Manganese Toxicity In Marigold

Iron/manganese (Fe/Mn) toxicity disorder in marigold has been related to high concentrations of Mn in the affected leaves.  In greenhouse crop production, preplant addition of micronutrients in the media combined with constant feed program and low medium pH, create favorable conditions for the development of Mn toxicity.  To investigate the effect of excess supply of Mn on marigold (Tagetes erecta L. ‘First Lady’) growth, appearance, and nutrient [Mn, Fe, Calcium (Ca) and magnesium (Mg)] concentrations, a solution culture study with various Mn concentrations was conducted.  Concentrations of Mn equal or above 6.5mg/L in the nutrient solution, resulted in stunted plants with significant number of small brown spots, interveinal chlorotic patches, and necrotic symptoms on the tips and margins of the leaves.  The concentration 4.5mg/L resulted in stunted plants free of symptoms.  Concentrations below 4.5mg/L Mn resulted in healthy looking plants.  The concentration 4.5mg/L Mn in the nutrient solution, may thus be considered critical toxicity concentration of Mn in marigold.  This concentration yielded 596 mg Mn/Kg total plant tissue dry weight, 1400 mg Mn/Kg dry weight in the tissue of the roots, 360 in the stems, 880 in the new leaves, and 1200 in the old leaves.  Using the methods of this experiment, excessive supply of Mn increased the concentration of Mn in the roots and leaves, increased the concentration of Fe in the roots, and increased the overall concentration of Ca and Mg in the plant.  Concentrations of Mn above 4.5mg/L in the nutrient solution, or above 596 mg Mn/Kg total plant tissue dry weight, result in Mn toxicity in marigold.