Plants can tolerate a specific amount of micronutrient. A slight lesser amount of it can cause deficiency symptom and a slight higher amount can cause toxicity. The mineral ion concentration which reduces the dry weight of a tissue by 10% is called toxic concentration.
This concentration is different micronutrients as well as for different plant, e.g., `Mn^(2+)` is toxic beyond `600 mu g g ^(-1)` for soyabean and beyond `5300mu g g^(-1)` for sunflower. It is very difficult to identify the toxicity symptoms of mineral ion. It is because excess uptake of one element can reduces the uptake of other element at a time.
e.g., manganese `(Mn^(2+))` becomes toxic when absorbed by plants in higher amounts. The toxicity is expressed in form of brown spots surrounded by chlorotic vein.
It is due to the following
(i) Reduction in uptake of `Fe^(3+)` and `Mg^(2+)`.
(ii) Inhibition of binding of `Mg^(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.
