Mechanism of manganese toxicity and tolerance of plants

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Abstract
Rice (Oryza sativa L. cv. Koshijiwase) seedlings were treated in nutrient solutions containing 0.32, 1.0, 3.2, 10, 32, and 100 ppm of Mn with Si (as 50 ppm SiO2) addition (+Si treatment) or without Si addition (-Si treatment) for 19 days. In the +Si treatment, the top dry weights of the plants grown in 1.0 to 32 ppm Mn solutions were higher than those of the plants grown in a 0.3 ppm Mn solution. In contrast, the top dry weights decreased with increasing Mn concentration in the solution in the -Si treatment. Silicon contents of the tops of the +Si plants showed considerably high values (ca. 12% as SiO2) compared with those of the -Si plants (ca. 0.1% as SiO2) for all the levels of Mn tested in the solution. Transpiration rates of the -Si plants were higher than those of the +Si plants for all the Mn levels in the solutions. Manganese contents of the roots increased and those of the tops decreased by the +Si treatment. In the solutions containing higher levels of Mn, the increased amount of Mn in the roots induced by the +Si treatment was less pronounced than the decreased amount of Mn in the tops. With increasing Mn concentration in the tissues up to ca. 3,000 ppm, the top dry weights of the +Si plants increased slightly whereas those of the -Si plants decreased slightly. The necrotic brown spots caused by Mn toxicity developed gradually from the lower leaves to the middle leaves and subsequently to the upper leaves. The necrotic brown spots were observed on the middle leaves of the -Si plants containing Mn of a concentration of more than ca. 2,000 ppm whereas no symptoms were observed on the leaves of the +Si plants containing a higher concentration of Mn (ca. 3,000 ppm). Peroxidase activities both in the tops and in the roots increased with increasing Mn level in the solution. At the same Mn concentration in the tissues, the peroxidase activities of the +Si plants were slightly lower than those of the -Si plants. For the plants having similar peroxidase activities, the necrotic brown spots were observed on the middle leaves of the -Si plants whereas no symptoms were observed on the leaves of the +Si plants. Neither Si nor Mn affected the peroxidase activity in vitro. It is suggested that the Si supply alleviates the Mn toxicity of rice plants not only by decreasing the Mn uptake by the plant but also by increasing the internal tolerance to an excessive amount of Mn in the tissues.