Global warming, C3 plants, and world's food production

For every scientific phenomenon, there are denials. However, there is no denying the fact that the planet is getting warmer by the day. Seeing, as the saying goes, is believing. The variation in the mean global temperature over the years is as depicted in the graph below:

^{By RCraig09 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=88535596}

What makes the above image interesting and global warming undeniable is that the data depicted is not just from one, but five independent research bodies. The pattern from the previous years even makes it possible to model future change to the global average temperature.

Apart from the popular consequences of continuous rise in global temperature such as erratic climate, rising sea levels, diminishing glaciers, melting of polar ice caps, etc, one possible effect that has not received as much attention is the effect rising temperature could have on carbon fixation in the plant world.

Fixation of carbon by plants happens during photosynthesis. Specifically, the conversion of carbon dioxide to organic carbon happens in the dark or light-independent reaction of photosynthesis. The process is one of the 3 processes in the Calvin cycle. The beginning of the fixation sees carbon dioxide diffuses into the stroma of the chloroplast and an enzyme-mediated conversion of the gas to two 3-carbon molecule compound known as phosphoglycerate (3-PGA). From there onward, the molecules proceed to the second and the third steps in the cycle.

The polyamorous enzyme in photorespiration

So, where does rising global temperature comes in? I know someone is already itching to know. The problem of increasing temperature has to do with the enzyme that mediates the conversion of carbon dioxide to two molecules of PGA. RuBP carboxylase/oxygenase, or rubisco, as it is shortly called, is that boy that just cannot keep to a single partner. He loves carbon dioxide and oxygen equally, but on a conditional basis. When the temperature is normal, carbon dioxide can sleep over, but once the temperature rises to a certain point, oxygen becomes the favourite.

In other words, at high temperatures, rubisco's affinity for carbon dioxide reduces and oxygen gradually becomes the most preferred madam. Instead of producing two molecules of 3-PGA, one molecule of 3-GPA and one molecule of phosphoglycolate is produced. The latter is a 2-carbon compound. While the single 3-GPA molecule continues with the remaining processes of the Calvin cycle, the phosphoglycolate cannot, and hence, exits the cycle to take a longer route where some of it is recovered as glycerate and returned to the cycle while the remaining is converted to carbon dioxide and released as wastes.

The process whereby rubisco binds with oxygen instead of carbon dioxide is what is scientifically known as photorespiration. Apart from high temperature, the ratio of carbon dioxide to the oxygen concentration in the cellular spaces where diffusion into the stroma occurs must be low. In other words, oxygen must be present at a higher concentration and the ambient temperature must be above normal in order for photorespiration to occur.

^{A depiction of photorespiration. By Rachel Purdon - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=24598606}

Global warming - a perfect recipe for photorespiration

One might say that the conditions for photorespiration to occur in plants might be difficult to come by. However, based on the current trend of events in the global climate, there is a likelihood that it is already happening in our plants. First, it is established that the average global temperature has been rising steadily. During high temperatures, plants generally limit evapotranspiration by closing their stomata pores. When this happens, carbon dioxide can no longer diffuse into leaves, hence, becomes a limiting reactant in the Calvin cycle.

Nevertheless, the closing of stomatal pores does not prevent oxygen from being produced and released into the intercellular spaces in leaves. Thus, high temperatures usually go with the closing of stomata and accumulation of oxygen in leaves - a perfect recipe for photorespiration.

Evolution from C3 to C4

Photorespiration is a problem that is specific to a group of plants known as carbon 3 or C3 plants. They are so-called because the first stable molecules to be formed during carbon dioxide fixation are 3 carbon molecules (PGA).

Recently, a new group of plants has been uncovered that do not suffer the problem of photorespiration. Instead of a 3 carbon compound molecule being the first to be formed during carbon dioxide fixation, a 4 carbon molecule known as malate, is formed. A compound known as oxaloacetate is initially formed before being reduced to malate. Of course, an entirely new enzyme, phosphoenolpyruvate carboxylase, mediates this process instead of the infidel known as rubisco. The malate formed undergoes some processing to yield carbon dioxide which is then fed into the normal Calvin cycle of C3 plants.

What makes C4 plants unique is that they have the ability to fix carbon even when the concentration of carbon dioxide is very low. They are able to do this due to their special leaf anatomy which is quite different from that of C3 plants. However, according to research, the proportion of C3 to C4 plants in our world is high with C3 plants making up as high as 95 percent.

Final words

With rising global temperatures, most of our crop plants may not be able to fix carbon dioxide as they should as a result of photorespiration. This may cause reduced yield and worsen global food crises. The implication is that the world might shift towards the production of C4-based foods such as millets, corn, sugarcane, etc. as global temperature continues to rise. The consequence? Perhaps a nutritionally deficient population in the future. Or am I going too far?

Let's hear your opinion in the comment section.

Thank you for reading.