Professor Colin Osborne, Associate Director of the Grantham Centre for Sustainable Futures, wrote the following article for Financial Chronicle. In it, he sets out some the scientific challenges in making crops more sustainable, and explains how new research from the University of Sheffield could help pave a way forward.
Find out more about Colin’s research
As the global population climbs from seven towards 10 billion and societies worldwide become more affluent, the global demand for food is climbing. Indeed, the UN Food and Agriculture Organization estimates that 50 per cent more food could be needed globally by 2030. There is no simple answer to this problem. It will require social, cultural and political changes, coupled with scientific and technological innovations. Breeding more productive crops will be necessary, but the yields of staples like wheat, rice and cassava are not currently increasing fast enough to meet future demands. The growing gap between crop production and demand for food has raised alarm amongst scientists, who have responded by thinking about radical ways to boost crop production.
Crop production can be reduced to a few simple principles. Plants use photosynthesis to convert solar energy into energy-rich sugar, starch or oils. The efficiency of that energy conversion depends on how much sunshine a crop intercepts, how efficiently it converts the sun’s energy into biomass, and what fraction of that biomass becomes an edible food product. The Green Revolution in crop breeding that started in the 1960s led to new crop varieties with improved sunshine interception and a greater edible fraction. These strategies were immensely successful, helping to double the production of staple crops like rice, maize and wheat within twenty years. But improvements in these areas are becoming increasingly limited, leading scientists to turn to photosynthetic energy conversion as the next major target for crop breeding.
Most of our crops convert about 3 per cent of intercepted solar energy into biomass. This leaves a lot of room for improvement because, in theory, the efficiency could be more than tripled. Approaches to close this gap by raising the efficiency of photosynthetic energy conversion have turned to nature for inspiration. Multiple groups of bacteria, algae and plants have boosted photosynthetic efficiency by evolving carbon dioxide pumps. These are incredibly diverse in the way they operate, but all use the sun’s energy to concentrate carbon dioxide, the raw material of photosynthesis, inside green cells. One of the best-known examples is C4 photosynthesis, which has evolved more than 60 times, and is used by tropical plants like maize and sugarcane to achieve greater efficiencies and crop yields than species like rice and wheat that use the more common C3 type of photosynthesis. Introducing C4 photosynthesis into a C3 crop like rice could raise efficiency by up to 50 per cent and this remarkable potential has inspired international groups of scientists to chase the long-term goal of C4 rice.
A number of scientific and technical problems must be solved before breeders can produce synthetic C4 crops. In particular, we still don’t know how C4 photosynthesis works within the whole plant to increase growth. This question is particularly important because, although we have known for a long time that C4 photosynthesis explains the high yields and growth rates of crops such as maize and sugarcane, many comparisons of C3 and C4 plants have not shown this expected difference. Our new research at the University of Sheffield has solved this problem with the biggest experiment of its kind yet attempted. We measured growth side-by-side in almost 400 wild grass and cereal crop species, including C3 and C4 plants from many evolutionary groups, with 10-fold differences in growth rates. Our intention was to discover how C4 photosynthesis changes growth rates in nature.
We found that C4 photosynthesis increases growth by 20 per cent in young plants and doubles the growth rate as plants get bigger. This happened in our experiment because relative growth slows down as plants get larger, and that slowdown is stronger in C3 plants. The research also turned up a few surprises, showing that C4 plants reorganise the way they grow. They produce lower density leaves that reduce the cost of carrying out photosynthesis, and instead invest heavily in growth beneath the soil, producing 50 per cent more roots than C3 plants. This reinvestment has important benefits for wild plants because it could allow greater access to soil water and nutrients. It also has important implications for a future synthetic C4 rice crop, showing potential benefits beyond production. Greater investment in roots might give these plants easier access to water and nutrients, allowing farmers to get better yields with less irrigation or fertiliser. The increasing threats of climate change to rainfall and groundwater supplies means that any such benefits from C4 rice would be good news for growers.
For now, innovative technologies for improving crop production by boosting photosynthetic efficiency remain on the scientific horizon. A more immediate priority is breeding climate-adapted varieties. As the accumulation of greenhouse gases in the atmosphere continues to drive climate change, we increasingly need to protect crops from high temperature events and droughts. However, rising carbon dioxide in the atmosphere also presents an opportunity for breeders, because it increases photosynthetic efficiency in the same way as higher carbon dioxide in the cells of C4 plants. Experiments growing crops in a high CO2 atmosphere show that greater photosynthetic efficiency has the biggest benefits in plants that can grow new roots, tubers or seeds to use the extra carbon they take up. This was a lesson learned millions of years ago by wild C4 plants, whose roots proliferated using the extra carbon. Now we need to breed a new generation of crops that can perform the same trick, and convert rising atmospheric carbon dioxide into larger harvests of grain or vegetables.