Plants need sunlight to survive, but what happens when their own leaves stop sunlight from penetrating through the plant? Too many leaves up top prevents photosynthesis from occurring in lower leaves and yield takes a hit.
Soybeans would be more productive with about 30% to 40% fewer leaves, says Praveen Kumar, University of Illinois professor.
Kumar lead research efforts to see how CO2 affects plants’ leaf area index (LAI) and yields at current and elevated levels to mimic anticipated increases in CO2 in coming years. Because higher CO2 levels typically produce a greater number of leaves, discovering soybeans already produce too many leaves indicates changes are needed in breeding selection.
“We think the plants might produce more leaves than they need because of their wild ancestors,” says Steve Long, co-author of the study. “In the wild, soybeans produced more leaves to compete with the plants around it.”
Wild soybeans also had to overcome insect feeding while still producing seeds. More leaves meant those plants had more opportunity to produce despite their circumstances.
Today, farmers manage competing weeds and insects for soybeans. “This allows us to trade off resilience for increased productivity,” Kumar adds.
Field testing showed a 5% reduction in leaf area created an 8% yield advantage at current CO2 levels. This 8% yield increase in the U.S. alone would provide 6.5 million more metric tons annually. In elevated CO2 levels, yield increased by 10%.
In field testing, researchers cut the emerging leaves off the plants between V10 and V16. The energy used to grow the smaller leaf is then reinvested into seed and pod development.
Midday, when sun is the most intense, soybeans with a greater LAI photosynthesize 13% more than soybeans with lower LAI. However, greater photosynthetic rates midday are overshadowed by a 25% increase in respiratory loss in soybeans with greater LAI. In addition, soybean leaves with lower LAI have 6% less shading, 9% greater light use efficiency and an 11% decrease in water demand under current CO2 levels.
Elevated CO2 levels mean more photosynthesis and about 25% more seed yield is possible. But decreasing leaf area by 17% under these conditions provides an 8% yield advantage, or 33% compared with yields today.
Researchers anticipate a 40% reduction in leaf area will do the most to increase yield. Decreasing soybean leaf area will need to be accomplished through genetic changes, not herbicidal or mechanical methods.
Research on the optimal number of leaves will continue on soybeans and other crops. As breeders focus efforts on higher yields, farmers might someday see soybeans with fewer leaves.