Nitrogen Fixation in Legumes
Legumes can use nitrogen from the air. Peas, beans, clover, and many trees in the bean family share this trait. They do not do the work alone. Soil bacteria called rhizobia live inside root nodules. Together they turn dinitrogen gas into a form the plant can use.
Most plants need nitrate or ammonium from the soil. Air is rich in N₂, but that molecule is tightly bound. Nitrogenase, a bacterial enzyme, breaks that bond. The reaction costs a lot of energy. The plant pays with sugars from photosynthesis. The bacteria pay with fixed nitrogen.
Infection starts in the rhizosphere. Roots release flavonoids. Matching rhizobia answer with Nod factors. Root hairs then curl. An infection thread guides the bacteria inward. Cortical cells divide and form a nodule. Inside, the bacteria become bacteroids. Those cells fix nitrogen.
A pink nodule is usually active. The colour comes from leghaemoglobin. That protein buffers oxygen. Nitrogenase fails if oxygen is high. The nodule therefore keeps a low-oxygen core and still lets respiration run. Green or white nodules often fix little nitrogen.
The partnership is specific. One host may accept only certain rhizobial strains. Soil pH, drought, and excess mineral nitrogen can weaken nodulation. Farmers therefore treat seed with the right inoculant when native strains are poor.
Fixed nitrogen moves into amino acids. The plant builds protein in leaves and seeds. When roots and nodules decay, some nitrogen enters the soil. A following cereal crop can benefit. That is why rotations often include a pulse or a forage legume.
Not all legumes nodulate equally. Some trees and wild herbs fix large amounts. Some crop varieties nodulate poorly. Breeders now look for hosts that form effective nodules under stress. Scientists also study the gene dialogue between plant and microbe.
Nitrogen fixation in legumes links microbiology to farming. The nodule is a controlled factory. Sugar goes in. Usable nitrogen comes out. That exchange reduces the need for synthetic fertiliser and helps keep soil fertile.