The two different processes plants use to generate carbon flows

Did you know that there are two different systems of photosynthesis used by plants? One is called the C3 pathway and the other is called the C4 pathway. The main difference between these two systems is the compounds used in turning carbon dioxide into starch and sugar.

Plants using the C3 pathway rely on the enzyme, rubisco, to fix carbon atoms from carbon dioxide in the air. The first stable product in this process is based on three carbon atoms, hence it’s called the C3 pathway. 

Plants using the C4 pathway use the enzyme, PEP carboxylase, to fix carbon from atmospheric carbon dioxide during photosynthesis. The first stable product of this process is a four carbon molecule, hence it’s called the C4 pathway.

The photosynthetic pathway that a plant uses will determine the conditions under which the plant will grow. The chart below is a guide to their likely requirements and performance under differing conditions.

The difference between the C3 and C4 photosynthetic pathways

The conditions each grows best under

Generally, C3 plants are more “temperate” plants, growing best in cool, moist conditions. They take up CO2 through open leaf pores or “stomates” and convert this to carbohydrates (leaves, stems, roots and seeds). In cool, moist conditions, this process is three times more efficient in C3s than in C4s. Unfortunately, for C3s, as the temperature increases, rubisco combines with oxygen instead of CO2. This wasteful process is called photorespiration. It produces photoglycolate, a useless product to the plant, and breaks down carbon compounds to release CO2. In warmer, drier conditions, C3 plants will photorespire more than they photosynthesize. In effect, they begin to die. In contrast to this, C4 plants hardly photorespire at all, even as the temperature rises.

C4 plants grow best in warmer, drier conditions. The C4 photosynthetic pathway needs more sunlight energy than the C3 pathway to convert CO2 to carbohydrates. In hot, dry climates, very low levels of photorespiration balance this higher energy requirement. Most of the CO2 absorbed by C4 plants is permanently converted to plant material in warmer conditions. But in cooler conditions, C3 plants are more efficient converters of water and CO2 to plant matter than C4 plants. In fact, below 15° C, many C4 species begin to hay off as the energy necessary for the C4 photosynthetic process becomes increasingly limited. C4 plants grow best when energy from sunlight is plentiful.

Some examples of C3 and C4 plants

C4 plants include the main grasses of the tropical savannahs, including black spear grass, kangaroo grass and golden beard grass, as well as crops like sugar cane, sorghum and corn. C3s crops include winter cereals, legumes, temperate pasture plants and all trees. C4 pasture plants are already more efficient than the C3s growing beside them and are thought not to gain as much from increased carbon dioxide levels in the atmosphere.

Quality versus quantity

The leaves of C3 plants are generally higher in nitrogen (protein) than those of C4 plants under the same conditions. This is because C3 plants have a higher concentration of rubisco (a protein) in their leaves. This explains why animals seek out C3 plants in preference to C4 plants putting them at more risk of being eaten out.

Annual grasses are C3s which is consistent with them being highly sought after by animals.

While C3 plants may produce higher quality feed, C4 plants nearly always produce more feed than C3 plants over a 12 month period. C4 plants are more water efficient, producing twice as much organic matter per litre of water to what C3 plants produce. A CSIRO scientist explained to me that on average, C4 plants require ¼ litre of water to produce 1 gram of pasture and C3 plants require ½ litre of water to produce 1 gram of pasture.

When paddocks degrade, the soil has a lower nutrient level. This reduces their ability to supply the nutrient requirements of C3 plants.

Complexity increases carbon flows

The key thing a farmer or grazier wants to achieve, is to create an ecosystem that harvests carbon/energy efficiently. This requires multiple options to capture carbon. Having both perennials and annuals and C3 and C4 plants, where climate allows, is the best option. Almost all “natural” systems configure themselves to do this, but human intervention disrupts this process. We always seem to want to simplify the system by reducing the number of pathways, and thus the total efficiency of the system. 

Conclusion

Diverse grasslands are more capable of supplying ongoing “green feed” because they make the most efficient use of water and nitrogen when it is available. They are also more successful at maintaining the health of the landscape and making it more profitable, because carbon is being introduced and cycled more often.

If the climate changes, as predicted, then this will change the balance between C3s and C4s in some pastoral regions.

While C3 plants are seen in a better light because of their higher nitrogen content, we should never lose sight of the fact that C4 plants can photosynthesise at higher temperatures, when the C3s shut down and achieve nothing. It is better to have inferior carbon compounds, that can be supplemented for livestock performance, than having no carbon in the paddock for livestock to consume.

There is a need for more emphasis on how different these two plant groups are in their manipulation of carbon.

Next week’s discussion: “Is pasture rest TIME or TIMING?”

Alan Lauder



WHY CARBON FLOWS?,