Life Support Systems
Section 4.3.5.
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Carbon Dioxide's Role in Plant Growth

With respect to CO2 utilization, plants are divided into two types: C3 plants and C4 plants. These names essentially distinguish two types of photosyntensis. C3 photosynthesis (so called because the photosynthetic process yields 3-carbon derivatives) has a problem in that sometimes O2 fills the role that CO2 is supposed to fill. When it does, much of the energy that goes into photosynthesis is wasted. C4 plants, on the other hand, starts with a gate, of sorts, that keeps much of the O2 out, so this waste happens less often.

Most plants, including plants used in agriculture, are C3 plants. This includes lemon trees (virtually all trees, in fact), sugar beets, and potatoes. Corn and surgarcane are C4 plants.

Each type of plant reacts to a change in CO2 concentrations differently. C4 plants already use CO2 efficiently. An increase in the concentration does not help them much. C3 plants, on the other hand, benefit greatly from increases in CO2 because less of the inefficient O2 photosynthesis occurs. Plants in a high CO2 environment increase their plant mass by 20 to 25%. Yields of some crops can be increased by up to 33%. This is the effect of doubling CO2 concentrations over Earth normal. Still higher concentrations can be expected to yield still better results.

Note, however, that the effects vary even among different types of C3 plants. Some are better able to take advantage of higher CO2 concentrations than others, and a few actually suffer if CO2 concentrations are raised.

But, there's a catch. These benefits occur only if the nutrient levels and the amount of water available also increase. CO2 alone does very little good. Consequently, to take advantage of a higher CO2 concentration, we must supply more water and bring in more nutrients (such as nitrogen).

In fact, there is more than one catch. As a plant's production of starch from CO2 increases, it seems to reach some sort of saturation point. It reaches a point where it can no longer take advantage of the greater abundance of CO2. Scientists suspect that this is because there is a bottleneck in the plant's metabolic system. It can manufacture more starch, but it can't get it to where it is needed - or it can't use what it is getting. At this point, you might as well bring the CO2 concentration back down to normal levels for all the good you're doing. Or, if this point is close to the plant's maturation point, you can harvest it and plant the next crop.

[Note: high conentrations of CO2 allows the plant to use water more efficiently. This is because the passageways that allow CO2 into the plant also let H2O out. Under higher CO2 concentrations, these passageways can be kept more tightly constricted, allowing less H2O to escape. But there is a tradeoff here between CO2 fertilization and efficient use of water. To the degree you have one, you must give up the other.]


Fakhri A. Bazzaz and Eric D. Fajer, "Plant Life in a CO2-Rich World," SCIENTIFIC AMERICAN, January, 1992, pp 68-74.

--R. Alonzo Fyfe

Life Support Systems

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