The Toona ciliata canopy in a forest in Australia. Photo Sophie Zwartsenberg Photosynthesis causes plants to grow. During photosynthesis, carbon dioxide (CO2) is transformed into glucose and oxygen with the help of light. The ever-increasing CO2 emissions cause the climate to change. Plants change accordingly; a study conducted by PhD candidate Sophie Zwartsenberg shows. Increased levels of CO2 in the atmosphere make their photosynthesis more efficient.
That improved efficiency results from Rubisco, the key enzyme in photosynthesis. Rubisco transforms CO2 into glucose. But, instead of extracting CO2 from the atmosphere, Rubisco can also extract oxygen, which prompts photorespiration. The difference between the two processes can be seen by the glucose that is formed.
‘Heavy glucose’
To make this distinction requires zooming in on a small part of the glucose, where a so-called heavy hydrogen (deuterium) atom is found. This is a hydrogen atom which contains an extra proton in its nucleus. NMR machines can distinguish between photosynthesis and photorespiration through the ‘heavy glucose’. Zwartsenberg uses this gift nature has given.
She studied glucose in Toona ciliata, the Australian Red Cedar, tree rings. This is a tropical tree from the Mahogany family. She used trees from Australia, Thailand, and Bangladesh for more than a century. The results left no doubt: in the last one hundred years, the elevated CO2 levels have caused increased photosynthesis in the trees.
The increased levels of CO2 in the atmosphere have shifted the ratio of carbon dioxide uptake (photosynthesis) to oxygen uptake (photorespiration) in favour of photosynthesis
Sophie Zwartsenberg, PhD candidate with the Forest Ecology and Management group
‘The tree’s physiology changed’, says Zwartsenberg. ‘The increased levels of CO2 in the atmosphere have shifted the ratio of carbon dioxide uptake (photosynthesis) to oxygen uptake (photorespiration) in favour of photosynthesis.’ In other words, photosynthesis has become more efficient. This increased efficiency can easily be seen as proof of CO2 fertilisation, a presumed positive effect climate change has on plant growth.
No proof
Zwartsenberg will not support this conclusion. ‘I don’t know what this means for the tree. The change we measure does not translate into information about growth, hence, this, in itself, does not prove increased growth through CO2 fertilisation, as was announced by some media. I have tried to avoid that suggestion, but putting out a nuanced message on this subject is challenging. I am not even sure whether this increased efficiency leads to the production of more glucose. It is likely, but I am not sure.’
A very limited dataset, yet with such a clear result
Sophie Zwartsenberg, Forest Ecology and Management group
Zwartsenberg explains that a tree can be efficient and still produce little glucose. ‘I believe that is the reason why smaller trees in this study are more efficient than larger ones. Smaller trees get less light, which limits the amount of photosynthesis, using less CO2. The concentration of CO2 in the leaf increases, Rubisco catches more CO2, making photosynthesis more efficient.’
Limitation
Zwartsenberg discovered physiological changes by studying just nine trees. Three per location, and only three to five samples per tree, distributed across different tree rings. A small dataset, and still, such a clear result.’ There is a practical reason for the limited dataset. ‘It takes two weeks to process four samples, and that is just the lab work, making glucose from wood.’
The glucose was analysed by the Umeå University, where the method was developed and applied to, among others, sugar beets. Zwartsenberg is the first scientist to apply the method to trees.
