In plants,the root system is well-developed due to:

Necrosis,or death of tissue,particularly leaf tissue,is due to the deficiency of ........

'Khaira disease of rice' is due to

Interveinal chlorosis of leaves is caused by the 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.

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: