A plants require magnesium for chlorophyllsynthesis All four sings bind with metalatom magnesium $\left(\mathrm{Mg}^{++}\right)$, Which remain present at the centre of chlorophyll molecule. Nitrogen is the main component for the synthesis of protein. Cell wall is made up cellulose, hemicellulose and pectin components.
Related Theory
Magnesium is highly mobile in the plant and deficiency symptoms first appear on the lower leaves. Symptoms are more severe on the lower leaves because magnesium is moved to the new growth. Deficiency symptoms consist of interveinal chlorosis (leaf veins stay green while the regions between them turn yellow).