Swedish researchers have discovered that increasing levels of carbon dioxide in the atmosphere have shifted photosynthetic metabolism in plants over the 20th century.

Fireweed (Chamerion angustifolium). Credit: Johan Gunséus

This study, carried out by scientists at Umea University and the Swedish University of Agricultural Sciences, is said to be the first of its kind worldwide that deduces biochemical regulation of plant metabolism from historical specimens.

The findings are now published in the leading journal PNAS and will have an impact on new models of future carbon dioxide concentration in the atmosphere.

In most plants, the uptake of CO2 through photosynthesis is reduced by a side reaction called photorespiration.

The research group has now found that the CO2 increase in the atmosphere over the 20th century has shifted the balance between photosynthesis and photorespiration toward photosynthesis.

This shift has so far contributed to the global vegetation’s ability to dampen climate change by absorbing a third of human-caused CO2 emissions.

The photorespiration pathway is known to increase with temperature, which means that temperature and CO2 effects predictably oppose one another. This implies that the CO2-driven metabolic shift will be counteracted by future temperature increases.

Vegetation’s ability to capture CO2 from the atmosphere through photosynthesis is not only a decisive factor for the global CO2 balance, but also in predicting future climate change and crop productivity.

By monitoring plant metabolism retrospectively using historic plant samples, this research group has quantified how much increased atmospheric CO2 levels during the 20th century have contributed to plants’ ability to capture the greenhouse gas carbon dioxide.

“Until recently, studying how plants respond to increases in CO2 on decadal to centennial time scales has relied on simulations based on short-term experiments, because methods to detect long-term metabolic changes were not available," says study leader, professor Jürgen Schleucher.

"By reconstructing past metabolic shifts in response to environmental changes, we lay the foundation for better modelling of future plant performance."

The researchers observed shifted photosynthetic metabolism in both wild plant species as well as crops.

"We suspected that photorespiration was stealing away a portion of photosynthesis. Now we know it was leaving fingerprints," says co author and Tree Physiology professor John Marshall.

Due to the fundamental biochemical origin of the observed shift, the same change should have occurred in most global vegetation.

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