The 2009 ASHS Annual Conference

Iron/manganese (Fe/Mn) toxicity disorder in marigold has been related to high concentrations of Mn and low concentrations of magnesium (Mg) in the affected leaves.  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 in greenhouse crops.  Deficiency of Mg is due in part to low medium pH and to lack of Mg in many of the fertilizers used in greenhouse production.  To investigate the effect of Mg on Mn toxicity effects (growth, appearance, and nutrient concentrations) on the plant, a solution culture study with various Mg and Mn concentrations was conducted in the greenhouse.
In this experiment, the treatments 10/6.5, 10/4.5, 5/6.5, and 5/4.5mg/L Mg/Mn solution concentrations resulted in stunted plants with significant number of small brown spots, interveinal chlorosis and necrotic symptoms on the tips and margins of the leaves, which are typical symptoms of Mn toxicity in marigold.  The treatments 10/0.5, 15/4.5, and 20/4.5, resulted in stunted plants free of symptoms.  Treatment 20/6.5 and 15/6.5 resulted in stunted plants with Mn toxicity symptoms. The treatment 20/0.5 and 15/0.5 resulted in fully-grown and healthy looking plants. 
Concentrations of Mg as high as the incipient deficiency concentration reduced the critical toxicity solution concentration of Mn in marigold from 4.5 to 2.5mg/L.
High concentrations of Mn (above 2.5mg/L) in the nutrient solution affected the plant appearance and reduced the dry weight of the plants.  High Mn also increased the concentration of Mn in the roots and leaves, increased the concentration of Fe in the roots, reduced the concentration of Fe in the stem and old leaves, reduced the concentration of Calcium (Ca) in the leaves, and reduced the concentration of Mg in the plant. 
Magnesium concentration in the nutrient solution had no significant effect on the effect of excessive Mn in the medium on Mn, Fe, Ca or Mg concentrations in the plant tissue. 
Under the conditions of this experiment, Mg did not affect the effect of excess Mn in the medium on Mn content in the plant tissue, but Mg deficiency reduced the medium critical toxicity concentration from 4.5 mg/L to 2.5/L.  Concentrations of Mg, equal or lower than the incipient deficiency concentration (10mg/L nutrient solution) increased the sensitivity of marigold to high concentrations of Mn in the medium (concentrations of Mn above 2.5mg/L nutrient solution.)