Nitrogen fixing bacteria are found in the root of :

Certain soil bacteria, most notably species of Rhizobium, can convert atmospheric nitrogen gas into a form plants can use — a process called biological nitrogen fixation. One important route for this is a symbiotic partnership in which the bacteria colonise specialised swellings called root nodules that develop on the roots of specific host plants; the plant supplies the bacteria with carbohydrates in return for the fixed nitrogen.

Among cultivated crops, this root-nodule partnership is the defining feature of leguminous plants — the pulses, such as gram, pea, moong and lentil. Their roots develop the nodules that Rhizobium colonises, so nitrogen-fixing bacteria are found in the roots of pulses.

Checking the remaining crops confirms this trait is not shared by every plant:

• Wheat is a cereal of the grass family; its roots never form nodules, so it cannot host this symbiosis and instead relies on nitrogen already present in the soil or added as fertiliser.

• Paddy (rice) is likewise a cereal grass; its roots do not develop nodules either, so it too lacks this particular root-based nitrogen fixation.

• Because only one of the three listed crops develops root nodules, the nodulation trait cannot be described as common to all of them.