| Manganese, an essential trace element that is found in varying amounts in all tissues, is one of the most widely used metals in industry. Manganese-contaminated soils and waters are becoming an environmental concern in China following increased awareness of the need for environmental protection. Manganese is however also an essential micronutrient and an activator of a number of enzymes involved in the tricarboxylic acid cycle. Higher plants can tolerate the presence of large amounts of metals in the substrates by two basic stragies:exclusion and accumulation. Hyperaccumulator plants are advantageous for phytoremediation.In the present work, the physiological indices, the antioxidant system, biomass, soluble manganese content, photosynthesis and chlorophyll fluorescence parameters of Phytolacca americana L.were determined by hydroponics with different concentrations of high-manganese and the manganese toxicity effect on its physiology and structure was discussed; the manganese toxicity effect on its structure was also discussed by microscopic and ultramicroscopic methods. The major results obtained are as follows:Seed germination rate, germination energy and mitotic index of root cells decreased with the increase of manganese level. High level of manganese had a greater effect on the seedling growth, especially on the roots, conductivities, MDA content, Pro content and soluble sugar content rose and the contents of AsA and chlorophyll decreased with the increase of manganese concentrations, while protein content increased at first, and then declined.POD and CAT contents increased significantly with the concentration of manganese varying from 1000μmol·L-1 to 8000μmol·L-1, and the trend of which slowed down when the concentration of manganese was higher than 8000μmol·L-1, while the SOD contents rose gradually. The content change of POD, CAT and SOD treated by manganese in 15 d was similar to the one in 30 d. H2O2 and O2- contents can effectively verify antioxidant boycotting the damage of generated free radicals from manganese toxicity.Determination of soluble manganese contents was taken during the growth environment with manganese toxicity. It was found that the root is the absorption and storage site of poisonous manganese, while the leaf is the storage site and the stem mainly is transfer part. The root became tubby firstly and then slim with the increase of manganese concentration, and the leaves were somewhat etiolated and browned. The effect on reproductive organs was more than on vegetative organs. At 15000μmol·L-1 concentration, the life-cycle can not be completed. At 12000μmol·L-1 concentration, the reproductive organs can not produce new generations.Determination of fluorescence parameters and photosynthetic characteristics was taken and found that manganese stress was likely to cause chlorophyll contents, some gas exchange parameters and chlorophyll fluorescence parameters to decline. The reason for net photosynthetic rate declining was nonstomatal limitation. Chlorophyll a/b value and the transpiration rate at manganese concentration of 1000μmol·L-1 were maximum. The maximum quantum yield of PSII photochemistry (Fv/Fm), the effective quantum yield of PSII photochemistry (EQY), Photosynthetic electron transport rate (ETR) were gradually decreased. Moreover the photochemical quenching (qP) reached the maximum at 5000μmol·L-1 of manganese concentration, and nonphotochemical quenching also reached the maximum at 5000 (μmol·L-1 of manganese concentration when photosynthetic active radiation (PAR) was higher than 239μmol photons·m-2·-1.High concentrations of manganese had a greater effect on the primary structure of root. Cortical cells were contracted and condutive tissues were undermined. The leaf structure was seriously undermined with the increase of manganese concentration. Secondary structure in root was less affected, while the mature leaf has obvious structural changes, especially when the manganese concentration was higher than 12000μmol·L-1. The chloroplast content in the palisade tissue and the spongy tissue in leaves was on a gradual decrease.Manganese toxicity had a greater effect on the ultramicroscopic structure. Cells, chloroplasts and mitochondria changed little in size and morphology. Nucleus disintegrated at manganese concentration of 12000μmol·L-1 and changed little at other concentrations. Black precipitate existed in vacuoles and cell gaps at higher manganese concentration. The internal structure of chloroplast changed, and granas and stroma lamella changed with the disorder and disintegration. Starch grains reduced and had indistinct edges, and the number of osmiophilic bodies increased continually.
|
PDF Full Text Request