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Rising carbon dioxide in the atmosphere is one of the very few aspects of global environmental change predicted to increase crop yields because more CO2 increases plant growth. 

But new research shows that, for soybean, a substantial part of the increased growth goes into leaves, rather than into seeds, which is the part of the plant used for food and animal feed. It suggests that reducing leaf growth could “significantly increase” food production.

Soybean is the world’s fourth largest food crop in terms of seed production, and the most important vegetable protein source, as well as being a key source of vegetable oil. 

In work led by Steve Long FRS, Professor of Crop Science at both the University of Illinois Urbana-Champaign and Lancaster University, researchers wanted to find out whether soybean crops need these extra leaves. 

They used computer modelling simulations to check thousands of combinations of plant leaf structure, CO2 level and environmental conditions. To their surprise it suggested soybean produces far too many leaves, not just under future predicted CO2 levels but under current levels as well, which are 40% higher than they were 100 years ago

“This was not what we expected, the original hypothesis was that natural selection will produce the ideal form of plant,” said Professor Long FRS, who was principal investigator on the research, which was supported by the Bill & Melinda Gates Foundation.

This unexpected finding was then tested in open air field conditions in Illinois, using Free Air Concentration Enrichment technology (FACE) to increase CO2 levels in some areas to that expected for the year 2050.

Every third emerging leaf was cut by hand from some soybean plants under both current and raised CO2 levels and the yield of these plants was compared to those where no leaves had been removed. The plants with removed leaves showed an 8% productivity increase at both current and 2050 CO2 levels.

Not only was leaf production diverting growth from seeds, but the higher leaves were shading the lower ones, reducing their ability to photosynthesise.

The research, published in Global Change Biology, is the first direct proof that a modern crop cultivar produces more leaf than is optimal for yield under today’s and future CO2 and that reducing leaf area would give higher yields.

“When we started looking for reasons, it began to make sense,” said Steve. “If you think of the wild ancestors of these cultivars they grew in a mixed community of plants. They might well have used their leaves to shade out the competition, and if a herbivore munches half of their leaves they have others in reserve. So they have evolved for particular conditions that do not apply in well managed farm fields where we do not want plants to waste resources in competing with each other.”

Having identified the ideal plant structure, the researchers believe that breeding or bioengineering plants with fewer leaves could contribute very significantly to meeting the growing demand for staple food crops and is urgently needed.

“The United Nations Food and Agricultural Organisation predicts a 70% increase in food needed by 2050. At our current rates of increase in food production we will not meet that goal. Between 2003 and 2013 the yield of wheat in the UK increased no more than 1%, even though it is our major arable crop.

“Innovations such as these will not be in farmers fields at scale for 20 years, so we need to start developing the technologies now."

Professor Longs' team want to investigate whether other major crops, such as wheat and maize, show a similar tendency to overproduce leaves.

“We are particularly interested in other legumes, such as black eyed pea, also known as cow pea and a very important source of protein and calories in sub Saharan Africa.”

The other researchers involved in the work were Dr Venkatraman Srinivasan and Dr Praveen Kumar, both from the University of Illinois at Urbana-Champaign.