Symbiotic nitrogen fixation in non-leguminous plants is carried out by

Nitrogen fixation in the root nodules of $Alnus$ is performed by:

Leghaemoglobin occurs in

Give scientific reasons:
$(1)$ Leguminous plants are rich in protein.

Select the correct statements from the given:
$(a)$ Nitrogenase is a heterodimeric protein.
$(b)$ Root hairs curl by action of nitrogenase and plant 'nod factors'.
$(c)$ During symbiotic $N_2$ fixation,$ATP$ is provided by the legume plant.

$A$ crop plant which can grow well even in nitrogen-deficient soil is: