Myriophyllum alterniflorum biochemical changes during in vitro Cu/Cd metal stress: Focusing on cell detoxifying enzymes.

Given the toxicity of trace metals, their concentration, speciation and bioavailability serve to induce various plant detoxification processes, which themselves are specific to several parameters like plant species, tissue type and developmental stage. In this study, Myriophyllum alterniflorum (or alternate watermilfoil) enzyme activities (ascorbate peroxidase, catalase, glutathione peroxidase and superoxide dismutase) from in vitro cultures was measured over 27 days in response to copper (Cu) or cadmium (Cd) stress. These enzymes are unique to reactive oxygen species (ROS) scavenging (mainly hydrogen peroxide H2O2 and superoxide anion O2-) and moreover showed specific or unspecific activity profiles, depending on the metal concentrations used. Our results suggest a higher-priority protection of chloroplasts during the initial days of exposure to both metals. At the same time, the increased catalase activity could indicate an H2O2 diffusion in peroxisome in order to protect other organelles from ROS accumulation. However, as opposed to the Cd effects, high Cu concentrations appear to induce a "limited oxidative threshold" for some antioxidant enzymes, which could suggest an ion absorption competition between Cu2+ and Fe2+. In spite of an overall analysis conducted of the scavenging processes occurring in plant cells, biochemical analyses still yielded relevant indications regarding the watermilfoil strategies used for ROS management.

Effects of ammonium pulse on the growth of three submerged macrophytes.

Ammonium pulse attributed to runoff of urban surface and agriculture following heavy rain is common in inland aquatic systems and can cause profoundly effects on the growth of macrophytes, especially when combined with low light. In this study, three patterns of NH4-N pulse (differing in magnitude and frequency) were applied to examine their effects on the growth of three submersed macrophytes, namely, Myriophyllum spicatum, Potamogeton maackianus, and Vallisneria natans, in terms of biomass, height, branch/ramet number, root length, leaf number, and total branch length under high and low light. Results showed that NH4-N pulse caused negative effects on the biomass of the submerged macrphytes even on the 13th day after releasing NH4-N pulse. The negative effects on M. spicatum were significantly greater than that on V. natans and P. maackianus. The effects of NH4-N pulse on specific species depended on the ammonium loading patterns. The negative effects of NH4-N pulse on P. maackianus were the strongest at high loading with low frequency, and on V. natans at moderate loading with moderate frequency. For M. spicatum, no significant differences were found among the three NH4-N pulse patterns. Low light availability did not significantly aggregate the negative effects of NH4-N pulse on the growth of the submersed macrophytes. Our study contributes to revealing the roles of NH4-N pulse on the growth of aquatic plants and its species specific effects on the dynamics of submerged macrophytes in lakes.

Comparative studies of the response of sensitive and tolerant submerged macrophytes to high ammonium concentration stress.

Three submerged macrophytes, Ceratophyllum demersum (CD), Myriophyllum spicatum (MS) and Myriophyllum aquaticum (MA), were treated with various concentrations of ammonia for different lengths of time. Ammonium ions (NH4+) in the medium severely inhibited plant growth and led to a reduction in total chlorophyll (chl a and b) in CD and MS. The addition of ammonia significantly decreased the soluble protein content and increased the free amino acid content of CD and MS in treatments with high concentrations of NH4+, but MA showed no significant physiological response. The antioxidant enzyme system of MA was activated, which in turn reduced the peroxidation level in the plant and maintained the plant's normal physiological activities when the ammonia nitrogen in the culture fluid increased. The study continued to use higher concentrations (25, 50, 100, 200 and 400 mg/L) of ammonium nitrogen to treat and observe the peroxidation level and corresponding enzyme production for this species of MA in vivo to explore its resistance mechanism. The experiments show that MA can normally live for a period of time in a high-ammonia environment of up to 100 mg/L. The results of the present study will assist in studies of the detoxification of high ammonium ion contents in submersed macrophytes and the selection of plants suitable for macrophyte recovery.

Effects of nanoplastics and microplastics on the growth of sediment-rooted macrophytes.

Plastic debris of all sizes has been detected in marine, terrestrial and freshwater habitats. Effects of plastic debris on macrophytes have hardly been studied, despite their importance in aquatic ecosystems. We provide the first experimental study exploring nano- and microplastic effects on the growth of sediment-rooted macrophytes. Myriophyllum spicatum and Elodea sp. were exposed to sediments amended with six doses of polystyrene (PS) nanoplastic (50-190 nm, up to 3% sediment dry weight) and PS microplastic (20-500 µm, up to 10% dry weight) under laboratory conditions. Both macrophyte species were tested for changes in root and shoot dry weight (DW), relative growth rate (RGR), shoot to root ratio (S:R), main shoot length and side shoot length. Microplastics did not produce consistent dose-effect relationships on the endpoints tested, except that main shoot length was reduced for M. spicatum with increasing microplastic concentration. Nanoplastic significantly reduced S:R for both macrophytes as a result of increased root biomass compared to shoot biomass. Nanoplastic also caused a decrease in M. spicatum main shoot length; however, shoot biomass was not affected. Elodea sp. side shoot length, root and shoot biomass and RGR were positively correlated to the nanoplastic concentration. All effects occurred at higher than environmentally realistic concentrations, suggesting no immediate implications for ecological risks. Our study did not aim for the elucidation of the exact mechanistic processes that cause the effects, however, particle size seems to play an important factor. CAPSULE: Nano- and microplastics affect growth of sediment-rooted macrophytes.

Comparative in vitro/in situ approaches to three biomarker responses of Myriophyllum alterniflorum exposed to metal stress.

Surface water pollution by trace metal elements constitutes problems for both public and terrestrial/aquatic ecosystem health. Myriophyllum alterniflorum (alternate watermilfoil), an aquatic macrophyte known for bioaccumulating this type of pollutant, is an attractive species for plant biomonitoring within the scope of environmental research. The two metal elements copper (Cu) and cadmium (Cd) are considered in the present study. Cu is essential for plant development at low concentrations, while very high Cu concentrations are detrimental or even lethal to most plants. On the other hand, Cd is usually toxic even at low concentrations since it adversely affects the physiological plant functions. In order to check whether watermilfoil could be used for the in situ biomonitoring of Cu or Cd pollution in rivers, the plant biomarker sensitivity is first tested during long-term in vitro assays. Three markers specific to oxidative stress (glucose-6-phosphate dehydrogenase, malondialdehyde and α-tocopherol) are evaluated by varying the pollutant concentration levels. Given the absence of effective correlations between Cu and all biomarkers, the response profiles actually reveal a dependency between Cd concentration and malondialdehyde or α-tocopherol biomarkers. Conversely, preliminary in situ assays performed at 14 different localities demonstrate some clear correlations between all biomarkers and Cu, whereas the scarcity of Cd-contaminated rivers prevents using the statistical data. Consequently, the three indicated biomarkers appear to be effective for purposes of metal exposure analyses; moreover, the in situ approach, although preliminary, proves to be paramount in developing water biomonitoring bases.

Cadmium accumulation and subcellular distribution in populations of Hylotelephium spectabile (Boreau) H. Ohba.

A pot experiment was conducted among six populations of Hylotelephium spectabile (Boreau) H. Ohba: four from Jiangsu province, one from Shandong province, and one from Shanxi province, China, to investigate the variation of Cd accumulation and subcellular distribution of this species (a newly reported Cd high accumulator). Under five different real Cd-contaminated soils (Cd: 0.93-97.97 mg/kg), results showed considerable differences in Cd concentration in (a) leaf (1.09-50.7 mg/kg), (b) stem (0.61-13.0 mg/kg), and (c) root (1.55-24.5 mg/kg) among the populations. Analysis of subcellular Cd distribution indicated that Cd accumulated in the leaves of H. spectabile was mainly in the cellular debris (44.1 to 53.5%), followed by heat-stable protein (HSP, 20.9 to 29.0%), Cd-rich granules (MRG, 9.9 to 19.5%), heat-denatured protein (6.0 to 8.5%), and organelle fractions (3.1 to 7.4%). The populations of H. spectabile with more Cd partitioned to cellular debris and biological detoxified metal (HSP + MRG) fractions have greater capacity to accumulate Cd, indicating the probable intrinsic mechanism to accumulate Cd. Therefore, H. spectabile has the considerable potential of phytoremediation for Cd-contaminated soils, but screening suitable populations according to soil Cd concentrations is necessary before used for phytoremediation of Cd-contaminated soils.

The adaptability of a wetland plant species Myriophyllum aquaticum to different nitrogen forms and nitrogen removal efficiency in constructed wetlands.

Constructed wetlands (CWs) cultivated with Myriophyllum aquaticum showed great potential for total nitrogen (TN) removal from aquatic ecosystems in previous studies. To evaluate the growth characteristics, photosynthetic pigment content, and antioxidative responses of M. aquaticum, as well as its TN removal efficiency in CWs, M. aquaticum was treated with different levels of ammonium (NH4+) and nitrate (NO3-) for 28 days. The results indicated that M. aquaticum had strong nitrogen stress tolerance and was more likely to be suppressed by high levels of NH4+ than NO3-. High levels of NH4+ also led to inhibition of synthesis of photosynthetic pigments and increased peroxidase activity in plant leaves, which was not found in the NO3- treatments. High levels of both NH4+ and NO3- generated obvious oxidative stress through elevation of malondialdehyde content while decreasing superoxide dismutase activity in the early stage. A sustainable increase of TN removal efficiency in most of the CWs indicated that M. aquaticum was a candidate species for treating wastewater with high levels of nitrogen because of its higher tolerance for NH4+ and NO3- stress. However, the increase of TN removal efficiency was hindered in the late stage when treated with high levels of NH4+ of 26 and 36 mmol/L, indicating that its tolerance to NH4+ stress might have a threshold. The results of this study will enrich the studies on detoxification of high ammonium ion content in NH4+-tolerant submerged plants and supply valuable reference data for proper vegetation of M. aquaticum in CWs.