Symptome von Mangan‐Überschuß bei Bohnen (Phaseolus vulgaris)

Symptoms of manganese toxicity in beans (Phaseolus vulgaris L) In water culture experiments with bush bean plants (Phaseolus vulgaris L. var. „Red Kidney”︁) the macro‐ and micro‐symptoms of manganese toxicity had been studied. Typical symptoms of Mn toxicity are small distinct blackish‐brown spots, first at the older leaves and at more severe toxicity also at the petals and stems. Using the autoradiographic technique on 54Mn‐distribution and comparing this with corresponding light microscopic studies it could be demonstrated that the blackish‐brown spots in the leaves resemble Mn‐precipitations. These precipitations are Mn‐compounds of higher oxidated form and are primarily localized in the walls of cells in vicinity of the vessels. The extend to which these precipitations occur is not only a function of the Mn‐content of the leaves but also of their age. At comparable Mn‐content the extend of the precipitation increases with the leaf age. As secondary symptom of Mn‐toxicity under certain conditions (presence of silicon e.g.) induced Fe deficiency (chlorosis of the youngest leaves) and Ca deficiency („crinkle leaf”︁) may occur. It is assumed that the process of Mn precipitation within the leaves is enzymatically regulated. The severe reduction in the growth of leaf blades at Mn toxicity is presumably related to an interference with the growth hormon balance.