Manganese toxicity in standard culture solutions

Summary

Atlas barley plants grown in standard Hoagland culture solutions developed dark brown necrotic spots on the older leaves. The symptoms varied from small freckle-like spots to large blotchy areas and were found to be associated with the concentrations of Mn and B in the culture solutions. An increase in the concentration of Mn or B in the solutions increased the intensity of the spotting. A decrease in the Mn- and B-concentrations to 0.025 ppm, or one-twentieth of the normal Hoagland value, caused the spots to almost completely disappear. Mn- and B-concentrations of 0.025 ppm are optimum only under a particular set of conditions. In deciding what concentration of Mn and B to use the number of plants, volume of solution, macro-salt concentration, and season must be taken into consideration. The recommended Hoagland concentrations of Mn and B were only slightly toxic to lettuce and non-toxic to tomato plants. Barley plants grown in the winter were able to tolerate much higher concentrations of Mn and B before showing toxicity symptoms. Mn- and B-toxicity symptoms on lettuce and barley are compared in photographs.