| Manganese (Mn) is a necessary element for plant growth, but excess Mn cause serious chlorosis and inhibite the plants growth, and dramatically increase accumulation of Mn in both shoots and roots, furthermore, inhibite the absorption of Ca, Mg and Zn. Mn toxicity is a serious agricultural problem in acid-soil area. In addition to aluminum (Al), manganese toxicity in acid soil become one of the most important factors which can limit plant growth. We used two kinds of relatively strong accumulation of manganese and manganese-resistant varieties of plants--Conyza canadensis and Solanum nigrum as materials in this test. The pot and hydroponic culture experiment was conducted under different Mn stress to explore plant growth characteristics and transport of manganese, to study the effects of root exudates on soil and rhizosphere micro effects, including soil microorganisms, soil enzyme activity and morphology of soil manganese, to reveal the resistance to manganese toxicity and manganese enrichment process of reconciliation of the two enrichment plants. Further more, it could provide a theoretical basis for the applications of manganese and promote further research of phytoremediation of contaminated soil.The results were summarized as follows:1. Leaf area, root length and survival rate of the two plants significantly decreased in various degree with the increase of Mn concentration, and plant height first increased compared with control(0.005 mmol/L) and then decreased gradually. Ascorbic Acid(ASA) concentrations in leaves of Solanum nigrum and Conyza canadensis significantly increased in low manganese concentration, then ASA contents had a reduced trend with the increasing manganese concentration, but were still greater than control. Dehydro ascorbate(DHA) content significant increased in manganese concentration of 16mmol/L The ASA content of Solanum nigrum was more than that of Conyza canadensis in various corresponding concentration of manganese, and the rate of increase levels of DHA was smaller than Conyza canadensis. Total ascorbic acid content of two plants was rising, the proportion of reduced ascorbic acid content of the total content ascorbic acid was decreased with the increasing manganese concentration.2. The Chlorophyll contents remarkably decreased with increasing Mn content. The responses of chlorophyll fluorescence parameters in leaves of these two species indicated that maximum quantum yield(Fv/Fm), maximum fluorescence(Fm), effective quantum yield of photosystem II(Yield), electron transfer rate(ETR) also obviously decreased with an increase of Mn. On one hand, their minmal fluorescence(Fo) first decreased and then increased with the increment of Mn concentration at the medium level, and on the other hand, there was an upward trend in non-photochemical quenching(NPQ)of these two species,but it did not show significant difference among various Mn concentrations. The results above indicated that electronic transmition process was inhibited and electronic transmition rate was decreased in Photosynthesis of these two species. Growth and development of Solanum nigrum and Conyza canadensis were influenced to different degrees by Mn toxicity, and Solanum nigrum showed stronger tolerance to Mn toxicity than Conyza canadensis, so it was more suitable for phytoremediation of Mn polluted areas.3. Mn concentration in the leaves stems roots of Conyza canadensis and Solanum nigrum significantly increased with increased Mn concentration (P<0.01) and Mn was preferentially accumulated in the leaf. With the increase of the concentration of manganese, Mn content in the organization of Solanum nigrum and Conyza canadensis significantly increased and reached the maximum at 8 mmol/L. The order of Mn absorption in tissue was:leaves>stems>roots. Mn contents in the organizations of Solanum nigrum was more than those of Conyza canadensis in various corresponds Mn concentrations. The absorption capacity of Mn of these two plants did not meet the requirements of being hyperaccumulator, but they showed good metal-enrichment of Mn and have good tolerant ability of Mn.4. Little Phytochelatins(PCs) could be detected in roots and leaves of these two plants under Mn stress, which could be caused by shorter inducing time or PCs were not responsive to Mn stress. The content of Glutathione(GSH) and Metallothionein-like(MTL) were more sensitive to Mn stress, and they presented in similar change patterns. MTL and GSH increased rapidly first and then dropped rapidly with the increase in the concentration of manganese and it indicated that there were some correlation between the physiochemical values and Mn concentrations in plants's tissues due to increasing the supply of Mn2+. The contents of Totle Non-protein SH(TNP-SH) and GSH were significantly (P<0.05)promoted in 8mmol/L Mn2+ when compared to the control respectively. The content of MTL in Solanum nigrum significantly increased under 8 mmol/LMn2+, while this of Conyza canadensis was promoted significantly under 2 mmol/L Mn2+ and 4 mmol/L Mn2+. The MTL and MTL were maintained at a higher level in Solanum nigrum in comparison with Conyza canadensis under the corresponding concentration. GSH and MTL could be effective response index to Mn stress. Solanum nigrum showed stronger tolerance to Mn toxicity than Conyza canadensis,so it was more suitable for phytoremediation of Mn polluted areas.5. Plant polyphenol content in leaves of these two plants increased under low manganese concentration, but it didn't have significant dose effect, and then the polyphenol content gradually decreased with increasing manganese concentration. There was a little change of tannin contentin in leaves of these two plants. Extractable Condensed Tannin(ECT), Protein bound Condensed tannin(PCT) and Fibre bound Condensed tannin(FCT) content in leaves increased with increasing concentration of manganese. With the increase of Mn concentration, ECT concentration showed a downward trend. The the proportion of ECT, PCT and FCT in the TCT were not significantly affected under Mn stress.6. The fungi quantity decreased gradually and a fluctuation in the population of bacteria and actinomyces was noticed with increasing Mn conten. Lower Mn concentration could stimulate the activities of urease and phosphatase, the higher Mn concentration could inhibit the activities of urease, protease, invertase, catalase and phosphatase to varying degrees and raise polyphenol oxidase activity. The correlation analysis of Conyza canadensis showed that there was a very significant negative correlation between different Mn stress and the activities of soil enzyme and the correlation degree was invertase> protease>polyphenol oxidase>urease>phosphatase>catalase.There was a significant positive correlation (P<0.05)in the activities of urease, polyphenol oxidase, protease, phosphatase, invertase, so these enzymes were of the similar adaptability to Mn stress. On the other hand, the activities of soil enzyme and the correlation degree of Solanum nigrum was urease> invertase> phosphatase> protein> catalase> polyphenol oxidase. There was a significant or very significant positive correlation among urease, protease, phosphatase, invertase of Solanum nigrum, while there was a significant correlation between catalase and urease, invertase, phosphatase. Activities of urease and invertase was mostly sensitive to Mn stress among these six soil enzymes, indicating that the invertase can be used as a biological index of manganese pollution.7. With the increase of Mn concentration, various forms of Mn content significantly increased.There was a significant or very significant negative correlation between different Mn chemical forms and 6 kinds of soil enzyme activity.There was not a significant negative correlation between total Mn content and soil enzyme activity, but there was little negative correlation between urease activity and total Mn content. Over all, there was a significant negative or positive correlation between urease, acid phosphatase, catalase activity and weakly adsorbed Mn, specific adsorption Mn. Because the correlation between the contents of different Mn forms and soil enzyme activities was higher than the correlation between total Mn content and soil enzyme activities, the relationship between the contents of different Mn forms can be used as the main biological index for evaluating Mn pollution degree of red roam. |
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