Stable Isotopes

by Laura Kent

Dr Hilary Stuart-Williams with a mass spectrometer used to measure the levels of different carbon isotopes in leaves in the 1980s

Stable isotopes are variations of the same element that have a different atomic mass but are not radioactive. Carbon exists in two stable forms; carbon-12 and carbon-13 while oxygen has three stable isotopes; oxygen-16, oxygen-17 and oxygen-18. The presence of certain isotopes can be used to study the photosynthetic pathway in plants. Plants have different levels of carbon-12 and carbon-13 in their leaves depending on whether the plant uses carbon-13 during photosynthesis. Rubisco often discriminates against carbon-13 when fixing CO2 but there is variation in the amount of discrimination within and between species. 

Plants have stomata or pores in their leaves that facilitate gas exchange for photosynthesis but also result in water loss. In 1982 ANU researchers Dr Graham Farquhar, Dr Marion O’Leary and Dr Joe Berry investigated how the stable isotopes of carbon could be used to measure photosynthetic metabolism. They showed that discrimination against carbon-13 was least in plants that fix the most carbon compared to the amount of water lost. Plants that use water more efficiently close their stomata more often leading to an increase in carbon-13 in the leaves of the plant that means the plant must use carbon-13 in photosynthetic reactions. Plants with higher ratios of carbon-13 to carbon-12 in their leaves don’t discriminate against carbon-13 during photosynthesis and were shown to also use water more efficiently.  The researchers showed that water efficiency was linked to carbon isotope discrimination as plants that were more likely to use carbon-13 in photosynthetic reactions didn’t need to open their stomata as often to obtain CO2 thus preventing the loss of water.

In 1993 Dr Farquhar and a team of researchers from ANU investigated the role of plants in storing excess CO2 from the atmosphere by tracing the distribution of oxygen-18. Plants also discriminate against oxygen-18 and CO2 molecules that contains oxygen-18 are heavier than other molecules and diffuse into and out of plants more slowly. The researchers created a complex mathematical model to describe how plants use oxygen-18 so they could use it to predict regions where CO2 was being stored in plants. 

The research done at ANU showed that stable isotopes could be used to screen for water efficiency in plants and by measuring the carbon isotope discrimination. This was a very important discovery that allowed researchers to locate crops that could use water more efficiently by measuring the isotope content of the leaves.

In 2017, we celebrate 50 years of Biology at ANU. This article is one of a set featuring the achievements and memorable occasions of ANU biologists in those first 50 years.
Read more at Biology at ANU – the first 50 years.

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