Genetic modification of maize over the last century has led to desirable shoot characteristics and increased yield - and that likely contributed to the evolution of root systems that are more efficient in acquiring nutrients, such as nitrogen, from the soil, according to a new study

About half of the yield gains in commercial corn in the last 100 years has come from improved plant genetics, explained Larry York, 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. 

Results of a new study suggest that future modifications that directly select for positive root traits could lead to yield gains needed to help feed a growing world population, while reducing pollution from excess nitrogen and reducing farmers' fertilizer costs

"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 said. "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." 

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 senior research manager Jeffrey R. Schussler, who 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 said. "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 conducted the field experiments at Penn State's Russell E. Larson Agricultural Research Center at Rock Springs.
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 co-author 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. 

"The laser ablation tomography combines the power of the laser for sectioning with a simultaneous image acquisition system to capture the internal organization of tissues, or anatomy, of entire root segments," Galindo explained. "Our laboratory's application of this technology in root biology allows us to study the root anatomical characteristics of thousands of samples of maize at high throughput."

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 said. "Although newer varieties were developed for use in high-nitrogen conditions, today's higher population densities mean plants have greater competition for available nutrients."

York said the results suggest that the maize root system has evolved to be more nitrogen efficient over the past century.

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.