Nitrogen Fixation and Legume Growth

Background

Bacteria as a domain have an incredibly versatile set of metabolic capabilities. Though more typical pathways like glycolysis are often explored, there are also unique reactions that are essential to all life on earth. Nitrogen fixation is an important example of these unfamiliar capabilities; basically, nitrogen gas from the atmosphere can be processed by certain bacteria which allow it to become available for life (called bioavailability) through the process of “fixing” the nitrogen into organic compounds. Furthermore, nitrogen fixing bacteria are often found in coordinated “hubs” at the tips of plant roots (called root nodules), where they help stimulate plant growth by providing an accessible source of nitrogen.

 The interaction of plants’ roots and microorganisms is an example of a symbiotic relationship called mutualism—both the bacteria and the plants benefit from their interaction. The plants provide nutrients to the bacteria in the form of sugars, vitamins, and growth factors, and the bacteria provide help to the plants in acquiring water, minerals, and elements that the plants can’t get alone. Furthermore, some of these bacteria secrete substances like plant growth hormones that help the plants to grow, stimulate the creation of root hairs, or even enhance germination rates! Each bacterial species has a particular host it needs to be in a relationship with— it’s a specific interaction. Not any bacteria can fix nitrogen and not just any plant can support these bacteria.

There are other mutualistic relationships between plant roots and fungi called mycorrhizae. While we will not be studying this kind of interaction it is worth mentioning that in this relationship, the plants are protected from certain kinds of pathogens that enter through the roots. In this way, plants can live in areas where they would otherwise not be able to survive. This also occurs in food crops: beans, tomatoes, strawberries, grapes, and even cotton.

Objective

Design an experiment that qualitatively tests the effect of nitrogen fixation on bean or pea plants.

Determine which kinds of bacteria are capable of forming a mutualistic relationship with bean or pea plants through the formation of root nodules.

Materials

Cups

Soil

Peas, beans, legumes, alfalfa, etc. 

Cultures: Commercial Legume Inoculant, Bacillus sp., Rhizobium sp.

Experimental Design

Create an experiment that will test the effect of nitrogen fixation on bean or pea plants. Be sure to consider that not all bacteria fix nitrogen or can form root nodules with plants. Your plants will be allowed to grow for a maximum of 3 weeks. Your results should be able to determine WHICH bacteria forms nodules by how it affects the plant growth.

1.   Write out the steps for approval by your instructor:

2.   Write (a sentence) about your expected results on nodule formation in plant growth: 

Record the Results

Appearance of each plant should be recorded, including the color and number of leaves. On the 3^rd week you should also explore the roots. (You obviously can’t examine the roots in the first two weeks without killing the plants.)

Questions

1.   Was your hypothesis supported?

2.   If you could redesign the experiment, what might you do differently?

3.   Which bacteria are responsible for creating a mutualistic relationship with bean/pea roots?

4.   Which plants grew better? When did you first observe this difference in growth?

5.   If modern chemical fertilizers are used to add nitrogen, phosphorous, and potassium in the soil, and they in fact inhibit the formation of root nodules and the ability of these bacteria to fix nitrogen, what do you think we should do to improve farming practices?

6.   Do you think you can use too much bacteria when inoculating the beans or peas? Too little? Why or why not?

References

Atlas, R.M., Bartha, R. (1998) Microbial Ecology: Fundamentals and Applications. New York: Addison Wesley Longman

Postma, J.J., Forch, M.G., (2014) Wageningen University & Research. Mycorrhiza for fighting Phytophthora in strawberries. https://www.wur.nl/en/newsarticle/Mycorrhiza-for-fighting-Phytophthora-in-strawberries.html