Question: 100: Which one of the following produces nitrogen fixing nodules on the roots of Alnus?
(1) Rhodospirillum
(2) Beijerinckia
(3) Rhizobium
(4) Frankia
Answer: Option (4)
Explanation:
Nitrogen fixation in plants can occur through symbiotic associations with specific microorganisms.
Alnus is a non-leguminous plant that forms symbiotic root nodules with the actinomycete Frankia.
Frankia fixes atmospheric nitrogen and supplies it to the host plant in usable forms.
Rhizobium forms nodules mainly in leguminous plants, not in Alnus.
Rhodospirillum and Beijerinckia are free-living nitrogen-fixing bacteria and do not form root nodules in Alnus.
Therefore, Frankia is responsible for producing nitrogen-fixing nodules on the roots of Alnus.