| To explore the concentrations of Si in different plants and the ecophysiological effect of Si on plant, the Si concentration in mature leaves of 48 species of vascular plants along the Zhangjiang River was studied. Furthermore, the effect of Si on the salt-tolerant ability of mangrove plant was studied with Aegiceras corniculatum seedlings cultivated in Hoagland solution for 120 days. We expected that the importance of Si would be recognised, and provided theoretical gist for understanding the mechanism of salt-tolerance of Aegiceras corniculatum and the practice of revegetation of degraded forests in tideland area. The results showed that:1. The average Si concentration in mature leaves of 48 plant species was 1.23%. Among these plants, the Si concentration in Hippochaete debile leave was the highest, up to 5.88%, and Rhodomyrtus tomentosa was the lowest, only 0.02%. The Si concentration varied significantly in different plant groups. The plants of Graminae, Equisetaceae, Osmundaceae, Pteridaceae, Commelinaceae and Moraceae were rich in Si. 17 of the 48 plant species were Si accumulators, in which, the average Si concentration was 2.53%, other 31 species were non-accumulators, in which, the average Si concentration was 0.28%.2. We also noticed that from the upper reaches to the lower reaches along the Zhangjiang River, with the increase of water concentration and decrease of the grain size of soil, the Si concentration in soil increased accordingly, and the salinity was higher in the mangroves tideland soil in lower reaches than the upper reaches. The Si concentrations of the Lantana camara and Commelina communis had positive correlations with the effective Si concentration in the soil, which distributed wide along the river; while the Si concentrations of Aegiceras corniculatum and Kandelia obovata had positive relationships with the salinity in the soil.3. The survival rate and the stem height of Aegiceras corniculatum seedlings were the highest under salinity 10‰. Aegiceras corniculatum seedlings can not grow well in the situations of higher or lower salinity. Both the survival rate and the stem height of the seedlings were promoted at each gradation in the group 200mg·L-1 Si was added, moreover the change of survival rate and the stem height were gentler, approaching the level under the medium salinity. Si may promote the survival rate of Aegiceras corniculatum seedlings and the stem height of seedlings under salt-stress (low salinity and high salinity), made it restore close to the value of salinity 10‰.4. With salinity increase, Chla, Chlb and total Chl concentrations all presented the tendency that rose first and then dropped, and the concentrations all reached the peak at 10‰salinity. But the Chla/b value was toward a tendency of droping along with salinity increase, although had a small scope fluctuation. Compared with the group without Si, the change tendency of Chla, Chlb and total Chl concentrations in the with Si group was similar, and the only difference was the change relatively gentler. But the Chla, Chlb and Chl concentrations at all salinity were much higher than the group without Si. After added Si, the Chla/b value displayed a tendency of droping with salinity increase, the lowest was at salinity 35‰, but the change was quite gentler compared to the shift of Chla, Chlb and total Chl concentrations.5. Regardless of with Si or without, the MDA concentration in the leaves of Aegiceras corniculatum seedlings was higher than in the roots. On the whole, in the situation with Si and without Si, the MDA concentrations in the leaves and roots presented a similar change tendency with salinity increase, that was dropped at first and then rose. In the treatment without Si, compared with CK, the MDA concentration in mature leaves of the medium salinity reduced obviously; but when added Si the change scope of MDA concentration was less in the leaves. Si kept the MDA concentration at a lower level in the leaves. The MDA concentration in the roots was lower than in the leaves, and it changed greatly in the group without Si. At salinity 10‰the MDA concentration was the lowest, and the increasing was obvious relative to CK when salinity was higher. After adds Si, the MDA concentration was much lower in the roots, while the change was gentler under each salinity.6. The Pro concentrations in the leaves and roots of the seedlings were not in line with salinity. In the situation with Si and without Si, the Pro concentration in the leaves and roots of the Aegiceras corniculatum seedlings all presented a tendency that rose first and then dropped with the salinity increase. Moreover in both situations, the Pro concentration in the leaves was higher than in the roots. When Si was absent, the Pro concentration in the leaves at medium salinity reduced obviously compared with CK. But in the situation of salinity 35‰, the Pro concentration was 1.21 times as high as CK. When Si was added, the Pro concentration in the leaves was in the trough at salinity 10‰, and the peak concentration was at salinity 35‰. When Si was added,Pro concentrations in the leaves and the roots reduced relatived to the group without Si at each salinity. The Pro concentration in the roots was lower than in the leaves, and the change was remarkable compared to the group without Si. After added Si, the Pro concentration in the roots reduced, moreover the change was slighter at each salinity.7. In the situation with Si and without Si, the SOD activity in the leaves presented rising with the salinity increase. It reached maximizations at salinity 10‰, then dropped. Moreover the SOD activities in the leaves of all salt-stressed groups were higher than the CK. Without Si, the SOD activity reached to the peak at salinity 10‰, and was 1.20 times as CK. At salinity 35‰the SOD activity was only 95.92% of the CK. When Si was added, the peak value at salinity 10‰was 1.17 times as CK, the trough was the CK itself.8. In the situation with Si and without Si, the change tendency of POD activity in the leaves was consistent basically, which rose first and then dropped with the salinity increase. But the salinity at the peak was different slightly. The POD activity of the group without Si reached the peak at salinity 15‰, but the activity at salinity 10‰was also high. The difference between them was not very clearly. In the situation with Si and without Si, the change tendency of POD activity in the roots was extremely similar to in the leaves, both reached the peak at salinity 10‰. The POD activity in the roots with Si was higher compared to without Si, but the superiority was not very obvious.9. With Si concentration increase, the survival rate and the stem height of the Aegiceras corniculatum seedlings reached the peak at Si concentration of 100mg·L-1. Before reaching the peak, the survival rate and the stem height rose fastly with Si concentration increase, but when the Si concentration increased ulteriorly, the survival rate and the stem height dropped gradually. It was very obvious that medium concentration of Si had a promoter action to the survival and the stem growth of Aegiceras corniculatum seedlings, but highly concentrated Si had a suppressant effect on seedlings.10. The Chla, Chlb and Chl concentrations at each Si gradient were higher than the CK in the leaves of the Aegiceras corniculatum seedlings, and all the three reached the peak when the Si concentrations was 100mg·L-1. But after that the Chla, Chlb and Chl concentrations all reduced with the Si concentration increase. When the Si concentration rose, the Chla/b value increased undulatorily in the leaves of the seedlings.11. With Si concentration increase, the MDA concentration in the leaves dropped first and then rose, and the lowest MDA concentration was 65.67% of the CK when the Si concentrations was 100mg·L-1. The change scope of MDA concentration in the roots was narrow. The minimum MDA concentration was 75.61% of the CK when the Si concentrations was 100mg·L-1. Under each Si gradient, the MDA concentration in the roots was lower than in the leaves. The effect of Si on the MDA concentration in the leaves was strong, but not so abvious in the roots.12. The Pro concentrations in the leaves and roots of Aegiceras corniculatum seedlings all presented the tendency that first dropped and then rose with Si concentration increase. When the Si concentrations was 100mg·L-1, the Pro concentrations in the leaves and roots were both lowest. But the Pro concentration in the roots was lower than in the leaves at each Si gradient, moreover the change was gentler relatively.13. Treated with different concentration of Si, the change tendency of SOD activity in the leaves and roots of Aegiceras corniculatum seedlings was the same approximately, first dropped and then rose with Si concentration increase. Though the SOD activity in the leaves rose and dropped at different Si concentrations treatment, but the undulation scope was not wide, and the minimum activity was 89.63% of CK. In the roots, the change of SOD activity was even slighter.14. With Si concentration increase, the POD activity in the leaves and roots of Aegiceras corniculatum seedlings dropped first, but it rose after reached the minimum when the Si concentrations was 100mg·L-1. In the roots, the change tendency of POD activity was similar to the leaves nearly. Although under various Si concentration, the POD activity in the roots was lower than in the leaves, but the change scope of POD activity in roots was wider than in the leaves.
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