New model uncovers carbon’s mysterious paths inside the cell

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Plants transform carbon from the atmosphere into biological matter by fixing carbon dioxide (CO2) during photosynthesis. The exchange of carbon between the plant cells and the atmosphere is central to our understanding of diverse fields ranging from agriculture and ecology to global climate.

One of the most popular tools to grasp this complex exchange is isotope analysis, better known for being one of the most accurate tools to determine the age of rocks, bones and fossils.

Carbon isotopes are molecules of carbon that have different masses; some are heavier than others. During photosynthesis, leaves preferentially assimilate the lighter 12CO2 while they discriminate against the heavier 13CO2. The analysis of carbon isotopes and how plants discriminate between them has opened a window to discover how plants interact with their environment.

Here is where mathematics makes a triumphant entry into this story, because mathematical models are the language scientists use to describe this complex process. In 1982, Australian plant scientists created a mathematical model that uses carbon isotope discrimination to describe how carbon moves inside the leaf during photosynthesis. Since then, the widely used Farquhar et al. model became the standard method to interpret the observed preference for the lighter carbon during photosynthesis.

However, the Farquhar et al. model has some problems, found by one of his team members and published in Nature Plants just two weeks ago. “We discovered that the model makes some assumptions largely related to respiration that are not correct,” says Dr Florian Busch.

“After many discussions and endless hours with the team improving a system to measure carbon isotope discrimination, we came out with a model which is a much better representation of the processes going on inside the leaf. We now have a model that will improve predictions involving carbon isotope discrimination made on any level, including studying photosynthesis and respiration on the global scale.”

The most important feature of this new model is that it permits scientists to split the contributions of CO2 into the different diffusion paths, giving you more accurate measurements and a better estimate of the CO2 concentration inside the chloroplast.

“This model is a better representation of what happens to CO2 inside the leaf, and give us a clearer picture of the resistances that CO2 finds along its travel towards fixation. The model can be used to more accurately study the water-use-efficiency of crops, which may lead to the future generation of plants that perform better under water-limited environments. It gives us a powerful tool to investigate the fundamental nature of these processes, such as which components, pathways and proteins are playing a role in them.”

“The great thing about working with mathematical models is that If the model works, it is good news, and if it doesn’t work, playing around with the model and solving the inaccuracies gives you the opportunity to learn something new.”

This research was published in March, 2020 here:

Busch FA, Holloway-Phillips m, Stuart-Williams H and Farquhar G. 2020 Revisiting carbon isotope discrimination in C3 plants shows respiration rules when photosynthesis is low. Nature Plants Read article