A team of engineers at Washington University in St. Louis has found a sustainable way to boost the growth of a protein-rich bean by improving the way it absorbs much-needed nutrients.

Research scientist Ramesh Raliya and Professor Pratim Biswas, both of the university's School of Engineering & Applied Science, discovered a method to reduce the use of fertilizer from rock phosphorus while yielding improvements in the growth of the food crop through the use of zinc oxide nanoparticles.

Food crops need phosphorus—a resource in limited supply—to grow, and farmers are using more and more phosphorus-based fertilizer as they increase crops to feed a growing world population. However, plants can only use about 42% of the phosphorus applied to the soil; the rest runs off into streams, where it grows algae that pollutes water resources.

"If farmers use the same amount of phosphorus as they're using now, the world's supply will be depleted in about 80 years," Raliya says. "Now is the time for the world to learn how to use phosphorus in a more sustainable manner."

Raliya and his team created zinc oxide nanoparticles from a fungus found around the mung bean plant's root that helps it mobilize and take up the nutrients in the soil. Zinc is an essential nutrient for plants because it interacts with three enzymes that mobilize the complex form of phosphorus in the soil into a form that plants can absorb.

When they applied the zinc nanoparticles to the leaves of the mung bean plant, it increased the uptake of the phosphorus by nearly 11% and the activity of the three enzymes by 84% to 108%. That means less phosphorus needs to be added to the soil, according to Raliya.

Applying zinc nanoparticles to mung bean plant leaves increased phosphorus uptake by 11%. Image credit: Washington University in St Louis.Applying zinc nanoparticles to mung bean plant leaves increased phosphorus uptake by 11%. Image credit: Washington University in St Louis. "When the enzyme activity increases, you don't need to apply the external phosphorus, because it's already in the soil, but not in an available form for the plant to uptake," he says. "When we apply these nanoparticles, it mobilizes the complex form of phosphorus to an available form."

The mung bean is a legume grown mainly in China, Southeast Asia and India, where much of the population is vegetarian and relies on plant-based protein sources. The bean is adaptable to a variety of climate conditions and is very affordable for people to grow.

Raliya says 45% of the phosphorus used globally for agriculture takes place in India and China. Much of the phosphorus supply in developing countries is imported from the United States and Morocco-based rock phosphate mines.

"We hope that this method of using zinc oxide nanoparticles can be deployed in developing countries where farmers are using a lot of phosphorus," Raliya says. "These countries are dependent on the U.S. to export phosphorus to them, but in the future the U.S. may have to help supply food, as well. If this crop can grow in a more sustainable manner, it will be helpful for everyone."

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