Various canopy structures affect crop yields

Researching variations in how crops absorb light and assimilate carbon dioxide could help make plants more productive

Researchers at the University of Illinois, Urbana-Champaign, hope to make plants more productive by looking at variations in how different crops absorb light and assimilate carbon dioxide.

They focused on cowpeas, also known as black-eyed peas.

“Cowpea is an important legume in Africa, especially West Africa, which is responsible for 80 percent of the cowpea world production,” said Lisa Ainsworth, research plant physiologist with the United States Department of Agriculture’s Agricultural Research Service based at the University of Illinois.

She said several studies have shown that the above-ground display of leaves, the plant canopy, is not always optimal for light interception and whole plant photosynthesis.

“In soybean, it was shown that leaves were produced in excess, leading to shading within the plant canopy and reduction of photosynthetic activity and productivity. We wanted to test if cowpea similarly had more leaf area than optimal and if altering canopy architecture was a potential target to improve productivity.”

The cowpea canopy was naturally diverse. She said many traits among the genotype lines included leaf shape and size, chlorophyll content, and erectness and width of the main stem, which affected the total above-ground biomass and the amount of carbon taken up.

The research team screened 50 cowpea genotypes from a multi-parent advanced generation inter-cross (MAGIC) population for canopy architecture traits, canopy photosynthesis, and water-use efficiency by using a canopy gas exchange chamber. The chamber was used to measure the rate by which plants converted CO2 into carbohydrates as energy for growth.

They used genetic engineering to achieve better precision in altering the genome, said Ainsworth.

“We are currently screening a collection of diverse cultivars in order to identify regions of the genome regulating some of the traits that combine to produce different plant architectures.”

Researchers found variations in how the plant population grew and organized their leaves and also in how leaves were displayed within the plant canopy. Ainsworth said some plants spread their leaves over a large area, while others had a smaller leaf area.

“This ability to expose leaves to light was affected by other traits of the canopy. Plants with a long and crawling stem, for instance, were more capable of spreading their leaves over a larger area.”

The density of leaves affects the amount of light penetration, greenness, height, and ultimately the efficiency of photosynthesis and water use.

“It was interesting that leaf photosynthesis explained less than 15 percent of the variation we observed in canopy photosynthetic activity. In contrast, almost 40 percent of the variation was explained by the canopy architecture.”

Researchers noted photosynthetic activity of the canopy increased with increased leaf area across the genotypes — but only to a point. After a certain threshold, they found a decrease in photosynthetic activity, which researchers attributed to an increase in self-shading within the canopy.

“There were two factors that could reduce the disadvantage of too much leaf area,” Ainsworth said. “The first was how much light the leaves absorbed. When we compared two genotypes with similar leaf area, the one with a wider canopy allowing greater exposure of leaves to sunlight usually performed better. The second factor that contributed to reducing the drawback of too many leaves was their greenness. When you measure photosynthesis at the leaf level, dark green leaves usually perform better than light green leaves because they have more chlorophyll, which absorbs light. However, at the canopy level, genotypes characterized by high leaf area had a higher photosynthetic activity when they also had lighter green leaves. We believe this is because light green leaves at the top of the canopy absorb less light and allow greater light penetration to the lower layers of the canopy, which improves the overall photosynthetic activity at the canopy level.”

The study also found that genotypes with a higher canopy photosynthetic activity used water more efficiently.

“Water-use efficiency refers to the amount of CO2 assimilated by a crop canopy relative to the amount of water that is lost by the canopy,» said first author Anthony Digrado, a USDA-ARS postdoctoral researcher in Ainsworth’s university lab. “The ideal for a crop is to be able to have a lot of carbon intake without losing too much water.”

The team next plans to study how canopy traits, especially total leaf area, biomass, leaf photosynthesis, and canopy photosynthesis, interact to determine yield.

“While carbon assimilation is an important trait affecting yield, it doesn’t tell the full story,” Ainsworth said.

“We have also started experiments to study how elevated carbon dioxide concentrations will affect the canopy architecture and canopy photosynthesis.”

The research was published in the journal Food and Energy Security.

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