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

Deficiency of which of the following elements causes the weakening of the pedicel and petiole?

Yellowing of tea leaves occurs due to the deficiency of:

$A$ lower concentration of a mineral element causes $...........$ and a higher concentration causes $.............$.

High fluoride concentration in plants typically results in necrosis or chlorosis in:

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: