Roots to Fruits

by Leonie Joubert

Computer-generated image representing the root system of a common bean plant at 40 days after germination (digital rendering by Johannes Postma)

This computer-generated image represents the root system of a common bean plant at 40 days after germination. Simulation models help researchers analyze root performance in accessing nutrients at varying soil depths and how root architecture may influence the acquisition of water and nutrients like phosphorus, nitrogen, and potassium. In Africa at the Ukulima Root Biology Center, plant nutritionist Jonathan Lynch and his research team search maize and bean germplasm looking for genetic lines that are more water and nutrient efficient. Such traits are in demand by a world where fertilizers and water are growing more expensive and scarce. Digital rendering by Johannes Postma

Five or so men stand in a loose huddle around a string of weedy-looking maize plants, speculating about their stunted growth and why the straplike leaves are shot through with streaks of purple. Occasionally, the sun cuts through the clouds moving drowsily overhead, which soften the heat and promise a late summer shower across the southern African bushveld.

“Disease?” one of the men speculates. Nope. This is beef and wildlife country. It’s unlikely that any maize disease would spill over from neighboring farms to infect this research plot. “Insects?” another ponders. Maybe. “Heat?” Quite likely. “It was hot over Christmas,” says Curtis Frederick, a 23-year-old horticulture graduate working as field agronomist and site manager on this South African farm where Penn State’s Ukulima Root Biology Center has a five-year research program (see “Letters from the Bushveld”). Maybe the young maize plants didn’t get enough water during the holiday season, one postdoctoral researcher speculates, his brow crinkled in thought.

Katie Barlow and Jonathan Lynch (photo by Robert Snyder)

Katie Barlow, graduate student in horticulture, and Jonathan Lynch discuss data-logging units that monitor volumetric water content over time at different soil depths. The information provides a basis to correlate moisture levels and root development and performance as part of ongoing drought studies. Photo by Robert Snyder

“No,” contests Shawn Kaeppler, maize geneticist and professor of agronomy at the University of Wisconsin–Madison. “If they hadn’t been watered enough at seedling stage, they wouldn’t have come up at all. My guess is that they need nutrients. But if I were a plant breeder, I wouldn’t select this genotype.”

Jonathan Lynch, head of the college’s plant nutrition lab conducting the research, calls for a leaf analysis to check for nutrient deficiency. The discussion ambles through the course of the morning as the scientists plod methodically from one row to another, inspecting various-sized maize and bean plants that fan neatly around a central-pivot irrigation system.

It’s February 2010 and the Southern Hemisphere growing season is drawing to a close. Lynch is in South Africa for two days to inspect the research plot on a farm two hours north of Johannesburg, the country’s commercial hub. The 50-acre plot has been made available to the Ukulima Root Biology Center by the Howard G. Buffett Foundation, which has further sponsored the research through a $1.5-million grant.

Monitoring root development (photo by Rodger Bosch)Here, in the sandy, rust-colored soils of the bushveld, Lynch’s crew of doctoral and postdoctoral researchers, along with agricultural technicians from neighboring Mozambique, are conducting field experiments on maize and bean plants to identify lines, or genotypes, that are resistant to drought or even thrive in low-nutrient conditions.

Along the rows of five thousand maize types, some are thriving, while others limp along. While observing plant performance provides important insights, the Ukulima team is really concerned with what’s happening below the ground, in the root systems of these plants. This project is all about “root architecture,” explains Lynch as he surveys the uneven growth around him.

Later that evening, at the farmhouse where his team lives for the season, Lynch recites some statistics on world hunger as boerewors (traditional farm-style sausage) sizzles and pops over the braai (barbecue) coals nearby: “A billion people are hungry around the world, and seven million children die of malnutrition every year as the human population increases. People say it’s because of war, poverty, AIDS, all kinds of social inequalities. But the fact is people in developing countries aren’t getting good crop yields.”

Monitoring root development (photo by Robert Snyder)Maize farmers in Africa get only 5 percent of their yield potential, Lynch explains. Paddy rice in many developing countries yields 20 percent of its potential, and the common bean produces a mere 10 percent. Low fertility and drought are major contributors to the underperformance.

This limited agricultural output exists despite the so-called Green Revolution, which saw massive increases in productivity with the industrialization of agriculture following World War II and, since the 1960s, the development of high-yield hybrid crops, synthetic fertilizers, and more advanced irrigation systems.

But, that abundance hasn’t spilled over into all countries. And now, even these advances are constrained by the limitations of natural resources. Dwindling reserves of oil and natural gas used in the manufacture of inorganic nitrogen fertilizer, along with increased costs of extracting these fossil fuels, are pushing up the price of fertilizer.

Likewise, phosphorus fertilizer comes from a mined ore, another dwindling nonrenewable resource. “At current rates of use, we’re going to run out of phosphorus before we run out of petroleum,” Lynch says. “So the idea that everybody can farm with fertilizers like they do in the United States and Europe—well, it’s not going to happen.”

Jimmy Burridge examines root density and angle (photo by Rodger Bosch)

Jimmy Burridge, Ph.D. candidate in horticulture, examines root density and angle on a specific maize genotype. Photo by Rodger Bosch

Worldwide, soils are degraded from erosion and deteriorate year after year. In Africa, more than 75 percent of cropland is severely degraded, and because of cost, farmers use negligible amounts of fertilizers. Limited irrigation infrastructure and access to water also hamper production for poorer farmers, and the increased likelihood of drought due to climate change will only amplify their vulnerability to water shortages. “It’s no longer possible to fertilize our way out of the problem,” Lynch says. So the next solution is to farm with plants that don’t need fertilizer or large amounts of water. This is what what Lynch calls the “second Green Revolution.”

The challenge is how to identify maize and bean lines that are genetically predisposed to be water and nutrient efficient, specifically in their root systems. So Ukulima Root Biology Center researchers planted five thousand maize genotypes in the sandy bushveld soil on the Buffett farm in December 2009. At points in the following growing season, they dug out three plants for each genotype, washed the soil from the roots, and systematically mapped each plant’s root structure, including the number and diameter of different root types, how they branched, and their growth angle. This was noted along with stem diameter and the general state of the plant’s health. Soil cores were taken alongside the plant’s growing site to establish the depth to which its roots grew. Root samples were sent to Penn State for analysis.

Similarly, stress tests were done on additional plots of maize and beans: 22 lines of maize were treated with high, medium, and low nitrogen levels to see how they’d respond. Fifty-seven lines of dry beans were subjected to drought stress, and 20 lines of beans received combinations of high and low phosphorus levels, while some were well watered and others subjected to drought conditions.

Genetic maps exist for all these plant lines. Once researchers record and analyze the plant’s physical structure, they can narrow down what belowground traits might make the plant hardier, and identify what genes might be responsible for those traits.

Back at the farmhouse, dusk has drawn in a bat, which dashes hither and thither, plucking insects from the air as they swarm around the veranda light. Lynch points to an animated computer model on his laptop that shows how quickly rain flushes nitrates down into the soil and the depths to which a plant’s roots must grow if they’re to reach the nutrients. “You can see how the roots are growing, trying to catch that nitrate,” he says, “but as the nitrates move, they’re getting out of reach of the roots. It’s a race.”

Jonathan Lynch (photo by Steve Williams)

Jonathan Lynch

Only half of the nitrogen that farmers apply as fertilizer is actually taken up by the plant. The rest is flushed away into the soil and wasted. But Lynch’s team has identified maize root traits that improve nitrogen uptake by 36 percent. Improving efficiency to this degree is a huge benefit both in terms of saving on the high cost of fertilizer and preventing nutrient pollution in the environment surrounding farmlands.

Experiments have shown that maize plants with lots of air space in their roots grow deeper, so they are more able to reach water. Even under forced drought stress, these plants stay healthy and yield noticeably better corn cobs relative to lines with lower root air spaces. Lots of air space is an advantage for the plant, Lynch explains, because those spaces aren’t taken up by cells that require energy, sugar, phosphorus, and nitrogen.

“It’s all about roots that are steep, cheap, and deep,” he says. Steep roots bore down to the nutrients; deep roots reach the water; and cheap roots, with large amounts of air present, demand fewer resources from the plant in order to grow and reach water and nutrients.

Once these traits are linked to specific genes, maize and bean breeders can use this information when selecting material for breeding future, tougher hybrids that can produce high yields even in a post-peak-oil world where the impacts of climate change are challenging many poor farmers.