Symbiotic nitrogen-fixing bacterium is . . . . . . .

Root nodules are found in which of the following plants?

Which of the following are the part or example of symbiotic mutualistic association?
$I.$ Yeast
$II.$ Rhizobium
$III.$ Mycorrhiza
$IV.$ Oscillatoria

Which of the following is a symbiotic nitrogen fixer?

What is the function of leghemoglobin?

We find that $Rhizobium$ forms nodules on the roots of leguminous plants. Also,$Frankia$,another microbe,forms nitrogen-fixing nodules on the roots of the non-leguminous plant $Alnus$. Is it necessary for a microbe to be in close association with a plant to provide mineral nutrition? Explain with the help of one example.