What is mycorrhiza?

The correct answer is Option (2) → Symbiotic association of fungi with roots of plants.

“Myco” means fungus and “rhiza” means root. A mycorrhiza is a symbiotic association between a fungus and the roots of a plant. Many members of the genus Glomus form it. The fungus absorbs phosphorus from the soil and passes it to the plant, and the plant gives the fungus carbohydrates.

Option (1) Fungi that fix atmospheric nitrogen. — Incorrect. Fixing atmospheric nitrogen is the work of bacteria and cyanobacteria, such as Rhizobium, Azotobacter and Anabaena. The fungus of a mycorrhiza takes up phosphorus, not nitrogen.

Option (2) Symbiotic association of fungi with roots of plants. — Correct. The fungus lives in association with the plant roots and both gain. Plants with mycorrhiza also show resistance to root-borne pathogens, tolerance to salinity and drought, and better growth.

Option (3) Symbiotic association of bacteria with plants. — Incorrect. An association of bacteria with plants, such as Rhizobium in the root nodules of legumes, is not a mycorrhiza. A mycorrhiza always has a fungus as the partner.

NCERT Extract:
Microbes as Biofertilisers – “… Fungi are also known to form symbiotic associations with plants (mycorrhiza). Many members of the genus Glomus form mycorrhiza. The fungal symbiont in these associations absorbs phosphorus from soil and passes it to the plant. Plants having such associations show other benefits also, such as resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in plant growth and development.”
Population Interactions – “… Similarly, the mycorrhizae are associations between fungi and the roots of higher plants. The fungi help the plant in the absorption of essential nutrients from the soil while the plant in turn provides the fungi with energy-yielding carbohydrates.”