Agronomy Degree Practice Exam

How can you determine if nitrogen-fixing bacteria have infected your legume crop?

1. Internodes are present

2. Nodes are present

3. Nodules are present

4. Root hairs are present

The correct answer is: Nodules are present

To determine if nitrogen-fixing bacteria have infected a legume crop, the presence of nodules is the key indicator. Nodules are specialized structures that form on the roots of legumes as a result of a symbiotic relationship between the plant and specific soil bacteria, primarily from the genus Rhizobium. These nodules are where the nitrogen-fixing process occurs, allowing the plant to convert atmospheric nitrogen into a form that can be utilized for growth and development. The formation of nodules is crucial because it signifies not only the presence of the bacteria but also an active interaction between the plant and these microorganisms. In healthy and properly functioning legume plants, these nodules will typically appear as small, rounded growths on the roots. Their color can vary based on the stage of the infection; fresh nodules may appear pink due to the presence of leghemoglobin, which helps maintain an anaerobic environment for the bacteria to carry out nitrogen fixation. Internodes, nodes, and root hairs do not specifically indicate the presence of nitrogen-fixing bacteria. While they are important structures for plant growth and nutrient uptake, their presence does not confirm a successful infection by nitrogen-fixing bacteria. Therefore, checking for nodules is the definitive method for assessing the infection and