Answer:
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 for different micronutrients as well as for different plant, e.g.,
is toxic beyond 
for soyabean and
beyond 
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 
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 
and 
.
(ii) Inhibition
of binding of to specific
enzymes.
(iii) Inhibition
of 
translocation in
shoot apex.
Thus, excess
of causes deficiency
of iron, magnesium and calcium.
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