Which mineral deficiency causes the 'Exanthema' disease in citrus plants?

Which of the following mineral deficiencies may cause a reduction in the growth of leaves?

Deficiency of . . . . . . causes death of shoot and root tips in plants.

Browning of cauliflower takes place due to deficiency of which one of the following elements?

Assertion : Deficiency of sulphur causes chlorosis in plants.
Reason : Sulphur is a constituent of chlorophyll,protein and nucleic acids.

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.