How do orange peels influence soybean growth?

Soybean is one of the most important crops cultivated around the world and it is the principal grain legume grown in the United States. It is valued because it has a high protein content – around 38%. And that high-protein content makes soybeans important for both human and animal nutrition. Our research team at Auburn is looking at ways to use orange peels to improve the growth and yield of soybeans.

Why more soybeans?

The world population is growing fast and is expected to reach 9.7 billion people by 2050. To feed all these people, we need to increase our crop production worldwide. The most feasible way to do this is to increase yield, in other words, boost the quantity of grain produced by area cultivated. We also need to do this in a sustainable way that allows for food production while protecting the environment.

Maria Leticia evaluating soybean plants in the field. Credit: Dylan Warner

Growth habits of soybeans

Besides having high protein content, soybeans are a valuable crop because they are in the legume family. The plants can pull nitrogen from the air into the soils and use it in the process of growing and producing the soybeans. But they don’t do this alone, they do it through a partnership with particular bacteria.

Soybeans partner with a type of bacteria called rhizobacteria to turn airborne nitrogen into a usable nutrient for the plant. The roots of the soybeans have nodules, providing a home to rhizobacteria. The bacteria metabolizes nitrogen into a form the soybeans can use and in return the soybean plant provides that home, and some sugars that help the bacteria growth. Other types of rhizobacteria can help plants absorb some nutrients and others can protect the plant against pathogens that cause diseases.

Past research showed that soybean seeds inoculated with rhizobacteria before planting grow better than non-inoculated seeds. Greenhouse studies have better results than field studies. This may be because the rhizobacteria are not able to survive very well in agricultural soils and gets outcompeted by the very numerous and diverse bacteria that are already present in that soil.

The extensive soybean roots on the right were treated with rhizobacteria plus orange peel. The soybean roots on the left had no treatment. Roots support plant growth and provide nutrients for good yields. Credit: Maria Leticia

Can orange peels influence soybean growth?

The rhizobacteria need “food” to grow and colonize the soybean roots in order to stimulate plant growth. One type of food source for some strains of rhizobacteria is a carbohydrate called pectin. Pectin is found in fruits and vegetables, and you may recognize it as an ingredient in jams and jellies.

Our research lab looked at reusing a by-product of the orange juice industry that contains pectin to find if it helped soybeans grow: orange peels. They have a high content of pectin. Because the peels are a by-product of orange juice production, they are inexpensive. This new use of orange peels is a possible sustainable material that can increase crop yield.

Comparing soybean growth with (right) and without (left) treatment with rhizobacteria plus orange peel. Credit: Maria Leticia

We are specifically looking for rhizobacteria strains that can survive and grow using the pectin from orange peel and improve the yield of soybean in the field. The results obtained in a greenhouse experiment show that some soybeans are very responsive to this new treatment.

During our research we also noticed that the rhizobacteria plus orange peel treatment could also increase the size of plant’s roots and the size and number of nodules. We think that this can have a positive influence in the amount of nitrogen that soybean can fix from the atmosphere, increasing plant growth and yield without the need of nitrogen fertilizers, improving the sustainability of the agricultural systems.

Answered by Maria Leticia Pacheco da Silva, Auburn University