African Experiment Shows Maize Crops May Benefit from Rising CO2 Levels

As maize is a staple food for billions of people, global maize production is worth billions of dollars each year. Because most maize production is dependent on natural rainfall, it is vulnerable to changing rainfall patterns. Because climate change is expected to result in lower rainfall in many regions, this limitation is likely to worsen in the future.

However, predicting the effects of climate change on crop yields is difficult. This is due to the fact that the effects of rainfall and temperature can interact in complex ways. Rising carbon dioxide (CO2) levels in the atmosphere, caused by industrialization, only add to the uncertainty. However, as our new South African research shows, it may mitigate some of the effects of drying and warming on maize crops in tropical growing regions such as those found throughout much of Africa.

Importance of CO2:

CO2 is an important resource for photosynthesis, and its scarcity in the atmosphere has been a major impediment to plant growth for millennia. As a result, some plant groups, particularly grasses, have evolved a photosynthetic pathway that concentrates CO2 and increases photosynthesis efficiency in low-CO2 environments.

This pathway, known as C4 photosynthesis, is also found in maize. Its growth is thus not limited by CO2 availability under warm and humid conditions, and it gains no direct benefit from increasing atmospheric CO2. Elevated CO2 levels, on the other hand, allow plants to absorb enough CO2 while keeping their leaf pores (stomata) partially closed. This reduces plant water loss and has the potential to increase maize's drought tolerance.

However, while CO2 can increase soil water availability and slow the effect of drought on photosynthesis, it cannot completely compensate for a lack of rainfall. Seasonality of rainfall is thus still an important factor in determining where maize can be grown. Our ability to predict this will improve as more data from tropical growing regions becomes available.

Series of Experiments Carried Out:

Experiments are needed to predict the interacting effects of increased drought and increased CO2 on maize yields; these studies allow scientists to manipulate each of these factors individually and in combination. While manipulating water is relatively simple, experimenting with atmospheric CO2 necessitates specialized and expensive equipment.

It is not surprising, then, that the most important experiments on the effects of temperature, water, and CO2 have been conducted in temperate regions of the northern hemisphere, where research resources are concentrated.

South Africa's Rhodes University opened Africa's first large-scale elevated CO2 plant research facility in 2018. We exposed six different maize cultivars bred for South African climates to drought and watering treatments in special open-top chambers under ambient, elevated CO2, and elevated temperature conditions.

Plants were grown during the summer and were either irrigated daily or allowed to grow with only a little rainfall that fell naturally. Because the study area receives insufficient summer rainfall to be viable maize growing region, we were able to simulate the effects of drought under hot and dry summer conditions.

To investigate the impact of atmospheric CO2, we compared current conditions of 400 parts per million (ppm) to those predicted for the end of the twenty-first century (800 ppm). The air temperature in the open-top chambers was 4-5°C higher than the ambient temperature, which corresponds to future climate predictions.

Findings of Experiment:

Plants had very low yields when exposed to ambient CO2 and were not watered. Irrigated plants with elevated CO2 yielded nearly four times more.

Adding elevated CO2 to unwatered plants resulted in the same growth and yield as ambient CO2 irrigation. This demonstrates that increased CO2 had the same effect on plants as daily irrigation, completely compensating for drought. Plants required less water when given more CO2 because they could partially close their leaf pores and avoid water loss.

With the addition of CO2, irrigated maize yields increased. This suggests that even when water is being irrigated, hot and dry weather can cause water stress and reduce productivity.

According to this study, future atmospheric CO2 concentrations could help mitigate the effects of warming and drought, even for irrigated agriculture. More research, however, is required to determine the effects of intermediate CO2 concentrations ranging from 400 to 800 ppm, which will be experienced between now and the end of the century.

(Source: The Conversation)