Impact of nitrogen form on iron uptake and distribution in maize seedlings in solution culture

Comparative studies on the effect of nitrogen (N) form on iron (Fe) uptake and distribution in maize (Zea mays L. cv Yellow 417) were carried out through three related experiments with different pretreatments. Experiment 1: plants were precultured in nutrient solution with 1.0×10^–4 M FeEDTA for 6 d and then exposed to NO₃–N or NH₄–N solution with 1.0×10^–4 M FeEDTA or without for 7 d. Experiment 2: plants were precultured with ⁵⁹FeEDTA for 6 d and were then transferred to the solution with different N forms, and 0 and 1.0×10^–4 M FeEDTA for 8 d. Experiment 3: half of roots were supplied with ⁵⁹FeEDTA for 5 d and then cut off, with further culturing in treatment concentrations for 7 d. In comparison to the NH₄-fed plants, young leaves of the NO₃-fed plants showed severe chlorosis under Fe deficiency. Nitrate supply caused Fe accumulation in roots, while NH₄–N supply resulted in a higher Fe concentration in young leaves and a lower Fe concentration in roots. HCl-extractable (active) Fe was a good indicator reflecting Fe nutrition status in maize plants. Compared with NO₃-fed plants, a higher proportion of ⁵⁹Fe was observed in young leaves of the Fe-deficient plants fed with NH₄–N. Ammonium supply greatly improved ⁵⁹Fe retranslocation from primary leaves and stem to young leaves. Under Fe deficiency, about 25% of Fe in primary leaves of the NH₄-fed plants was mobilized and retranslocated to young leaves. Exogenous Fe supply decreased the efficiency of such ⁵⁹Fe retranslocation. The results suggest that Fe can be remobilized from old to young tissues in maize plants but the remobilization depends on the form of N supply as well as supply of exogenous Fe.     

Growth response to subsurface soil acidity of wheat genotypes differing in aluminium tolerance

Subsurface soil acidity coupled with high levels of toxic Al is a major limiting factor in wheat production in many areas of the world. This study examined the effect of subsurface soil acidity on the growth and yield of two near-isogenic wheat genotypes differing in Al tolerance at a single genetic locus in reconstructed soil columns. In one experiment, plants were grown in columns with limed topsoil and limed or acidic subsurface soils, and received water only to the subsurface soil at a late part of the growth period. While shoot dry weight, ear number and grain yield of Al-tolerant genotype (ET8) were not affected by subsurface soil acidity, liming subsurface soil increased shoot weight and grain yield of Al-sensitive genotype (ES8) by 60% and ear number by 32%. Similarly, root length density of ET8 was the same in the limed and acidic subsurface soils, while the root length density of ES8 in the acidic subsurface soil was only half of that in the limed subsurface soil. In another experiment, plants were grown with limed topsoil and acidic subsurface soil under two watering regimes. Both genotypes supplied with water throughout the soil column produced almost twice the dry weight of those receiving water only in the subsurface soil. The tolerant genotype ET8 had shoot biomass and grain yield one-third higher than ES8 when supplied with water throughout the whole column, and had yield 11% higher when receiving water in the subsurface soil only. The tolerant genotype ET8 produced more than five times the root length in the acidic subsurface soil compared to ES8. Irrespective of watering regime, the amount of water added to maintain field capacity of the soil was up to 2-fold higher under ET8 than under ES8. The results suggest that the genotypic variation in growth and yield of wheat grown with subsurface soil acidity results from the difference in root proliferation in the subsurface soil and hence in utilizing nutrient and water reserves in the subsurface soil layer.     

Differential transport of Zn, Me and sucrose along the longitudinal axis of developing wheat grains

The hypothesis that Zn and Mn are transported within the grain in a similar manner to sucrose was investigated in the developing wheat grain. Detached ears were cultured in solution containing ⁶⁵Zn, ⁵⁴Mn and [¹⁴C]-sucrose for 10 to 120 min at 18–22 days post-anthesis. At different times the grain was cut transversely into 1-mm sections and the radioactivity in each section determined The embryo region was damaged in some grains to investigate the effect of reduced accumulation rate on the transport of ⁶⁵Za, ⁵⁴Mn and [¹⁴C]-sucrose to the embryo. The distribution of ⁶⁵Zn. ⁵⁴Mn and [¹⁴C]-sucrose between the endosperm cavity sap. endosperm, embryo and pericarp in grains labelled for 2.5 and 6 h at 18–22 days post-anthesis was also determined. [¹⁴C]-su-crose was initially high in the first, embryo-containing section of the grain but decreased progressively to the distal end of the grain. The amount of ⁶⁵Zn along the longitudinal axis of the grain was distributed evenly in each 1-mm section, whilst ⁵⁴Mn accumulated exponentially in the first proximal 1-mm section of the grain and was distributed evenly in the remaining sections. Damaging the embryo had no effect on ⁶⁵Zn and ⁵⁴Mn transport to the section containing the embryo. The pericarp contained almost all of the grain ⁶⁵Za and ⁵⁴Mn, with small amounts found in the embryo, endosperm and endosperm cavity sap. Increasing amounts of [¹⁴C]-sucrose were found in the endosperm as time progressed. The rate of accumulation of ⁶⁵Zn, ⁵⁴Mn and [¹⁴C]-sucrose was much higher in the embiyo than the endosperm: the difference between the embryo and endosperm was especially large for ⁶⁵Zn and ⁵⁴Mn. It is suggested that ⁶⁵Zn and ⁵⁴Mn are not transported within the grain in the same way as [¹⁴C]-sucrose. [¹⁴C]-sucrose moves laterally out of the vascular system of the crease into the endosperm cavity and is subsequently taken up and stored in the endosperm. In contrast, ⁶⁵Zn and ⁵⁴Mn appear to be retained within the vascular system of the crease and may be transported more slowly to grain parts such as the embryo and pericarp tissue.     

Kinetics of ammonium, nitrate and phosphorus uptake by Canna indica and Schoenoplectus validus

Canna indica L. is an upright perennial rhizomatous herb, and Schoenoplectus validus (Vahl) A. Löve and D. Löve is a tall, perennial, herbaceous sedge. The nutrient uptake kinetics of C. indica and S. validus were investigated using the modified depletion method after plants were grown for 4 weeks in simulated secondary-treated wastewater. The maximum uptake rate (I_max) and Michaelis–Menten constant (K_m) were estimated by iterative curve fitting. The I_max for NH₄N (623 μmol g⁻¹ dry root weight h⁻¹) was significantly higher than that for NO₃N (338 μmol g⁻¹ dry root weight h⁻¹) in S. validus. In contrast, no difference was observed in C. indica. The I_max values for NO₃N and NH₄N were higher in S. validus than in C. indica. A significantly lower K_m was detected for NO₃N uptake in C. indica (385 μmol L⁻¹) compared to that in S. validus (1908 μmol L⁻¹). The I_max for PO₄P did not differ between the plant species. The K_m for PO₄P was significantly higher in C. indica (157 μmol L⁻¹) than in S. validus (60 μmol L⁻¹). In conclusion, we found that S. validus preferred NH₄N over NO₃N, had greater capacity for N uptake and higher affinity for PO₄P, but C. indica had greater affinity for NO₃N. Nutrient uptake capacity is likely related to habitat preference, and is influenced by the structure of roots and rhizomes.     

Development of concise two-step catalytic approach towards lasofoxifene precursor nafoxidine

We have elaborated a two-step catalytic approach to nafoxidine, a key precursor to lasofoxifene. Firstly, an efficient α-arylation of 6-methoxy-3,4-dihydronaphthalen-1(2H)-one with chlorobenzene was developed, which operates at low 0.1?mol% Pd-132 catalyst loading in the presence of 1.9 equivalents of sodium tert-butoxide at 60?°C in 1,4-dioxane and provides 6-methoxy-2-phenyl-3,4-dihydronaphthalen-1(2H)-one in 90% yield. Secondly, we have demonstrated that 6-methoxy-2-phenyl-3,4-dihydronaphthalen-1(2H)-one can be converted to nafoxidine in 61% yield via CeCl₃ promoted reaction with (4-(2-(pyrrolidin-1-yl)ethoxy)phenyl)lithium, which is formed in-situ from the corresponding arylbromide precursor and n-butyllithium. Altogether, the shortest two-step approach to nafoxidine from simple tetralone commodity starting material has been developed with overall 55% yield. The developed synthetic approach to nafoxidine has several beneficial aspects over the one used in the synthetic route primarily developed for the preparation of lasofoxifene.     

Quenching of chlorophyll triplet states by carotenoids in algal light-harvesting complexes related to fucoxanthin-chlorophyll protein

Photosynthesis Research - We have used time-resolved absorption and fluorescence spectroscopy with nanosecond resolution to study triplet energy transfer from chlorophylls to carotenoids in a...     

Alternate pathways of malonylCoA formation in Streptomyces aureofaciens

Alternate metabolic pathways for the formation of malonylCoA in the actinomycete Streptomyces aureofaciens are examined. Comparison of the specific activities of pyruvate kinase, pyruvate dehydrogenase, and phosphoenol-pyruvate carboxylase during cultivation, the degree of incorporation of individual radioactive substrates into the tetracycline molecule, and the high randomization of acetate-[2-¹⁴C], indicate that the malonylCoA used in tetracycline biosynthesis does not appear solely through the carboxylation of acetylCoA. The role of the phosphoenolpyruvate carboxylase and oxaloacetate dehydrogenase systems in the formation of malonylCoA is established, and using radio-GLC, a cell-free preparation of S. aureofaciens mycelium is shown to form malonate from oxaloacetate. The reaction requires HSCoA and NAD⁺.     

Physiological responses of wheat genotypes grown in chelator-buffered nutrient solutions with increasing concentrations of excess HEDTA

The chelator-buffered nutrient solutions containing excess chelator have been used frequently in the micronutrient research, but potential toxicity of the excess chelator has not been ascertained. The present study was conducted to test effects of four concentrations of excess HEDTA [ N-(2-hydroxyethyl)ethylenedinitrilotriacetic acid] and two levels of total Zn on growth, root exudation, and nutrient uptake and transport by Triticum aestivum L. (cv. Aroona) and Triticum turgidum L. conv. durum (Desf.) MacKey (cv. Durati) genotypes differing in tolerance to Zn deficiency. Excess HEDTA at 50 μM reduced root and shoot growth and caused visual toxicity symptoms (necrotic lesions) on leaves; these effects were generally absent at lower concentrations of excess HEDTA. Root exudation of phytosiderophores increased with increasing concentrations of excess HEDTA at deficient and sufficient Zn levels, and was higher in Zn-deficiency-tolerant Aroona than in Zn-deficiency-sensitive Durati wheat. Shoot and root Zn concentrations showed a saturable response to increasing Zn²⁺ activities in solution. Excess HEDTA at 50 μM caused an increase in shoot concentrations of Fe and a decrease in concentrations of Mn and Cu. An average rate of Zn uptake increased with an increase in Zn²⁺ ionic activity in solution, with Zn-deficiency-tolerant Aroona having a higher rate of Zn uptake than Zn-deficiency-sensitive Durati in the deficiency range of Zn²⁺ activities. Average uptake rates of Mn and Cu decreased with an increase in concentration of excess HEDTA. Similar observations were noted for transport of Mn and Cu to shoots, while Zn transport to shoots was proportional to Zn²⁺ activities in solution. It was concluded that excess HEDTA at 50 μM adversely affects wheat growth and physiology, while excess of 25 μM or less does not cause measurable toxicity. This revised version was published online in June 2006 with corrections to the Cover Date.     

Membrane fluxes and comparative toxicities of aluminium, scandium and gallium

Measurements were made of the membrane fluxes and toxicitiesof three cations with trivalent forms, Al, Ga and Sc, in internodalcells of the giant alga Chara corallina. With this species itwas possible to separate the cell wall from the cell contentsto obtain membrane fluxes which were not complicated by adsorptionof cations to the cell wall. Net uptake of Al was low, approximately1.5 pmol m^–2 s^–1, compared to the influxes of thedivalent cation ⁴⁵Ca of 82 pmol m^–2 s^–1 and themonovalent cation ²²Na of 1100 pmol m^–2 s^–1 at thesame external concentration. Traditional desorption methodsfor removing cell wall cations were found to be relatively ineffectivein the case of trivalent cations and, consequently, influx measuredwithout separating the cell wall component would greatly overestimatethe true membrane flux, possibly by several orders of magnitude.Al, Ga and Sc all inhibited growth at 20 mmol m^–3 at pH4.4. Toxicity decreased in the order Sc>Al>Ga. Sc andAl were also toxic to mature non-growing cells. Influx of ⁴⁶Scincreased with increasing pH, consistent with membrane permeationby hydroxy Sc rather than Sc³⁺. However, Sc was more toxic atlow pH where Sc³⁺ was the dominant species and where influxwas low and binding to cell walls was high. These results argueagainst Sc acting intracellularly and favour a toxicity mechanismwhich is initiated extracellularly. Key words: Aluminium toxicity, trivalent cations, Chara corallina, scandium influx, gallium