Grey spots of oat are caused by the deficiency of which element?

Define the following terms:
$(1)$ Chlorosis
$(2)$ Necrosis

'Petiole crack' is caused by the deficiency of

The deficiency of this micronutrient results in little leaf disease.

It is observed that the deficiency of a particular element shows its symptoms initially in older leaves and then in younger leaves. Name two elements that are highly mobile and two that are relatively immobile.

'Mottled chlorosis' on the leaves occurs due to the deficiency of