Clarification of role of silicon on the enhancement of manganese tolerance in higher plants.

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Mechanisms of Si-enhanced Mn tolerance were investigated in leaves of pumpkin and cowpea.When pumpkin plants were supplied with Si under excess Mn, toxicity symptoms were clearly avoided without decreasing the total Mn. Manganese in the apoplastic washing fluids (AWF) of the plants supplied with Si were lower than 10% of those without Si supply. Fractionation of Mn in the cell walls into exchangeable, Mn oxide, and residual forms indicated that at 2 days after excess Mn treatment, the exchangeable Mn of the plants supplied with Si was larger than that of the plants without Si supply. At 4 days after the treatment, larger amounts of Mn oxide were found for the plants supplied with Si. These plants also showed larger amounts of Si and Mn in the cellulose fraction from their cell walls. Thus, Si supply enhances Mn tolerance of the pumpkin plants by decreasing the apoplastic soluble Mn through the incorporation of excess Mn into the cell walls, in addition to the enhanced adsorption of Mn on, the cell walls.Silicon supply also alleviated Mn toxicity of cowpea without decreasing the total leaf Mn. The Mn in the AWF of the plants supplied with Si was about 40% of those without Si supply. The AWF were extracted from the leaves of the plants treated at different timing of Si supply under excess Mn to examine the relationships between the severities of Mn toxicity and the concentrations of Mn and Si in the AWF. The severities of Mn toxicity were not significantly correlated with Mn in the AWF but were significantly correlated with Si. The results indicated that the decrease of Mn in the AWF caused by Si supply was insufficient to explain the enhanced Mn tolerance in cowpea. An important role of apoplastic soluble Si was suggested.