Soybean root nodules that are actively fixing nitrogen are what color on the inside?

The inside of soybean root nodules that are actively fixing nitrogen is typically pink or red. This coloration is indicative of the presence of leghemoglobin, a protein that serves to facilitate the transport of oxygen to the nitrogen-fixing bacteria within the nodules. Leghemoglobin also protects these bacteria from oxygen, as they operate in anaerobic conditions to convert atmospheric nitrogen into a form that the plant can utilize. The pink or red color is an important visual marker for farmers and agronomists to assess the health and nitrogen-fixing capability of legumes such as soybeans.

This vibrant color is due to the iron-containing heme group in leghemoglobin, similar to the red color in hemoglobin in animal blood. It signifies that the bacteria are actively metabolizing and that the plant is benefiting from this symbiotic relationship, essential for optimal growth and yield.