"Maize is the largest crop in the United States and around the world," says Jonathan Lynch. "By the same token, nitrogen is the biggest environmental, economic, and energy cost of maize production." (Credit: Science and Plants for Schools/Flickr)

agriculture

How corn roots got better by accident

While selectively breeding maize for other desirable qualities, scientists created root systems that are more efficient at getting nutrients, such as nitrogen, from the soil.

A new study suggests that future breeding efforts that directly select for positive root traits could lead to higher yield—which is needed to help feed a growing world population, while reducing pollution from excess nitrogen and reducing farmers’ fertilizer costs.

About half of the yield gains in commercial corn hybrids in the last 100 years have come from improved plant genetics, explains Larry York, recent PhD graduate in ecology, now a postdoctoral research fellow at the University of Nottingham. The other half came largely from agronomic practices, such as fertilizer use and higher planting densities.

corn roots
Using a computer program called SimRoot, researchers modeled the average root architecture of modern corn hybrids (shown) to help compare it to that of older varieties. (Credit: Larry York)

“A lot of research has focused on the shoots of maize plants, such as the direction of the leaves and how they capture light, or how the plants divide matter into ears and kernels,” York says. “We all know roots are responsible for the uptake of water and nutrients. However, relatively little is known about how roots do that.

“If we understand how roots have evolved and which specific root traits increase the plant’s efficiency, then we can take the next step in breeding that can help decrease pollution, save farmers money and make more yield.”

Biggest crop in the world

Finding new ways to improve plant nutrient uptake has global implications, according to coauthor Jonathan Lynch, professor of plant nutrition at Penn State’s College of Agricultural Sciences.

“Maize is the largest crop in the United States and around the world,” Lynch says. “By the same token, nitrogen is the biggest environmental, economic, and energy cost of maize production.

“Not only can crop varieties with improved root systems increase yields and reduce hunger in impoverished regions of the world with nutrient-poor soils, they also can decrease excess nitrogen where water quality is a critical issue, such as in the Chesapeake Bay watershed.”

Corn history

The researchers hypothesized that during a century of corn breeding aimed at increasing yields, root systems were indirectly selected for architecture and anatomy that are more efficient for nitrogen acquisition. To test this, they collaborated with DuPont Pioneer, which supplied 16 varieties from the company’s collection representing maize grown commercially in the United States from the early 1900s to the present.

“During this time period, agronomic practices changed immensely, from using horses to plant corn at low population densities with barely any added fertilizer, to using tractors to plant high-population-density fields with significant amounts of applied fertilizers,” York says. “Planting density increased about fourfold during this time, and nitrogen fertilizer use has tripled since World War II.”

The researchers grew all 16 varieties in both high- and low-nitrogen plots at three different densities, representing both historical and modern growing environments.

They measured shoot mass and yield and used a technique known as “shovelomics” to dig up the top portion of the roots so they could measure root quantity, angles, diameters, degree of lateral branching, and length.

Using a technology called laser ablation tomography, study coauthor Tania Galindo, doctoral candidate in horticulture, measured root anatomical traits, such as the size and number of cells, the size and number of xylem vessels that transport water and nutrients, and the percentage of roots that were aerenchyma, which are air-filled spaces that allow for the exchange of gases between the roots and shoots.

Better roots

The researchers found that the newest commercial varieties performed better in every agronomic environment. These varieties also had root characteristics known from previous Penn State research to make plants more efficient at acquiring nitrogen from the soil, including fewer nodal roots, longer lateral roots, and larger cortical cells. They published their results online in the Journal of Experimental Botany.

“That the newer material performed better in low-nitrogen environments is a novel result, since researchers tend to focus on high-input cropping systems.” York says. “Although newer varieties were developed for use in high-nitrogen conditions, today’s higher population densities mean plants have greater competition for available nutrients.”

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York says the results suggest that the maize root system has evolved to be more nitrogen efficient over the past century. He notes that the research did not look directly at plant water uptake, but the results may indicate greater water-use efficiency as well. “Water use will become more important because climate change is causing greater variability in rainfall events.”

The researchers say more in-depth research is needed on how specific root properties affect nitrogen uptake and on how these traits influence acquisition of other nutrients, such as phosphorous. “In addition, we need to know how the optimal root phenotype will depend on different environments—do we need the same roots in Pennsylvania as in Africa?” York says.

“Creating more nitrogen- and water-efficient crops is a major goal in a world that will see population reach 10 billion people within 50 years, requiring a 50 percent increase in food production without using more land,” he says. “We believe roots will be central to meeting this challenge.”

The National Science Foundation’s Basic Research to Enhance Agricultural Development program and the Agriculture and Food Research Initiative of the USDA National Institute of Food and Agriculture supported this work.

Source: Penn State

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