Stable Isotopes

by Laura Kent and the Farquhar Lab

Stable isotopes are variations of the same element that have a different atomic mass but are not radioactive. Most of the light elements used by living things exist as several isotopes. 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 relative abundance of certain isotopes can be used to study the photosynthetic pathways in plants. For example C3 plants (such as wheat and apples) have enzymes that react faster with carbon-12 so that more ends up in the products: sugars etc.  C4 plants (e.g. Sugar cane and maize) use the Hatch-Slack pathway that is less selective for carbon-12 and are consequently more carbon-13 enriched.

Plants have stomata or pores in their leaves that facilitate carbon dioxide uptake 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 in C3 plants 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 of the time and increase the relative concentration of carbon-13 in their leaves as carbon-12 is removed.  Eventually they are forced to use more carbon-13 in photosynthetic reactions. This carbon-13 signal is preserved in the carbohydrates in the leaves, so by analysing the ratio of carbon-13 to carbon-12 it is possible to identify individuals that have had their stomata closed more to save water but have also used carbon dioxide efficiently.

In 1993 Dr Farquhar and a team of researchers from ANU investigated the role of plants in removing CO2 from the atmosphere by tracing the distribution of oxygen-18. Plants also discriminate against oxygen-18, and CO2 molecules that contain oxygen-18 diffuse into and out of plants more slowly. Additionally in wetter high northern latitudes the carbon dioxide loses a lot of its oxygen-18 by exchange with depleted water in the soil and plants.  The researchers created a complex mathematical model to describe uptake of naturally oxygen-18 tagged CO2 so that they could use it to identify regions where relatively more CO2 was being taken up by plants. 

The research done at ANU showed that stable isotopes could be used to screen for water use efficiency in plants by measuring the carbon isotope discrimination. Plants in a field that have grown to full size but have a greater content of carbon-13 are making the best of their water supply: a valuable attribute in Australia and many other regions!

This article is one of a set featuring the achievements and memorable occasions in the History of Biology at ANU.