbon dioxide is a stable, inert gas.

To make this gas more reactive and useful for the plant, it needs to be ‘reduced’ or ‘fixated’ into carbohydrates, better known as sugars.

A lot of energy is needed for the reduction of carbon dioxide. Plants (and photosynthetic bacteria) use sunlight and the necessary enzymes (biocatalysts) for this purpose. RuBisCO (Ribulose-1,5-bisphosphate carboxylase oxygenase) is such an enzyme that plays a role in the reduction process of carbon dioxide.

A simple prediction

By using Le Chatelier's principle one can make a simple prediction.

Le Chatelier’s principle can be summarized as: If a chemical system at equilibrium experiences a change in pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established. 

Applied to the chemical reaction that is catalyzed by RuBisCO:

CO2 + Ribulose-1,5-bisphosphate <- -> 2 (3 phosphoglycerate)

When the concentration of CO2 raises, a new equilibrium is established by shifting the reaction more to the right, this means more Ribulose-1,5-bisphosphate will react with carbon dioxide, resulting in more 3 phosphoglycerate. This will result in more carbohydrates, thus more biomass.

Is it that easy?

Evolution of plants through history

The concentration of CO2 is one of the key elements that drives the evolution of plants. When the concentration of CO2 was high (>5%), more than 445 million years ago, plants didn’t had leaves. They were simple leafless stems that contained a few stomata (openings for gas exchange). When the CO2 concentration dropped (<1%), plants needed more stomata to take up the scarce CO2. The solution for this problem was the development of leaves. Leaves have a big surface area, resulting in a higher density of stomata.

For 20 million years, plants have adapted themselves to the low CO2 concentration (about 240 ppm). When the Industrial Revolution started, the concentration of CO2 started to rise, now it is still rising and in the future it will still be rising. Some predictions suggest a concentration of more than 700 ppm by the end of this century.

How will plants react on this new diet that is given to them?

Experiments, conducted by several scientists, on various plants and diverse test conditions come with the following conclusions:

1)      The higher concentrations of CO2 is no problem for the photosynthetic machinery. It can handle far higher concentrations of CO2 (up to 1000 ppm) than the current concentrations.  

2)      The higher CO2 uptake and fixation doesn’t automatically lead to more biomass production.

The last conclusion can be easily illustrated as follows: when building a house, one needs a construction plan and building materials (bricks, steal,…). When on one day the building materials become very cheap, and you don’t change the construction plan, is it wise to buy more building materials than needed? Will the house become bigger when you buy more bricks, but don’t change the construction plan? The answer is obviously no. This is the same with plants. Cell growth is a complex regulated system. The construction plan is written in the DNA and it takes millions of years to change the construction plan (see previous part). Researchers are still searching which genes play a role in plant growth and how they are interacting with each other, in the hope of using transgene (or cisgene) techniques to skip the million years of evolution.


A higher concentration of carbon dioxide will probably lead to a raised level of biomass production and help agriculture, but this will be less than expected because plant growth is a regulated and complex system that is not yet fully understood.

More about this author: Geert Hesters

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