CO2 and temperature challenge photosynthesis

Carl Bernacchi works on a crop heating array control system.  |  University of Illinois photo

Heat may impair the enzymes that drive photosynthesis, hindering plants’ abilities to regulate carbon uptake and water loss

A recent scientific review explored how rising temperatures can affect plant uptake of carbon dioxide.

CO2 drives plant productivity, but excessive heat could reduce the efficiency of enzymes driving photosynthesis, which may hinder the plant ability to regulate carbon uptake and water loss, impacting crop yields. The effect of one can complicate the function of the other, which led researchers at the University of Illinois at Urbana-Champaign, to study the impacts of rising CO2 and rising temperatures.

Carl Bernacchi, professor of plant biology and crop sciences at the university said earlier work focussing on rising CO2 suggested general patterns of responses for most crops.

“However, rising CO2 is coupled with warming temperatures so crop responses to these factors must be studied together.”

He said heat stress has a significant impact on photosynthesis and yields. Warming crops in cooler climates may increase photosynthesis and yields, whereas the warming of crops in hotter environments can have an opposite effect. Short-term heating, such as with heat waves, has a much different impact compared to increases in temperatures averaged over entire growing seasons.

“Even within a growing season, heat waves during early crop growth seems to have a much smaller impact on photosynthesis and yields than heating during later in the season,” he said. “So much of this complexity is driven by temperature impacting so many aspects of crop physiology from photosynthesis that drives plant growth to plant respiration to reproductive development that ultimately determines final yields. Temperature and the drying potential of the atmosphere are intricately linked such that warmer air generally causes plants to use more water. So much of the temperature responses we observed are also linked with humidity and it is challenging to disentangle these factors.”

Plants lose a lot of water in the process of absorbing CO2 into their leaves, a requirement for photosynthesis. Plants also orient their leaves to maximize light capture, which causes leaves to heat up. He said there is a never-ending balance between photosynthesis versus water loss and light absorption versus heat. However, global warming is presenting new challenges as rapidly increasing temperatures coupled with much drier atmospheres shift the balances.

“The world is getting hotter at a shocking rate,” said colleague Amanda Cavanagh, a University of Illinois alumna currently with the University of Essex, U.K. “And we know from global models that each increase in gross temperature degree Celsius can cause three percent to seven percent losses in yield of our four main crops.”

The review outlines strategies from studying enzymes to the plant canopy to plant management opportunities.

“Most of these strategies fall within the context of optimizing how light is captured, improving the efficiency of metabolic processes, such as photosynthesis, or improving the resilience of reproductive structures to be less temperature sensitive,” said Bernacchi.

He referred to Rubisco, the enzyme that represents the entry point of carbon dioxide into plant metabolism. The enzyme can drive two reactions. The first is to fix CO2 by binding it to sugars to drive photosynthesis and the second is that Rubisco will sometimes fix oxygen, initiating a pathway that wastes a plant’s resources.

“As temperatures get higher, Rubisco tends to increasingly favour the wasteful oxygenation reaction,” he said. “At temperatures that get too high, the enzyme function begins to taper off, catalyzing neither reaction. Because of this, strategies to both improve Rubisco’s specificity for carbon dioxide and its ability to function at higher temperatures are ideal for adapting crops to global warming.”

Rubisco’s limit for heat is confined to a low temperature. He said that most crops will start to see declines in photosynthesis because of Rubisco inefficiencies at around 30 C. However, crops grown in hot or arid conditions have greater resilience at high temperatures. Clearly, certain Rubiscos can handle the heat; it is a question of adapting cooler-based enzymes so that Rubisco can function better in temperate climates.

Studies to explore ways to enhance crops’ resilience are ongoing at the University of Illinois and other institutions affiliated with the work.

Lead author of the study, Caitlin Moore, research fellow at the University of Western Australia and an affiliate research fellow at the University of Illinois, said new tools can help screen plants on a large scale.

Satellites that can detect changes in chlorophyll fluorescence in plants will indicate whether a crop is under heat stress even before the plants show outward signs of becoming warm such as browning leaves. These tools would enable farmers to take pro-active measures.

Cavanagh, who studies the molecular biology and physiology of plants, said in the news release that some plants are more heat tolerant than others and scientists are searching their genomes for clues to their success.

She said that wild Australian relatives of rice grow in harsher climates than most paddy rices. They are exploring the options of transferring heat-tolerant genes to cultivated rice varieties.

Bernacchi said that research groups around the world are engineering plants and trying to introduce new Rubiscos or modifying organelles to be more efficient under current and future environmental conditions.

Other strategies include engineering structures that pump more CO2 to the site of carbon fixation to improve Rubisco efficiency, altering the light-gathering properties of leaves at the top and bottom of plants to even out sunlight distribution and maintain moisture levels, and changing the density of stomata to improve their control of CO2 inflow as well as moisture loss.

The review was published in Journal of Experimental Botany.

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