Nitrate Reductase Enzyme Research Articles

Lighting up phytoplankton cells with quantum dots

Quantum dots (Qdots) are semiconducting nanocrystals composed of periodic elements with different intrinsic band‐gap energies that yield unique fluorescent signatures. Unlike conventional organic fluorophores, Qdots are photo‐chemically stable and have a wide absorption spectrum, but a narrow, tunable emission spectrum. Multiple colors can be imaged from a single excitation wavelength allowing for labeling of many different target sites (e.g., membrane proteins) simultaneously.We conjugated Qdots to primary antibodies specific for the soluble enzyme nitrate reductase (NR) and a light‐harvesting structural protein localized in the chloroplast, the fucoxanthin chlorophyll a/c protein (FCP), in the marine diatom Skeletonema costatum. By fluorescence microscopy, we successfully detected NR and FCP in single cells of S. costatum with a clarity and definition that was not obtainable with conventional organic fluorophores. Biotinylated cells labeled with Qdot‐strepavidin conjugate and Qdot‐FCP immuno‐labeled cells were detected by flow cytometry. Qdot bioconjugates provide an alternative photostable probe for surface or intracellular protein immuno‐localization in the study of marine bacteria and phytoplankton metabolism and physiology.

Growth on nitrate and occurrence of nitrate reductase‐encoding genes in a phylogenetically diverse range of ectomycorrhizal fungi

Ectomycorrhizal (ECM) fungi are often considered to be most prevalent under conditions where organic sources of N predominate. However, ECM fungi are increasingly exposed to nitrate from anthropogenic sources. Currently, the ability of ECM fungi to metabolize this nitrate is poorly understood. Here, growth was examined among 106 isolates, representing 68 species, of ECM fungi on nitrate as the sole N source. In addition, the occurrence of genes coding for the nitrate reductase enzyme (nar gene) in a broad range of ectomycorrhizal fungi was investigated. All isolates grew on nitrate, but there was a strong taxonomic signature in the biomass production, with the Russulaceae and Amanita showing the lowest growth. Thirty-five partial nar sequences were obtained from 43 tested strains comprising 31 species and 10 genera. These taxa represent three out of the four clades of the Agaricales within which ECM fungi occur. No nar sequences were recovered from the Russulaceae and Amanita, but Southern hybridization showed that the genes were present. The results demonstrate that the ability to utilize nitrate as an N source is widespread in ECM fungi, even in those fungi from boreal forests where the supply of nitrate may be very low.

Phosphorylation of the Yeast Nitrate Transporter Ynt1 Is Essential for Delivery to the Plasma Membrane during Nitrogen Limitation

Ynt1 is the sole high affinity nitrate transporter of the yeast Hansenula polymorpha. It is highly regulated by the nitrogen source, by being down-regulated in response to glutamine by repression of the YNT1 gene and Ynt1 ubiquitinylation, endocytosis, and vacuolar degradation. On the contrary, we show that nitrogen limitation stabilizes Ynt1 levels at the plasma membrane, requiring phosphorylation of the transporter. We determined that Ser-246 in the central intracellular loop plays a key role in the phosphorylation of Ynt1 and that the nitrogen permease reactivator 1 kinase (Npr1) is necessary for Ynt1 phosphorylation. Abolition of phosphorylation led Ynt1 to the vacuole by a pep12-dependent end4-independent pathway, which is also dependent on ubiquitinylation, whereas Ynt1 protein lacking ubiquitinylation sites does not follow this pathway. We found that, under nitrogen limitation, Ynt1 phosphorylation is essential for rapid induction of nitrate assimilation genes. Our results suggest that, under nitrogen limitation, phosphorylation prevents Ynt1 delivery from the secretion route to the vacuole, which, aided by reduced ubiquitinylation, accumulates Ynt1 at the plasma membrane. This mechanism could be part of the response that allows nitrate-assimilatory organisms to cope with nitrogen depletion.

Involvement of nitrate reductase in auxin-induced NO synthesis

It is well known for a long time, that nitric oxide (NO) functions in variable physiological and developmental processes in plants, however the source of this signaling molecule in the diverse plant responses is a very obscure.1 Although existance of nitric oxide sythase (NOS) in plants is still questionable, LNMMA (NG–monomethyl-L-arginine) -sensitive NO generation was observed in different plant species.2,3 In addition, nitrate reductase (NR) is confirmed to have a major role as source of NO.4,5 This multifaced molecule acts also in auxin-induced lateral root (LR) formation, since exogenous auxin enhanced NO levels in regions of Arabidopsis LR initiatives. Our results pointed out the involvement of nitrate reductase enzyme in auxin-induced NO formation. In this addendum, we speculate on auxin-induced NO production in lateral root primordial formation.Addendum to: Kolbert Zs, Bartha B, Erdei L. Exogenous auxin-induced NO synthesis is nitrate reductase-associated in Arabidopsis thaliana root primordia. J Plant Physiol 2008; 165:967-75.

Impacts of some divalent cations on periplasmic nitrate reductase and dehydrogenase enzymes of Escherichia, Pseudomonas and Acinetobacter species

The impacts of Hg2+, Cd2+ and Zn2+ on the activities of periplasmic nitrate reductase (NAP) and dehydrogenase (DHA) enzymes of three organisms isolated from soil and sediment-water interface were analysed in liquid culture studies. NAP and DHA activities were estimated from nitrite and triphenyl formazan were produced respectively after 4h incubation at 28 ± 2oC. Hg2+ completely inhibited NAP activity in Escherichia and Pseudomonas spp at all the concentrations (0.2 – 1mM) while progressive inhibitions of NAP activity were observed in Escherichia and Pseudomonas spp with increasing concentrations of Zn2+ and Cd2+. Both metals were stimulatory to NAP of Acinetobacter sp at 0.2 – 1mM. Apart from stimulation of DHA activity by Zn2+ (0.2 – 1mM) in Escherichia sp, Cd2+ (0.4 -1.0mM) in Acinetobacter sp and (1.0mM) in Pseudomonas sp, all the metals progressively inhibited DHA activities in the three organisms. In Escherichia sp, the activities of the two enzymes were negatively correlated on exposure to Zn2+ (r = -0.91) and positively correlated (r = >0.90) on exposure to Cd2+ and Hg2+. Based on IC50 values of the metals for the DHA and NAP enzymes, the most resistant of the three organisms were Escherichia sp and Acinetobacter sp respectively. Quantitatively, NAP with its lower IC50 values than DHA was a more sensitive toxicity measure for Hg2+ in all the organisms. The sensitivity of microbial metabolic enzymes to the toxic effects of metals varies with the type of enzyme, metal and the microorganism involved.

Heat Stress Stimulates Nitric Oxide Production in Symbiodinium microadriaticum: A Possible Linkage between Nitric Oxide and the Coral Bleaching Phenomenon

Nitric oxide (NO) is a gas displaying multiple physiological functions in plants, animals and bacteria. The enzymes nitrate reductase and NO synthase have been suggested to be involved in the production of NO in plants and algae, but the implication of those enzymes in NO production under physiological conditions remains obscure. Symbiodinium microadriaticum, commonly referred to as zooxanthellae, is a marine microalga commonly found in symbiotic association with a cnidarian host including reef-building corals. Here we demonstrate NO production in zooxanthellae upon supplementation of either sodium nitrite or L-arginine as a substrate. The nitrite-dependent NO production was detected electrochemically and confirmed by the application of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a specific NO scavenger. Cells stained with the diaminofluorescein, DAF-2 DA, an NO fluorescent probe, showed an increase in fluorescence intensity upon supplementation of both sodium nitrite and L-arginine. Microscopic observations of DAF-stained cells verified that NO was produced inside the cells. NO production in S. microadriaticum was found to increase upon exposure of cells to an acute heat stress which also caused a decline in the photosynthetic efficiency of PSII (F(v)/F(m)). This study provides substantial evidence to confirm that zooxanthellae can synthesize NO even when they are not in a symbiotic association with a coral host. The increase in NO production at high temperatures suggests that heat stress stimulates the microalgal NO production in a temperature-dependent manner. The implications of these findings are discussed in the light of the coral bleaching phenomenon which is associated with elevated sea surface temperature due to global warming.

Role of Campylobacter jejuni Respiratory Oxidases and Reductases in Host Colonization

Campylobacter jejuni is the leading cause of human food-borne bacterial gastroenteritis. The C. jejuni genome sequence predicts a branched electron transport chain capable of utilizing multiple electron acceptors. Mutants were constructed by disrupting the coding regions of the respiratory enzymes nitrate reductase (napA::Cm), nitrite reductase (nrfA::Cm), dimethyl sulfoxide, and trimethylamine N-oxide reductase (termed Cj0264::Cm) and the two terminal oxidases, a cyanide-insensitive oxidase (cydA::Cm) and cbb3-type oxidase (ccoN::Cm). Each strain was characterized for the loss of the associated enzymatic function in vitro. The strains were then inoculated into 1-week-old chicks, and the cecal contents were assayed for the presence of C. jejuni 2 weeks postinoculation. cydA::Cm and Cj0264c::Cm strains colonized as well as the wild type; napA::Cm and nrfA::Cm strains colonized at levels significantly lower than the wild type. The ccoN::Cm strain was unable to colonize the chicken; no colonies were recovered at the end of the experiment. While there appears to be a role for anaerobic respiration in host colonization, oxygen is the most important respiratory acceptor for C. jejuni in the chicken cecum.

Nitrogen metabolism enzymes, soluble protein and free proline content in soybean genotypes and their F1 hybrids

Nitrate reductase and glutamine synthetase activity, as well as free proline and soluble protein content were measured in eight soybean parent genotypes and six F1 hybrids. The aim of this study was to determine variability and the mode of inheritance for these parameters, and point out the genotypes of interest for future breeding programs. Analysed genotypes and their F1 hybrids expressed significant differences in activities of nitrate reductase and glutamine synthetase enzymes, as well as in soluble proteins and free proline contents. Since mode of inheritance for all investigated traits was in most cases dominance or heterosis, it can be concluded that these parameters are under control of dominant genes. The obtained results suggest that genotypes with favorable traits, such as variety Linda, line 1511, and F1 hybrids (Linda x LN92-7369) and (Balkan x BL-8), could be of interest as a good starting material for further breeding programs.

A high-affinity molybdate transporter in eukaryotes

Molybdenum is an essential element for almost all living beings, which, in the form of a molybdopterin-cofactor, participates in the active site of enzymes involved in key reactions of carbon, nitrogen, and sulfur metabolism. This metal is taken up by cells in form of the oxyanion molybdate. Bacteria acquire molybdate by an ATP-binding-cassette (ABC) transport system in a widely studied process, but how eukaryotic cells take up molybdenum is unknown because molybdate transporters have not been identified so far. Here, we report a eukaryotic high-affinity molybdate transporter, encoded by the green alga Chlamydomonas reinhardtii gene MoT1. An antisense RNA strategy over the MoT1 gene showed that interference of the expression of this gene leads to the inhibition of molybdate transport activity and, in turn, of the Mo-containing enzyme nitrate reductase, indicating a function of MoT1 in molybdate transport. MOT1 functionality was also shown by heterologous expression in Saccharomyces cerevisiae. Molybdate uptake mediated by MOT1 showed a K(m) of approximately 6 nM, which is the range of the lowest K(m) values reported and was activated in the presence of nitrate. Analysis of deduced sequence from the putative protein coded by MoT1 showed motifs specifically conserved in similar proteins present in the databases, and defines a family of membrane proteins in both eukaryotes and prokaryotes probably involved in molybdate transport and distantly related to plant sulfate transporters SULTR. These findings represent an important step in the understanding of molybdate transport, a crucial process in eukaryotic cells.

Nitric oxide synthesis and signalling in plants

As with all organisms, plants must respond to a plethora of external environmental cues. Individual plant cells must also perceive and respond to a wide range of internal signals. It is now well-accepted that nitric oxide (NO) is a component of the repertoire of signals that a plant uses to both thrive and survive. Recent experimental data have shown, or at least implicated, the involvement of NO in reproductive processes, control of development and in the regulation of physiological responses such as stomatal closure. However, although studies concerning NO synthesis and signalling in animals are well-advanced, in plants there are still fundamental questions concerning how NO is produced and used that need to be answered. For example, there is a range of potential NO-generating enzymes in plants, but no obvious plant nitric oxide synthase (NOS) homolog has yet been identified. Some studies have shown the importance of NOS-like enzymes in mediating NO responses in plants, while other studies suggest that the enzyme nitrate reductase (NR) is more important. Still, more published work suggests the involvement of completely different enzymes in plant NO synthesis. Similarly, it is not always clear how NO mediates its responses. Although it appears that in plants, as in animals, NO can lead to an increase in the signal cGMP which leads to altered ion channel activity and gene expression, it is not understood how this actually occurs. NO is a relatively reactive compound, and it is not always easy to study. Furthermore, its biological activity needs to be considered in conjunction with that of other compounds such as reactive oxygen species (ROS) which can have a profound effect on both its accumulation and function. In this paper, we will review the present understanding of how NO is produced in plants, how it is removed when its signal is no longer required and how it may be both perceived and acted upon.

Regulation of nitrate reductase by nitric oxide in Chinese cabbage pakchoi (Brassica chinensis L.)

Nitrate reductase (NR), a committed enzyme in nitrate assimilation, involves generation of nitric oxide (NO) in plants. Here we show that the NR activity was significantly enhanced by the addition of NO donors sodium nitroprusside (SNP) and NONOate (diethylamine NONOate sodium) to the culturing solution, whereas it was decreased by NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (cPTIO). Interestingly, both NO gas and SNP directly enhanced but cPTIO inhibited the NR activities of crude enzyme extracts and purified NR enzyme. The cPTIO terminated the interaction between NR-generated NO and the NR itself. Furthermore, the NR protein content was not affected by the SNP treatment. The investigation of the partial reactions catalysed by purified NR using various electron donors and acceptors indicated that the haem and molybdenum centres in NR were the two sites activated by NO. The results suggest that the activation of NR activity by NO is regulated at the post-translational level, probably via a direct interaction mechanism. Accordingly, the concentration of nitrate both in leaves and roots was decreased after 2 weeks of cultivation with SNP. The present study identifies a new mechanism of NR regulation and nitrate assimilation, which provides important new insights into the complex regulation of N-metabolism in plants.

Exogenous auxin-induced NO synthesis is nitrate reductase-associated in Arabidopsis thaliana root primordia

Nitric oxide (NO) functions in various physiological and developmental processes in plants. However, the source of this signaling molecule in the diversity of plant responses is not well understood. It is known that NO mediates auxin-induced adventitious and lateral root (LR) formation. In this paper, we provide genetic and pharmacological evidence that the production of NO is associated with the nitrate reductase (NR) enzyme during indole-3-butyric acid (IBA)-induced lateral root development in Arabidopsis thaliana L. NO production was detected using 4,5-diaminofluorescein diacetate (DAF-2DA) in the NR-deficient nia1, nia2 and Atnoa1 (former Atnos1) mutants of A. thaliana. An inhibitor for nitric oxide synthase (NOS) N(G)-monomethyl-l-arginine (l-NMMA) was applied. Our data clearly show that IBA increased LR frequency in the wild-type plant and the LR initials emitted intensive NO-dependent fluorescence of the triazol product of NO and DAF-2DA. Increased levels of NO were restricted only to the LR initials in contrast to primary root (PR) sections, where NO remained at the control level. The mutants had different NO levels in their control state (i.e. without IBA treatment): nia1, nia2 showed lower NO fluorescence than Atnoa1 or the wild-type plant. The role of NR in IBA-induced NO formation in the wild type was shown by the zero effects of the NOS inhibitors l-NMMA. Finally, it was clearly demonstrated that IBA was able to induce NO generation in both the wild-type and Atnoa1 plants, but failed to induce NO in the NR-deficient mutant. It is concluded that the IBA-induced NO production is nitrate reductase-associated during lateral root development in A. thaliana.

Involvement of carbon monoxide produced by heme oxygenase in ABA-induced stomatal closure in Vicia faba and its proposed signal transduction pathway

Carbon monoxide (CO) has recently proven to be an important bioactive or signaling molecule in mammalian cells. Its effects are mainly mediated by nitric oxide (NO) and cyclic GMP (cGMP). In Vicia faba leaves, CO production and heme oxygenase (HO) activity, an important CO synthetic enzyme, are first reported to increase in response to ABA treatment, which could result in stomatal closure. Interestingly, ABA-induced stomatal closure in V. faba guard cells is partially blocked when the synthetic CO inhibitor ZnPP, or the CO/NO scavenger Hb is added. Furthermore, we show that, exogenously applied CO donor, hematin, and CO aqueous solution not only result in the enhancement of CO release, but also time-dependently induce stomatal closure, and the latter is mimicked by the application of an NO donor SNP. The above-mentioned stomatal closure effects are differentially reversed by the addition of tungstate, a potent inhibitor of NO synthetic enzyme nitrate reductase (NR), the specific NO scavenger cPTIO, ZnPP, or Hb. During treatment for 4 h, SNP, 0.01% CO aqueous solution or hematin significantly triggers NO synthesis, whereas cPTIO, or tungstate approximately fully inhibits NO fluorescence. Additionally, application of the GC inhibitor ODQ blocks CO-induced stomatal closure. This inhibition could be reversed when 8-Br-cGMP is added. Thus, the above results suggest that CO produced by HO is involved in ABA-induced stomatal closure, and NO and cGMP may function as downstream intermediates in the CO signaling responsible for stomatal closure.

Concurrent nitrite oxidation and aerobic denitrification in activated sludge exposed to volatile fatty acids

The goal of this research was to investigate the simultaneous occurrence of nitrification and denitrification by activated sludge exposed to volatile fatty acids (VFAs) during aerobic wastewater treatment using a single-stage reactor. A mixture of VFAs was spiked directly into a continuous-stirred tank reactor (CSTR) to assess subsequent impacts on nitrite removal, nitrate formation, CO(2) fixation, total bacterial density, and dominant nitrite oxidizing bacteria (NOB) concentration (i.e., Nitrospira). The activity of the periplasmic nitrate reductase (NAP) enzyme and the presence of nap gene were also measured. A rapid decrease in the nitrate formation rate (>70% reduction) was measured for activated sludge exposed to VFAs; however, the nitrite removal rate was not reduced. The total bacterial density and Nitrospira concentration remained essentially constant; therefore, the reduction in nitrate formation rate was likely not due to heterotrophic uptake of nitrogen or to a decrease in the dominant NOB population. Additionally, VFA exposure did not impact microbial CO(2) fixation efficiency. The activity of NAP enzyme increased in the presence of VFAs suggesting that nitrate produced as a consequence of nitrite oxidation was likely further reduced to gaseous denitrification products via catalysis by NAP. Little, if any, nitrogen was discharged in the aqueous effluent of the CSTR after exposure to VFAs demonstrating that activated sludge treatment yielded compounds other than those typically produced solely by nitrification.

The role of quantum chemistry in the elucidation of the elementary mechanisms of catalytic processes: from atoms, to surfaces, to enzymes

The recent activity of the laboratory of quantum and computational chemistry of the University of Calabria in the field of catalysis is shortly reviewed. Theoretical determinations of the potential energy profiles for the cyclotrimerization of acetylene catalyzed by bare and supported niobium atom and the reduction mechanism of nitrate to nitrite by nitrate reductase enzyme are presented as examples of studies in which a certain number of investigation methods mostly used in this field, are applied.

Short‐term Response of Soil Iron to Nitrate Addition

The inhibition of soil Fe(III) reduction by fertilizer NO3− applications is complex and not completely understood. This inhibition is important to study because of the potential impact on soil physicochemical properties. We investigated the effect of adding NO3− to a moderately well‐drained agricultural soil (Sadler silt loam) under Fe(III)‐reducing (anoxic) conditions. Stirred‐batch experiments were conducted where NO3− was added (0.05 and 1 mM) to anoxic slurries and changes in dissolved Fe(II) and Fe(III), oxalate‐extractable Fe(II), and dissolved NO3− were monitored as a function of time. Addition of 1 mM NO3− inhibited Fe(II) production sharply with reaction time, from 10% after 1 h to 85% after 24 h. The duration of inhibition in Fe(II) production was closely related to the presence of available NO3−, suggesting preferential use of NO3− by nitrate reductase enzyme. Active nitrate reductase was confirmed by the fivefold decline in NO3− reduction rates in the presence of tungstate (WO42−), a well‐known inhibitor of nitrate reductase. In addition, NO3−–dependent Fe(II) oxidation was observed to contribute to the inhibition in Fe(II) production. This finding was attributed to a combination of chemical reoxidation of Fe(II) by NO2−– and NO3−–dependent Fe(II) oxidation by autotrophic bacteria. These two processes became more important at a greater initial oxalate‐Fe(II)/NO3− concentration ratio. The inhibitory effects in Fe(II) production were short‐term in the sense that once NO3− was depleted, Fe(II) production resumed. These results underscore the complexity of the coupled N–Fe redox system in soils.

Ammonium, Nitrate and Nitrite Nitrogen and Nitrate Reductase Enzyme Activity in Groundnut (Arachis hypogaea L.) Fields After Diazinon, Imidacloprid and Lindane Treatments

Impacts of diazinon (O,O-diethyl O-2-isopropyl-6-methylpyrimidin-4-yl phosphorothioate), imidacloprid [1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine] and lindane (1,2,3,4,5.6-hexachlorocyclohexane) treatments on ammonium, nitrate, and nitrite nitrogen and nitrate reductase enzyme activities were determined in groundnut (Arachis hypogaea L.) field for three consecutive years (1997 to 1999). Diazinon was applied for both seed- and soil-treatments but imidacloprid and lindane were used for seed treatments only at recommended rates. Diazinon residues persisted for 60 days in both the cases. Average half-lives (t1/2) of diazinon were found 29.3 and 34.8 days respectively in seed and soil treatments. In diazinon seed treatment, NH4 +, NO3 −, and NO2 − nitrogen and nitrate reductase activity were not affected. Whereas, diazinon soil treatment indicated significant increase in NH4 +-N in a 1-day sample, which continued until 90 days. Some declines in NO3 −N were found from 15 to 60 days. Along with this decline, significant increases in NO2 −N and nitrate reductase activity were found between 1 and 30 days. Imidacloprid and lindane persisted for 90 and 120 days with average half-lives (t1/2) of 40.9 and 53.3 days, respectively. Within 90 days, imidacloprid residues lost by 73.17% to 82.49% while such losses for lindane residues were found 78.19% to 79.86 % within 120 days. In imidacloprid seed-treated field, stimulation of NO3 −N and the decline in NH4 +NO2 −-N and nitrate reductase enzyme activity were observed between 15 to 90 days. However, lindane seed treatment indicated significant increases in NH4 +-N, NO2 −-N and nitrate reductase activity and some adverse effects on NO3 −N between 15 and 90 days.

Low Level Nitrate or Nitroethane Preconditioning Enhances the Bactericidal Effect of Suboptimal Experimental Chlorate Treatment AgainstEscherichia coliandSalmonellaTyphimurium but NotCampylobacterin Swine

An experimental chlorate product that targets the respiratory nitrate reductase enzyme of bacteria such as Salmonella and Escherichia coli has shown promising results in reducing concentrations of these bacteria in the gut of food animals. Because expression of the target enzyme is induced by nitrate, we administered short-duration, low level nitrate or nitroethane preconditioning treatments to finishing swine to see if these would enhance the ability of an experimental chlorate product to kill these bacteria. Results from these studies showed that preconditioning the gut microflora of swine with low levels of nitrate or nitrocompounds enhanced (more than tenfold) the ability of the chlorate product to kill Salmonella and E. coli, but not Campylobacter. Further studies are needed before these compounds can be fed as feed additives to animals, although it is likely that nitrate preconditioning may be more near to market than the nitrocompounds, which may require more comprehensive review by regulatory authorities.

Brassinosteroid enhanced the level of antioxidants under cadmium stress in Brassica juncea

The change in plant growth, photosynthesis, carbonic anhydrase, nitrate reductase and antioxidative enzymes resulting from the feeding of cadmium and/or 28-homobrassinolide (HBL) to Brassica juncea were studied in 60-day-old plants. One-week old seedlings were supplied with 50, 100 or 150 μM of cadmium along with the nutrient solution. Subsequent seedlings, at day 30, were sprayed with 0.01 μM of HBL to their foliage. The plants fed with cadmium alone exhibited a decline in growth, the levels of carbonic anhydrase (E.C. 4.2.1.1) and chlorophyll pigments and net photosynthetic rate. Moreover, nitrate content, the activity of nitrate reductase (E.C. 1.6.6.1) and the level of carbohydrate, both in the leaves and roots, decreased as the concentration of cadmium, in nutrient solution, increased. Compared with the leaves, roots possessed a larger quantity of nitrate. However, the trend was reversed in case of nitrate reductase and the level of carbohydrates in the two plant organs. The toxic effect, generated by cadmium was overcome if the stressed plants were sprayed with HBL. The activities of antioxidative enzymes [viz. catalase (E.C. 1.11.1.6), peroxidase (E.C. 1.11.1.7) and superoxide dismutase (E.C. 1.15.1.1)] and the contents of proline increased, over the control, irrespective of the treatment. Their level increased further, if the plants supplied with cadmium were also supplemented with HBL, both in the roots and the aerial parts. Nevertheless, the contents of proline in roots were higher than the leaves.

Potassium-induced inhibition of nitrogen and phosphorus metabolism as a strategy of controlling Microcystis blooms

To explore potassium toxicity in Microcystis sp., growth, chlorophyll a, carotenoid and phycocyanin content, uptake of nitrate, phosphate and ammonium and activities of the assimilatory enzymes nitrate reductase, alkaline phosphatase and glutamine synthetase (GS) were studied. Nitrate, phosphate, ammonium uptakes and chlorophyll a and phycocyanin contents decreased with increase in the concentration of potassium, but carotenoid content registered an increase at increasing potassium concentration. Alkaline phosphatase and GS activities followed the trend of inhibition of their respective nutrients, whereas nitrate and nitrate reductase showed negative correlation (p < 0.01). Potassium was found to inhibit the activities of all the assimilatory enzymes in a non-competitive manner. Inhibitions of these parameters support the view that potassium has the potential to regulate Microcystis blooms in an eco-friendly manner.