Effects of short term iron citrate treatments at different pH values on roots of iron-deficient cucumber: a Mössbauer analysis.

Alkaline pH values and bicarbonate greatly reduce the mobility and uptake of Fe, causing Fe deficiency chlorosis. In the present work, the effects of pH and bicarbonate on the uptake and accumulation of Fe in the roots of cucumber were studied by Mössbauer spectroscopy combined with physiological tests and diaminobenzidine enhanced Perls staining. Mössbauer spectra of Fe-deficient cucumber roots supplied with 500 µM (57)Fe(III)-citrate at different pH values showed the presence of an Fe(II) and an Fe(III) component. As the pH was increased from 4.5 to 7.5, the root ferric chelate reductase (FCR) activity decreased significantly and a structural change in the Fe(III) component was observed. While at pH 4.5 the radial intrusion of Fe reached the endodermis, at pH 7.5, Fe was found only in the outer cortical cell layers. The Mössbauer spectra of Fe-deficient plants supplied with Fe(III)-citrate in the presence of bicarbonate (pH 7.0 and 7.5) showed similar Fe components, but the relative Fe(II) concentration compared to that measured at pH values 6.5 and 7.5 was greater. The Mössbauer parameters calculated for the Fe(II) component in the presence of bicarbonate were slightly different from those of Fe(II) alone at pH 6.5-7.5, whereas the FCR activity was similarly low. Fe incorporation into the root apoplast involved only the outer cortical cell layers, as in the roots treated at pH 7.5. In Fe-sufficient plants grown with Fe(III)-citrate and 1mM bicarbonate, Fe precipitated as granules and was in diffusely scattered grains on the root surface. The "bicarbonate effect" may involve a pH component, decreasing both the FCR activity and the acidification of the apoplast and a mineralization effect leading to the slow accumulation of extraplasmatic Fe particles, forming an Fe plaque and trapping Fe and other minerals in biologically unavailable forms.

Influence of pH, iron source, and Fe/ligand ratio on iron speciation in lignosulfonate complexes studied using Mössbauer spectroscopy. Implications on their fertilizer properties.

Iron chlorosis is a very common nutritional disorder in plants that can be treated using iron fertilizers. Synthetic chelates have been used to correct this problem, but nowadays environmental concerns have enforced the search for new, more environmentally friendly ligands, such as lignosulfonates. In this paper, Fe coordination environment and speciation in lignosulfonate (LS) complexes prepared under different experimental conditions were studied by (57)Fe Mössbauer spectroscopy in relation to the Fe-complexing capacities, chemical characteristics of the different products, and efficiency to provide iron in agronomic conditions. It has been observed that the complex formation between iron and lignosulfonates involves different coordination sites. When Fe(2+) is used to prepare the iron-LS product, complexes form weak adducts and are sensitive to oxidation, especially at neutral or alkaline pH. However, when Fe(3+) is used to form the complexes, both Fe(2+) and Fe(3+) are found. Reductive sugars, normally present in lignosulfonates, favor a relatively high content of Fe(2+) even in those complexes prepared using Fe(3+). The formation of amorphous ferrihydrite is also possible. With respect to the agronomical relevance of the Fe(2+)/Fe(3+) speciation provided by the Mössbauer spectra, it seems that the strong Fe(3+)-LS complexes are preferred when they are applied to the leaf, whereas root uptake in hydroponics could be more related with the presence of weak bonding sites.

Uptake and incorporation of iron in sugar beet chloroplasts.

Chloroplasts contain 80-90% of iron taken up by plant cells. Though some iron transport-related envelope proteins were identified recently, the mechanism of iron uptake into chloroplasts remained unresolved. To shed more light on the process of chloroplast iron uptake, trials were performed with isolated intact chloroplasts of sugar beet (Beta vulgaris). Iron uptake was followed by measuring the iron content of chloroplasts in the form of ferrous-bathophenantroline-disulphonate complex after solubilising the chloroplasts in reducing environment. Ferric citrate was preferred to ferrous citrate as substrate for chloroplasts. Strong dependency of ferric citrate uptake on photosynthetic electron transport activity suggests that ferric chelate reductase uses NADPH, and is localised in the inner envelope membrane. The K(m) for iron uptake from ferric-citrate pool was 14.65 ± 3.13 µM Fe((III))-citrate. The relatively fast incorporation of (57)Fe isotope into Fe-S clusters/heme, detected by Mössbauer spectroscopy, showed the efficiency of the biosynthetic machinery of these cofactors in isolated chloroplasts. The negative correlation between the chloroplast iron concentration and the rate of iron uptake refers to a strong feedback regulation of the uptake.

One metal-two pathways to the carboxylate-enhanced, iron-containing quercetinase mimics.

Mononuclear iron(iii) flavonolate was synthesized as synthetic enzyme-substrate complex, and its direct and carboxylate-enhanced dioxygenation as biomimetic functional models with relevance to flavonol 2,4-dioxygenase are briefly described.

Investigation of iron pools in cucumber roots by Mössbauer spectroscopy: direct evidence for the Strategy I iron uptake mechanism.

Distinct chemical species of iron were investigated by Mössbauer spectroscopy during iron uptake into cucumber roots grown in unbuffered nutrient solution with or without 57Fe-citrate. Mössbauer spectra of iron deficient roots supplied with 10-500 microM 57Fe-citrate for 30-180 min and 24 h and iron-sufficient ones, were recorded. The roots were analysed for Fe concentration and Fe reductase activity. The Mössbauer parameters in the case of iron-sufficient roots revealed high-spin iron(III) components suggesting the presence of FeIII-carboxylate complexes, hydrous ferric oxides and sulfate-hydroxide containing species. No FeII was detected in these roots. However, iron-deficient roots supplied with 0.5 mM 57FeIII-citrate for 30 min contained significant amount of FeII in a hexaaqua complex form. This is a direct evidence for the Strategy I iron uptake mechanism. Correlation was found between the decrease in Fe reductase activity and the ratio of FeII-FeIII components as the time of iron supply was increased. The data may refer to a higher iron reduction rate as compared to its uptake/reoxidation in the cytoplasm in accordance with the increased reduction rate in iron deficient Strategy I plants.

The structure of Fe(III) ions in strongly alkaline aqueous solutions from EXAFS and Mössbauer spectroscopy.

To establish the structure of ferric ions in strongly alkaline (pH > 13) environments, aqueous NaOH solutions supersaturated with respect to Fe(III) and the solid ferric-hydroxo complex salts precipitating from them have been characterized with a variety of experimental techniques. From UV measurements, in solutions of pH > 13, only one kind of Fe(III)-hydroxo complex species was found to be present. The micro crystals obtained from such solutions were proven to be a new, so far unidentified solid phase. Mössbauer spectra of the quick-frozen solution and that of the complex salt indicated a highly symmetrical ferric environment in both systems From the EXAFS and XANES spectra, the environment of the ferric ion in these solutions (both native and quick-frozen) and in the complex salt was found to be different. In the complex salt, the bond lengths are consistent with an octahedral coordination around the ferric centres. In solution, the coordination geometry of Fe(III) is most probably tetrahedral. Our results demonstrate that in strongly alkaline aqueous solutions, ferric ions behave very similarly to other structurally related tervalent ions, like Al(III) or Ga(III).

Structural characterization of glutamine synthetase from Azospirillum brasilense.

CD spectroscopic study of the secondary structure of partly adenylylated glutamine synthetase (GS) of the bacterium Azospirillum brasilense showed both the native and cation-free (EDTA-treated) enzyme to be highly structured (58 and 49% as alpha-helices, 10 and 20% as beta-structure, respectively). Mg(2+), Mn(2+), or Co(2+), when added to the native GS, had little effect on its CD spectrum, whereas their effects on the cation-free GS were more pronounced. Emission ((57)Co) Mössbauer spectroscopic (EMS) study of (57)Co(2+)-doped cation-free GS in frozen solution and in the dried state gave similar spectra and Mössbauer parameters for the corresponding spectral components, reflecting the ability of the Co(2+)-enzyme complex to retain its properties upon drying. The EMS data show that (a) A. brasilense GS has 2 cation-binding sites per active center and (b) one site has a higher affinity to Co(2+) than the other, in line with the data on other bacterial GSs.