Growth Characteristics And Roles Of 11 Species Of Wetland Plants In Polluted Water Purification

As an important component of the aquatic ecosystem, wetland plants have important ecological functions, In recent years, phytoremediation technology is increasingly being used in the purification and restoration of heavily polluted water because of its low cost and high efficiency. Previous studies have showed significant corelation between water purification ability and physioecological characteristics, such as growth characteristics, root features, photo synthetic efficiency, root oxygen loss, and so on. But it remains unclear how these factors effect on water purification, and which one plays the leading role. Therefore, this study was carried out to investigate the adaptation mechanism and roles of wetland plants in wastewater purification from the angle of physiological ecology. The aim was to provide theory basis for the application of phytoremediation technology in wastewater treatment.We have drawn conclusions as follows after hydroponic experiments under greenhouse conditions and filed application, which were related to the activities for the growing features, physioecological characteristics and decontamination efficiency of 11 species of wetland plants. These plants were three kinds of rhizomatic root plants (Canna indica, Iris pseudacorus and Pontederia cordata), three kinds of fibril root plants (Cyperus alterniflius, Vetiveria zizanioiaes and Pennisetum americanum) and five mangroves (Sonneratia apetala, Acanthus ilicifolius, Kandelia candel, Aegiceras corniculatum and Bruguiera gymnorrhiza).Firstly, proline can be used as an indicator to indicate the tolerance ability of plants. The change of poline concentration in these 11 species of plants have showed that, the stress to wetland plants grown in high concentration sewage was higher than that in low concentration, while the stress to mangroves was higher than emerged plants. This stress was found mainly to reduce the above-water biomass of wetland plants. The relative growth rates (RGR) order of the 11 species of wetland plants was C. indica≈V. zizanioiaes≈I. pseudacorus> P. americanum≈C. alterniflius> P. cordata> S. apetala≈A. ilicifolius≈K. candel> A. corniculatum≈B. gymnorrhiza.Secondly, plants can absorb soluble nitrogen and phosphorus during growth process. The total gross absorption for N and P was estimated to be 1.05 to 146.55 and 0.34 to 13.91 mg/m2·d. The difference of N and P accumulation by plants mainly came from the biomass differences. The plants have showed good biomass distribution, the stem and leaf biomass accout for over 70% of the total biomass.Thirdly, there was significant positive correlation between root porosity and proline accumulation (p< 0.01). The root porosity also can be used as a tolerance index of plants in a way. Of all these plants, mangroves stood out for its highest root porosity (17.70 to 43.79%), followed by rhizomatic root plants (10.75 to 32.49%). The lowest one was the fibril root plants, only 9.95 to 15.43%. There was significant positive correlation between the root of radial oxygen loss (ROL) rate and porosity of 6 species of emerged plants (p< 0.01), ie the ROL rate of high root porosity (rhizomatic root plants) was higher than the low one (fibril root plants), which were 84.69 to 205.25 and 12.5 to 111.23μmolO2d-1g-1DWroot, respectively. The larger the total ROL amount, the more active the aerobic rhizospherebacterial, which was also useful for the root to absorb more soluble nitrogen and phosphorus.Fourthly, PSⅡmaximum photochemical efficiency (Fv/Fm) value, reflected the potential maximal photosynthetic capacity of plants. For C. indica, I. pseudacorus, P. cordata, C. alterniflius and V. zizanioiaes, their Fv/Fm values have maintained between 0.80 to 0.85, showing the normal physiological condition. The Fv/Fm value of mangroves decreased firstly and then increased, it suggested the adaptability of pollutants. The Yield value reflected the actual photosynthetic efficiency, so the higher the Yield value, the higher the photosynthetic efficiency. Of all these plants,I. pseudacorus, C. indica, P. cordata and C. alterniflius stood out for their highest photosynthetic efficiency, and then were five mangroves; the lowest were V. zizanioiaes and P. americanum. For the high photosynthetic efficiency plant, it was much easier for the above-water part of the plant to transport oxygen down to the root system to help the root to reach deeper anoxic area. Therefore, for I. pseudacorus, C. indica and P. cordata, their roots can reach upper 15 to 20 cm layer, and V. zizanioiaes, C. alterniflius and five kinds of mangroves, can reach upper 10 to 15 cm layer, but the root of P. americanum was below 6 cm. In addition, as the roots distribution deepened, the aerobic and anaerobic biodegradation of nitrate nitrogen and biochemical organic compounds could take place at the same time and show better performance.Fifthly, Stepwise regression analysis showed that:antioxidant enzyme in plant tissues was a very efficient and important mechanism for tolerance and adaptability of wastewater by wetland plants. Nitrogen and phosphorus accumulation in stem and leaf have made an important contribution to nitrogen and phosphorus removal. Root biomass and the total ROL amount had important effect on nitrification, and photosynthesis plays an important role in degradation of nitrate nitrogen and biochemical organic compounds.Sixthly, Principal component analysis (PCA) showed that:(a) the performance of these 11 species of wetland plants was better in low concentration sewage than in high concentration; (b) the 6 species of emerged plants was better than the 5 mangroves in low concentration sewage, of which C. indica have the highest photosynthetic efficiency, the largest biomass and the deepest root distribution. And the P. americanum performed the worst in high concentration sewage.Seventhly, we have been trying to establish an enhanced ecological floating bed system by using fillers and wetland plants, hoping to restore the water quality of the heavily polluted south urban tidal rivers in situ. As a result of our observation and monitoring, the RGR order of the wetland plants was C. indica> Thalia dealbata≈C. alterniflius> P. cordata≈I. pseudacorus. N and P were removed by mowing the above water-part of the plants, the total gross was estimated to be 136.99 to 356.16 and 21.37 to 63.01 mg/m2·d, respectively. Wetland plants showed good root distribution that can reach upper 20 cm layer. Finally, the oxygen-transfer rates was estimated, which was 35.85 to 57.08 gO2·m-2·d-1. Wetland plants showed the advantages of improving the treatment effect of pollutants.In conclusion, antioxidant enzymes in plant tissues combined ROL from roots are very efficient and important mechanisms for tolerance and adaptability of pollutants (N, P and organic compounds) by wetland plants. Well developed root system keeps the nitrogen and phosphorus absorption, root oxygen loss improves the aerobic biodegradation, and photosynthesis plays an important role in degradation of nitrate nitrogen and biochemical organic compounds. This study can provide reference for application of phytoremediation technology.