Induction Of Phytochelatins By Arsenic Species In Hydrilla Verticillata(Linn.F.) Royle. And Its Influencing Factors

In an aquatic ecosystem, aquatic plants are very important since they maintain the stability of ecosystem. In order to reconstruct and restore the structure and function of aquatic ecosystem which was contaminated by heavy metals, it is necessary to understand the tolerance mechanisms of aquatic plants in polluted environment. The inductive synthesis of phytochelatins is one of adaptive mechanisms of plants to tolerate heavy metal pollution. However,the research of phytochelatins(PCs) mainly focused on lower algae and terrestrial plants, and little information is available regarding the mechanisms of PC synthesis in aquatic plants. In the present research, Hydrilla verticillata (Linn.f.) Royle. was chosen for tested plants, and reversed phase high performance liquid chromatography (RP-HPLC) was used. The hydroponic experiments were conducted to investigate the effects of different As speciations [As(III), As(V) and DMA] and influencing factors (Pb, Cd, NH4+-N, NO3--N and pH) on As uptake of H. verticillata and on PCs synthesis. Then the cell ultrastructure of this plant and the subcellular distribution of arsenic were examined, aiming to investigate where the3species of arsenic located in leaf cells of H. verticillata. This objectives of this paper is:(a) reveal the effects of As stress on the growth of H. verticillata;(b) find the potential functions of PCs in the process of aquatic plant resisting to heavy metal stress; and (c) provide scientific basis for pollution control and evaluation of arsenic-polluted water body. The main results are listed as follows:(1) An obvious uptake and accumulation of arsenic was observed in H. verticillata. Low concentrations of arsenic promoted plant growth and the uptake of As(III) was significantly higher than that of As(V) or DMA. A significant synthesis of GSH, PC2and PC4was noted in As(III) and As(V) treatment, respectively, and their contents were significantly positively correlated to the concentrations of As in Hydrilla verticillata (P<0.05). Phytochelatins were not sensitive to DMA treatment, with a synthesis only in0.3mg·L-1treatment. With the increasing concentrations of As(III), an increase at first but decrease then later of GSH and PC2synthesis was observed, but the PC4synthesis increased; with the increasing concentrations of As(V), an increase at first but decrease then later of GSH synthesis was observed, but the PC2and PC4synthesis increased. Very few PC3was synthesized under different concentrations of As speciation, with a small quantity detected only in0.3mg·L-1treatment. The results showed that the synthesis of PCs was sensitive to the stress of As(III) and As(V), and PCs could be selected as an biomarker under the stress of these two As speciation.(2) Different external factors had different effects on As uptake of H. verticillata and on PCs synthesis.①Under the compounded contamination of As, Pb and Cd, an independent action was found among these three heavy metals. There were no influences each other for the contents of heavy metals in H. verticillata. Arsenic concentrations significantly affected the contents of GSH, PC2and PC4, while Pb concentrations have no impact on that. Different concentrations of Cd have notable influence on the contents of PC2but neither GSH nor PC4.②The concentrations of As(III) was significantly higher at NH4+-N treatment than at NO3--N treatment in H. verticillata when exposed to3.0mg As·L-1. When the pH value of culture solution was adjusted to pH=5, both the As uptake and PCs synthesis were remarkably increased, but they significantly decreased when the pH=9.③When the plants of H. verticillata were cultured in the3.0mg·As(V) L-1, NO3--N was better to promote the uptake of As(V) than NH4+-N at1.0and10.0mg·L-1N treatments. But the result is reversed when the N concentration was designed as5.0mg-L"1. The effects of pH values on As uptake and PCs synthesis were the same as As(Ⅲ) treatments.(3) Different As speciations affected the distribution of As in leaf cells of H. verticillata. Arsenic was mainly distributed in cell wall and cytoplasm but little in chloroplast and mitochondria when As(III) or As(V) was added. However, arsenic was mainly distributed in the cell wall but little in cytoplasm at DMA treatments. The leaf cells of H.verticillata exposed to3.0As(III) treatment was severely damaged compared with the same concentrations of As(V) and DMA after7-day culture. It was discovered that the toxic effect of As on H. verticillata was not the destruction of one organelle, but the overall damage of the membrane and non-membrane structures. Different organelles had different tolerances to As. The tolerance of chloroplast is the best but the dictyosome is the worst..