Acquisition of nitrogen by external hyphae of arbuscular mycorrhizal fungi associated with Zea mays L

The objective of this study was to examine the ability of arbuscular mycorrhizal (AM) fungi to take up nitrogen from soil and transport it to the host plant. Maize (Zea mays L.) associated with Glomus intraradices Schenck and Smith or left uninoculated was grown in containers which were divided by a nylon net into a root compartment and a hyphal compartment. A 40 μm pore size nylon net was used to exclude plant roots while allowing fungal hyphae to grow into soil confined by the net. ¹⁵ N tracer was supplied either as inorganic N or as organic N to the hyphal compartment at a distance of 5 cm from the net. Inoculation with the AM fungus increased the ¹⁵ N content of maize compared to the non-mycorrhizal controls when N was applied as (¹⁵ NH₄ )₂ SO₄ . However, there was no conclusive evidence that AM hyphae could derive N from organic ¹⁵ N sources. Most of the increased N uptake of mycorrhizal plants occurred by hyphal translocation from the hyphal compartment to the root compartment. Higher N uptake by mycorrhizal plants with access to the hyphal compartment was indicated by depletion of total ¹⁵ N in the soil of that compartment. Cutting the extraradical hyphae in the hyphal compartment in order to sever their connection with the host roots decreased the ¹⁵ N uptake of the maize plants. A time-course study with inorganic ¹⁵ N over 26 d showed that G. intraradices transported most of the ¹⁵ N between 10 and 15 d after ¹⁵ N application, whereas the non-mycorrhizal control plants had a consistently low concentration of ¹⁵ N throughout the period of sampling. Nitrogen transport by external hyphae of three AM fungi, G. intraradices, Acaulospora laevis Gerdemann and Trappe and Gigaspora margarita Becker and Hall associated with maize, was further investigated. The results indicated that different isolates of AM fungi differ in the efficiency of hyphal N transport as a consequence of the different patterns of hyphal spread in the soil or of the different capacity for uptake by unit length of hyphae.

Neutron Depth Profiling: Overview and Description of NIST Facilities

The Cold Neutron Depth Profiling (CNDP) instrument at the NIST Cold Neutron Research Facility (CNRF) is now operational. The neutron beam originates from a 16 L D₂O ice cold source and passes through a filter of 135 mm of single crystal sapphire. The neutron energy spectrum may be described by a 65 K Maxwellian distribution. The sample chamber configuration allows for remote controlled scanning of 150 × 150 mm sample areas including the varying of both sample and detector angle. The improved sensitivity over the current thermal depth profiling instrument has permitted the first nondestructive measurements of ¹⁷O profiles. This paper describes the CNDP instrument, illustrates the neutron depth profiling (NDP) technique with examples, and gives a separate bibliography of NDP publications.

Molar Heat Capacity (Cv) for Saturated and Compressed Liquid and Vapor Nitrogen from 65 to 300 K at Pressures to 35 MPa

Molar heat capacities at constant volume (Cv ,) for nitrogen have been measured with an automated adiabatic calorimeter. The temperatures ranged from 65 to 300 K, while pressures were as high as 35 MPa. Calorimetric data were obtained for a total of 276 state conditions on 14 isochores. Extensive results which were obtained in the saturated liquid region (Cv(2) and Cσ ) demonstrate the internal consistency of the Cv (ρ,T) data and also show satisfactory agreement with published heat capacity data. The overall uncertainty of the Cv values ranges from 2% in the vapor to 0.5% in the liquid.

seedlings

The effects of soil nitrogen availability and chronic ozone stress on carbon and nutrient economy were investigated in loblolly pine (Pinus. taeda L.) and yellow-poplar (Liriodendron tulipifera L.). One-year-old seedlings were planted individually in pots in forest soil of low (58 μg g^-1 ), medium (96 μg g^-1 ) or high (172 μg g^-1 ) initial concentrations of soluble nitrogen. The seedlings were exposed to ozone in open-top field chambers at sub-ambient (charcoal-filtered air), ambient, and elevated (ambient + 60 nl 1^-1 O₃ ) (32, 56, 108 nl 1^-1 O₃ , 1 h seasonal mean, respectively) levels for 18 weeks. At final harvest loblolly pine dry matter increased by 50% at the highest soil K level relative to the low with the largest gains in new needle biomass. Elevated ozone reduced the biomass of current-year needles by 20% in plants grown at the highest N level. Higher soil N supply increased the concentration of nitrogen in needles, stimulated current-year needle photosynthesis and increased needle and whole-plant water-use efficiencies. Ozone treatment had no significant effect on photosynthesis or water-use efficiency in either species, although ozone exposure tended to reduce- stomatal conductance in loblolly pine. The low N treatment increased the proportion of dry matter allocated to fine roots in yellow-poplar, but whole-plant dry weight had not responded to N fertilization at the final harvest, suggesting other limitations on growth. Ozone exposure increased leaf abscission and doubled leaf turnover m yellow-poplar. Although yellow-poplar was highly sensitive to ozone-induced leaf abscission, final whole-plant dry weights were not affected. The indeterminate growth habit of yellow-poplar permitted compensatory leaf growth which may have ameliorated effects of chronic ozone stress on biomass gain. Ozone exposure also decreased shoot weight more than root weight, resulting in higher root:leaf ratios in loblolly pine and a similar trend m higher fine roor:leaf ratios in yellow-poplar. Greater proportional allocation of carbon to roots in response to nutrient deficiency may preclude an increased allocation to shoots often observed in response to air pollution stress. Interspecific differences in growth response to chronic ozone and nutrient stress may be influenced by differences in leaf growth habit.

A High-Temperature Transient Hot-Wire Thermal Conductivity Apparatus for Fluids

A new apparatus for measuring both the thermal conductivity and thermal diffusivity of fluids at temperatures from 220 to 775 K at pressures to 70 MPa is described. The instrument is based on the step-power-forced transient hot-wire technique. Two hot wires are arranged in different arms of a Wheatstone bridge such that the response of the shorter compensating wire is subtracted from the response of the primary wire. Both hot wires are 12.7 µm diameter platinum wire and are simultaneously used as electrical heat sources and as resistance thermometers. A microcomputer controls bridge nulling, applies the power pulse, monitors the bridge response, and stores the results. Performance of the instrument was verified with measurements on liquid toluene as well as argon and nitrogen gas. In particular, new data for the thermal conductivity of liquid toluene near the saturation line, between 298 and 550 K, are presented. These new data can be used to illustrate the importance of radiative heat transfer in transient hot-wire measurements. Thermal conductivity data for liquid toluene, which are corrected for radiation, are reported. The precision of the thermal conductivity data is ± 0.3% and the accuracy is about ±1%. The accuracy of the thermal diffusivity data is about ± 5%. From the measured thermal conductivity and thermal diffusivity, we can calculate the specific heat, Cp , of the fluid, provided that the density is measured, or available through an equation of state.

Physiological and growth responses to changes in ozone and nitrogen

Experiments were conducted to determine the impact of nitrogen and ozone (O₃ ) stress on the growth of domestic radish Raphanus sativus L. cv. Cherry Belle. Plants were grown in field chambers with sub-, optimal and supra-optimal levels of nitrogenous fertilizer. Chamber air was either charcoal-filtered, or supplemented with one of two levels of O₃ . The highest O₃ treatment resulted in significant reduction in weight of hypocotyls and roots while elevated nitrogen treatments resulted in increased weight of all plant parts. Ozone did not affect the weight of plant foliage at any nitrogen level. Plants grown with lower levels of nitrogen had less leaf biomass but the tissue accounted for a greater percentage total weight than did the foliage of higher nitrogen treatments. Relative growth rate of whole plants was not affected by O₃ or nitrogen treatments reflecting compensation in response to both stresses. Ozone-induced depression in biomass was observed in O₃ -treated plants grown with higher nitrogen supply but not in those grown with limiting nitrogen. This observation could reflect compensation at the lower levels of nitrogen supply or inability to detect changes in biomass due to reduced weights of plants grown at the lowest nitrogen supply. The dry weight ratio of sink organs (hypocotyl plus root)/shoot was significantly correlated with the total non-structural carbohydrate (TNC) content of these organs, regardless of treatment. Initially, O₃ induced a significant decrease and nitrogen an increase in percent TNC of sink organs. At later sampling times, plants adjusted to stress as effects on percent TNC were no longer evident.

Ecophysiology of xerophytic and halophytic vegetation of a coastal alluvial plain in northern Venezuela: III. Bromelia humilis Jacq., a terrestrial CAM bromeliad

The terrestrial CAM plant Bromelia humilis was examined in the salinas of the Ciénega el Ostional, on the north coast of Venezuela, in the rainy and dry seasons. Three colour forms were distinguished; yellow (in full sun), green exposed (also in sun) and green shaded (beneath woodland). Plant size decreased with increasing irradiance. An examination was made of the three phenotypes in terms of CO₂ exchange (J_co2 ), dawn-dusk changes in titratable acidity (ΔH⁺ ) and malate and citrate levels, osmotic pressure, xylem tension, sugar and amino acids levels, nitrogen and ion concentrations and ambient temperature fluorescence. All phenotypes exhibited lowered J_co2 and ΔH⁺ in the dry as compared to the rainy season. Citrate, as well as malate, showed dawn-dusk fluctuations. Soluble sugars were the major source of carbon skeletons for nocturnal organic acid production. The dawn-dusk changes in osmotic pressure were negligible. Yellow plants performed poorly in contrast to shaded plants in both seasons. The former showed higher dawn-dusk changes of citrate levels and contained much less nitrogen than shaded plants. Nocturnal recycling of respiratory CO₂ was more important in yellow plants and, in the dry season, reached 87%. These differences were reflected in the overall productivity, shaded plants showing increases in size whereas yellow plants utilized energy mainly for leaf replacement. Water availability and nitrogen supply appear to be the overriding factors determining higher productivity and CO₂ assimilation in partially shaded plants as compared with plants in full sun.

EFFECTS OF SALINITY AND NITROGEN ON GROWTH AND WATER RELATIONS IN THE MANGROVE, AVICENNIA MARINA (FORSK.) VIERH

Young plants of Avicennia marina (Forsk.) Vierh, were subjected to a factorial experiment with three concentrations of NaCl (0.1, 0.3, 0.5 M) and three of NH₄ C1 (0.14, 1.4, 14 mg N I^-1 ) in solution culture for three months. Nitrogen and salinity had significant effects on dry matter accumulation in shoots and roots. In general, growth of shoots and roots was significantly greater at 0.1 M NaCl than at 0.3 and 0.5 M NaCl. Added nitrogen at 14 mg N I^-1 significantly increased shoot growth at 0.1 and 0.3 M NaCl. In roots, differences were significant at 0.3 M NaCl and 14 mg N I^-1 . Nitrogen had no significant effect on shoot or root growth at 0.5 M NaCl. At lower salinities there was greater allocation of resources to shoots at 14 mg NI^-1 . Increasing salinity decreased stomatal conductance, tissue water potentials and the concentrations of nitrogen and potassium in tissues. Nitrogen levels had no effect on tissue water potentials. These results are discussed in relation to the effects of salinity and nitrogen on productivity, nutrient uptake and on plant water relations.

A Transient Hot Wire Thermal Conductivity Apparatus for Fluids

A new apparatus for measuring the thermal conductivity of fluids is described. This is an absolute method utilizing a transient hot wire. Measurements are made with a 12.7 μm diameter platinum wire at real times of up to 1 second. The data acquisition system includes a minicomputer and a digital voltmeter. The experimental core of the system incorporates a compensating hot wire in a Wheatstone bridge circuit. The cell containing the core of the apparatus is designed to accommodate pressures from 0 to 70 MPa and temperatures from 70 to 320 K. Oxygen was measured over a wide range of physical states including the dilute gas, the moderately dense gas, the near critical region, the compressed liquid states, and the vapor at temperatures below the critical temperature. Performance checks of the apparatus were conducted with nitrogen, helium and argon. Measurement of rare gases allows a direct comparison to the kinetic theory of gases through the viscosity. A second check looks at the variation of the measured thermal conductivity as a function of the applied power. The precision (2 σ) of the new system is between 0.5 and 0.8 percent for wire temperature transients of 4 to 5 K, while the accuracy is estimated at around 1.5 percent.

Generation of Controlled Low Pressures of Nitrogen by Means of Dissociation Equilibria

It is shown that fixed low pressures of nitrogen in the vacuum region can be generated by chemical dissociation in a system at equilibrium at constant temperature. Dissociation pressures ranging from 2 × 10-5 to 0.7 torr (3 × 10-3 to 90 N · m -2), corresponding to temperatures of 740 to 1150 K, for the reactionBa 3 N 2 ⇄ 3 2 Ba 2 N + 1 4 N 2 have been measured. The pressures, p, in N · m-2 are represented by the equationln  p = ( 22.02 ± 0.14 ) - ( 20 , 080 ± 140 ) T , where T is the absolute temperature, and the uncertainties are least squares estimates of the standard deviations of the parameters.

Nonanalytic Vapor Pressure Equation With Data for Nitrogen and Oxygen

The specific heat of a two phase liquid-vapor system at constant volume apparently increases without limit at temperatures approaching the critical point, suggesting (via a thermodynamic relation) that the vapor pressure derivative d 2 P/dT 2 may behave similarly. This nonanalytic behavior at the critical point is used in the present vapor pressure formula to gain simplicity and accuracy, as seen by use of data for nitrogen and oxygen.

Measurements of Gaseous Diffusion Coefficients by a Gas Chromatographic Technique

A method is presented for measuring gaseous diffusion coefficients using a gas chromatographic technique. Diffusion coefficients were measured for the systems: argon, krypton, oxygen, and nitrogen diffusing into helium at temperatures from 77 to 400 K.