Death of leaf tissues in plants is caused by the deficiency of:

The die-back disease in Citrus is due to deficiency of . . . . . . .

$A$: Manganese toxicity causes brown spots,chlorotic veins,and inhibition of calcium translocation to the shoot apex.
$R$: Toxic effects may be due to direct excess of micronutrients or their interference in the absorption and functioning of other nutrients.

In plants,where does chlorosis occur due to magnesium deficiency?

Deficiency symptoms of mineral elements appear first in young leaves. - $P$
Deficiency symptoms of mineral elements appear first in senescent (older) leaves. - $Q$
$P$ and $Q$ are respectively:

Absence of $Mg^{++}$ ions from plant tissue results in