Mechanism of Manganese Toxicity and Tolerance of Plants

Abstract

Seedlings of rice (Oryza saliva L.), barley (Hordeum vulgare L.), alfalfa (Medicago sativa L.), cucumber (Cucumis sativus L.), pumpkin (Cucurbita moschata Duch), and tomato (Lycopersicon esculentum Mill.) were grown in culture solutions containing 0.3, 1.0, 3.2, 10, 32, and 100 ppm Mn for 19 days. After the treatment, Mn contents of the tops and the roots were determined. The presence of oxidized Mn on the roots and the leaves was demonstrated by the blue staining test with benzidine, the solubility and bleaching of oxidized Mn in the oxalic-sulfuric acid solution, and the absorption spectrum of manganipyrophosphate. Rice and pumpkin accumulated a higher amount of Mn in the tops than in the roots for all the levels of Mn tested in the culture solutions. Rice accumulated a remarkably high amount of Mn in the tops even at lower levels of Mn in the solution and the tops were able to tolerate a high Mn concentration in their tissues. On the other hand, Mn contents of alfalfa, tomato, and barley were higher in the roots than in the tops. The tops of alfalfa were sensitive to a high Mn concentration in the tissues and the dry weight was reduced to half of the level of the control at a concentration of 1,000 ppm Mn in the tissues. Cucumber accumulated higher amounts of Mn in the roots only at higher Mn levels in the culture solutions. Seedlings of wheat (Triticum aestivum L.) were grown in culture solutions containing 0.3 and 32 ppm Mn, and the amounts of oxidized Mn in the roots were estimated. The oxidized Mn accounted for more than half of the total amount of Mn in the roots at concentrations of both 0.3 and 32 ppm Mn in the solutions. Deposition of oxidized Mn was observed on the roots which displayed a dark brown color and accumulated a large amount of Mn. Deposition of oxidized Mn on the tops was observed only around the trichomes of cucumber. It is suggested that the oxidation of Mn by the roots may play a role in the retention of an excessive amount of Mn in the roots. Excretion of oxidized Mn around the trichomes may account for part of the Mn tolerance mechanism of cucumber leaves.