Manganese `(Mn^(2+))` becomes toxic when absorbed by plants in higher amounts. The toxicity expressed in form of brown spots surrounded by chlorotic vein.
It is due to the following reasons
(i) Reduction in uptake of `Fe^(3+)` and `Mn^(2+)`.
(ii) Inhibition of binding of `Mn^(2+)` to specific enzymes.
(iii) Inhibition of `Ca^(2+)` translocation in shoot apex.
Thus, excess of `Mn^(2+)` causes deficiency of iron, magnesium and calcium .
