•Any mineral ion concentration in tissues that reduces the dry weight of tissues by about 10 per cent is considered toxic.
•Many a times, excess of an element may inhibit the uptake of another element.•For example, the prominent symptom of manganese toxicity is the appearance of brown spots surrounded by chlorotic veins.
•It is important to know that manganese competes with iron and magnesium for uptake and with magnesium for binding with enzymes
• Manganese also inhibit calcium translocation in shoot apex.
•Therefore, excess of manganese may, in fact, induce deficiencies of iron, magnesium and calcium
•Thus, what appears as symptoms of manganese toxicity may actually be the deficiency symptoms of iron, magnesium and calcium
•Micronutrients or trace elements, are needed in very small amounts (less than 10 mmole Kg^ –1 of dry matter). These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.
•Manganese: It is absorbed in the form of manganous ions (Mn2+). It activates many enzymes involved in photosynthesis, respiration and nitrogen metabolism
•The best defined function of manganese is in the splitting of water to liberate oxygen during photosynthesis.
