The effect of silicon on organ growth and its mechanisms of action were studied in rice ( Oryza sativa L. cv. Koshihikari), oat ( Avena sativa L. cv. Victory), and wheat ( Triticum aestivum L. cv. Daichino-Minori) seedlings grown in the dark. Applying silicon in the form of silicic acid to these seedlings via culture solution resulted in growth promotion of third (rice) or second (oat and wheat) leaves. The optimal concentration of silicon was 5-10 mM. No growth promotion was observed in early organs, such as coleoptiles or first leaves. In silicon-treated rice third leaves, the epidermal cell length increased, especially in the basal regions, without any effect on the number of cells, showing that silicon promoted cell elongation but not cell division. Silicon also increased the cell wall extensibility significantly in the basal regions of rice third leaves. These results indicate that silicon stimulates growth of rice and some other Poaceae leaves by increasing cell wall extensibility.
The effect of lead (Pb) on growth and mechanical properties of cell wall was investigated in rice seedlings. Caryopses of rice were germinated and grown in various concentrations of lead nitrate for 5 days at 250C in the dark. Growth of rice seedlings was suppressed by Pb ions; significant suppression was caused by low concentration of Pb as 1 渭M. Growth suppression was prominent in roots, but not clear in shoot organs, such as coleoptiles or first leaves, suggesting that roots are the primary target of Pb toxicity. The analysis of the cell wall extensibility of rice roots grown in Pb solution indicated that the cell wall extensibility was greatly decreased with increased concentration of Pb ions. These results suggest that Pb may influence the synthesis of cell wall polysaccharides, thereby decreasing the cell wall extensibility, resulting in growth suppression in rice roots.
Salt tolerance in relation to water status and plant nutrients of two mungbean varieties, BARImung 2 (salinity sensitive) and BUmung 2 (salinity tolerant) was evaluated. The seeds were grown in pots and treated with NaCl levels of 0 (control), 100 and 200 mM. Different parameters related to water relations as well as mineral nutritients were measured. The exudation rate and relative water content were decreased but water saturation deficit was increased by salinity in both the varieties. In BARImung 2 plants, the exudation rate and relative water content were lower but water saturation deficit was higher than those in BUmung 2 at both 100 and 200 mM NaCl levels. Salinity also influenced the accumulation of Na, K, Ca and Mg in leaves, stems and roots of the two said mungbean varieties. Sodium accumulation was inceseased in all the plant-parts of both the varieties in the order of stem > root > leaf but in BUmung 2 the accumulation was lower than that of BARImung 2 except in root. Potassium accumulation deceresed in all parts of both the mungbean varieties but that was lower in BUmung 2 than that of BARImung 2. The contents of Ca and Mg in all the plant-parts increased more in BUmung 2 than those of BARImung 2 with the increase of salinity levels. All these results indicated that high salt tolerance in BUmung 2 was associated with its better water status, more or less uniform mineral nutrient (Ca and Mg) distribution in different plantparts than that in BARImung 2.
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