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Achieving net-zero greenhouse gas emissions in cement production requires major reductions in both process and energy emissions. This study proposes an integrated low emission cement and power production (LECAPP) system that incorporates external reforming molten carbonate fuel cells to capture the CO2 emissions from a natural gas-fired cement plant. The system uses either natural gas or high-density polyethylene to generate the hydrogen demanded by the fuel cells while producing both low-carbon electricity (1,201 kWh/t clinker with a carbon intensity of 52 kgCO2/MWh, of which 1,000 kWh/t clinker is available for export) and a CO2 stream for sequestration. The carbon intensity assigned to clinker production (57 kgCO2/t clinker) is a 92% reduction from a clinker plant without carbon management. When plastics are used to generate hydrogen for the fuel cells, 144 kg plastics/t clinker would be diverted from landfills. Compared to other carbon capture methods, the LECAPP system performs better and its overall specific primary energy consumption is estimated to be in the range of 1.52–5.94 gigajoules per tCO2 avoided. The LECAPP system offers promise as a viable technology in the transition to net zero-emission energy systems.

Cement-making is an energy-intensive industrial process that contributes 8% of the global CO2 emissions. This study develops a thermal energy flow model (TEF) for 4200 tonnes of clinker per day, a natural gas-fired cement plant in which 50% of the pre-calciner energy requirements can be supplied by alternative fuels (AF) including biomass, plastics, etc. The TEF shows that the lower heating value (LHV dry), oxygen content (dry basis) and moisture content of the AF, as well as the flue gas oxygen concentration [O2] needed for complete combustion, affect the thermal energy intensity (TEI) and total air demand (AD). Compared to the reference system fueled by natural gas (NG) that burns completely at 1% [O2] in flue gas, all AFs require more total air and thermal energy demand, especially when the solid AFs need a higher [O2] in the flue gas (typically 3%) to ensure complete combustion. Due to the higher carbon (C) intensity, co-firing AFs could increase the total CO2 emissions by 1% to 18%. For the wood dust with 100% biogenic C, 50% NG replacement in pre-calciner could avoid 55.5 and 43.1 kgCO2/t clinker at 1% and 3% [O2] in flue gas respectively, an equivalent of 7.5% and 5.8% decrease in total GHG emissions relative to the reference case. Results of the TEF model from 24 diverse AFs were used to generate regressions that link fuel properties and flue gas oxygen requirement with TEI, making it possible to compare highly diverse AFs for their likely performance in the clinker-making.

Electric vehicles (EVs) may add new challenges during mass evacuations. Understanding the magnitude of the impacts EVs may have during the pre-departure stage of mass evacuations is an essential first step when planning for mass evacuations in a future where EVs are more common. In this paper, a generalized framework based on a G/G/c/N queueing model (general arrival process, general service process, c charging stations, and N EVs) was developed to estimate the number of vehicles that can be charged in the pre-departure evacuation stage and thus assess the pre-departure impacts. The model outputs are the number of vehicles that have or have not been served during the evacuation period, as well as average queue times and maximum queue lengths. This model is tested using the current electric vehicle fleet and charging infrastructure of Prince George, British Columbia, as a case study with a hypothetical short notice forest fire scenario. It was found that for the present-day case of Prince George, there is not enough charging network capacity to service all vehicles before departure. Increasing the number of charging stations, providing earlier evacuation notices, and ensuring a balanced makeup of level 3 fast-charging of different types were all found to be effective in increasing the number of EVs that received adequate charging before departure.

This study evaluated the efficacy of using a combined biofilm-biochar approach to remove organic (naphthenic acids (NAs)) and inorganic (metals) contaminants from process water (OSPW) generated by Canada’s oil sands mining operations. A microbial community sourced from an OSPW sample was cultured as biofilms on several carbonaceous materials. Two biochar samples, from softwood bark (SB) and Aspen wood (N3), facilitated the most microbial growth (measured by protein assays) and were used for NA removal studies performed with and without biofilms, and in the presence and absence of contaminating metals. Similar NA removal was seen in 6-day sterile N3 and SB assays (>30%), while biodegradation by SB-associated biofilms increased NA removal to 87% in the presence of metals. Metal sorption was also observed, with up to four times more immobilization of Fe, Al, and As on biofilm-associated biochar. These results suggest this combined approach may be a promising treatment for OSPW.

Between 1995 and 2010, the total energy intensity (E/GDP, PJ/Gross Domestic Product in 2002$) of the Canadian economy declined by 23% or − 2.64 MJ/$. To understand why, the Logarithmic Mean Divisia Index (LMD-I) method was used to decompose a large body of government statistical data supporting the observed E/GDP decline. The analysis shows that (a) 48% (1.27 MJ/$) of the decline was associated with an inter-sector structural change in the economy (i.e. an increased contribution to the total GDP of the low energy-using commercial and institutional sector compared with the high energy-using manufacturing and heavy industry sectors); (b) 24% (0.62 MJ/$) was attributed to the impact of the Canadian GDP growing faster than population; (c) 22% (0.58 MJ/$) of the decline was associated with an overall decrease in business energy intensity. A deeper analysis of business sectors shows a positive impact of 0.4 MJ/$ from increased energy intensity in the oil and gas sector, offset by a 0.98 MJ/$ decline due to energy intensity declines in the other business sectors; (d) 6.3% (0.17 MJ/$) of the decline was associated with an improvement in the energy intensity of households, mostly from residential energy use rather than personal transportation energy use. These results provide insights for policy makers regarding those aspects of the Canadian economy that contribute to, or work against, efforts to transform energy systems toward sustainability.

Energy systems are under pressure to transform to address concerns about climate change. The modeling and visualization of energy systems can play an important role in communicating the costs, benefits and trade-offs of energy systems choices. We introduce EnergyViz, a visualization system that provides an interface for exploring time-varying, multi-attribute and spatial properties of a particular energy system. EnergyViz integrates several visualization techniques to facilitate exploration of a particular energy system. These techniques include flow diagram representation to show energy flow, 3D interaction with flow diagrams for expanding viewable data attributes such as emissions and an interactive map integrated with flow diagrams for simultaneous exploration of spatial and abstract information. We also perform level-of-detail exploration on flow diagrams and use smooth animation across the visualizations to represent time-varying data. Finally, we include evaluation results of EnergyViz collected from expert and inexperienced participants.

This study explored the feasibility of using residual biomass to both mitigate greenhouse gas (GHG) emissions and remediate water contaminated by hydrocarbons. Using produced (process-affected) water from Canada’s oil sands operations as a case study, activated biochar (ACB) was found to have a higher affinity to organics than activated coal and removed 75 % of total organic carbon (TOC) from produced water in steam-assisted gravity drainage (SAGD) operations or 90 % of the TOC from synthetic tailings (ST) water sample. Up to 6 Tg dry biomass year−1 would be required to treat the waters associated with the 93 × 106-m3 of bitumen recovered per year. Landfilling the spent ACB and flaring any biogas produced were estimated to provide a greater GHG benefit than the combustion of the biochar + organics for heat to offset natural gas demand. Net costs for the ACB were about 13.84 $ m−3 bitumen for SAGD operations and 1.76 $ m−3 bitumen for mining operations. The values for mining operations justify further work to create a value chain that will integrate bioprocesses into the fossil fuel industry.

The influence of KOH addition and air pretreatment on co-pyrolysis (600 °C) of a mixture of bio-oil and biomass (aspen wood) was investigated with the goal of increasing biochar yield. The bio-oil was produced as a byproduct of the pyrolysis of biomass and recycled in subsequent runs. Co-pyrolysis of the biomass with the recycled bio-oil resulted in a 16% mass increase in produced biochar. The yields were further increased by either air pretreatment or KOH addition prior to co-pyrolysis. Air pretreatment at 220 °C for 3 h resulted in the highest mass increase (32%) compared to the base case of pyrolysis of biomass only. No synergistic benefit was observed by combining KOH addition with air pretreatment. In fact, KOH catalyzed reactions that increased the bed temperature resulting in carbon loss via formation of CO and CO2.

Greenhouse gas accounting for agricultural systems consider methane and nitrous oxide emissions, carbon emissions from liming and urea use, as well as carbon stock changes, but it ignores gross flows of bio-based energy and carbon. This study compiled data for Canada’s agri-food system over the 2010-13 period, from food supply and disposition to crop processing, animal production, and crop/pasture production. The data were converted to units of energy and carbon, tracked through the agri-food system and compared in scale and conversion efficiency with Canadian crude oil recovery to the production of refined petroleum products. Results showed domestic photosynthesis-derived energy and carbon flow equivalent to 75% and 98%, respectively, of the fossil fuel-derived energy and carbon in the crude oil recovered in Canada. This magnitude is substantial since Canada is a nation with high per capita oil demand that exports over 50% of its own production. Only 14% of the agri-food energy and carbon, respectively, emerged in agri-food products, compared to 91% of the energy and carbon in crude oil that resulted in refined petroleum products. The low conversion efficiency of the agri-food system derived, in part, from 40% of bio-based energy and carbon being diverted to crop and animal residues or waste. Per unit of energy in end products, the other energy inputs (e.g. electricity, fuels) needed to support the agri-food system were 5.3-folds higher in the agri-food system than in the crude oil to products system. This study highlights the need to develop strategies to better utilize the energy and carbon flows of the agri-food system, thereby reducing fossil energy use and greenhouse gas emissions associated with human activities.

The objective of this study was to determine if biochar yield could be increased by the deposition of volatile pyrolysis species within the bed during production, without negatively influencing the microporosity and adsorption properties. Aspen ( Populus tremuloides ) wood chips were loaded into three vertically stacked zones within a reactor and heated in nitrogen to temperatures between 420 and 650 °C (i.e., pyrolyzed). The yield did increase from the zone at the reactor inlet to the subsequent zones as volatile species deposited and carbonized, and importantly, the carbonized deposits had a similar microporous structure and organic vapor uptake (1,1,1,2-tetrafluoroethane) to that of the primary biochar. Based on these results, bio-oil from previous runs at 600 °C was recycled to the bed, which further increased the yield while maintaining the desirable adsorption properties of the biochar.

The early atmospheric detection of carbon dioxide (CO2) leaks from carbon capture and storage (CCS) sites is important both to inform remediation efforts and to build and maintain public support for CCS in mitigating greenhouse gas emissions. A gas analysis system was developed to assess the origin of plumes of air enriched in CO2, as to whether CO2 is from a CCS site or from the oxidation of carbon compounds. The system measured CO2 and O2 concentrations for different plume samples relative to background air and calculated the gas differential concentration ratio (GDCR = −ΔO2/ΔCO2). The experimental results were in good agreement with theoretical calculations that placed GDCR values for a CO2 leak at 0.21, compared with GDCR values of 1–1.8 for the combustion of carbon compounds. Although some combustion plume samples deviated in GDCR from theoretical, the very low GDCR values associated with plumes from CO2 leaks provided confidence that this technology holds promise in providing a tool for the early detection of CO2 leaks from CCS sites. Implications: This work contributes to the development of a cost-effective technology for the early detection of leaks from sites where CO2 has been injected into the subsurface to enhance oil recovery or to permanently store the gas as a strategy for mitigating climate change. Such technology will be important in building public confidence regarding the safety and security of carbon capture and storage sites.

Air temperature freeze–thaw cycles often occur during the early spring period directly after snowmelt and before budbreak in low arctic tundra. This early spring period may be associated with nitrogen (N) and carbon (C) loss from soils as leachate or as trace gases, due to the detrimental impact of soil freeze–thaw cycles and a developing active layer on soil microorganisms. We measured soil and microbial pools of C and N in early spring during a period of fluctuating air temperature (ranging from −4 to +10°C) and in midsummer, in low arctic birch hummock tundra. In addition we measured N2O, CH4 and CO2 production in the early spring. All of these biogeochemical variables were also measured in long-term snowfence (deepened snow) and N-addition plots to characterize climate-change related controls on these variables. Microbial and soil solution pools of C and N, and trace gas production varied among the five early spring sample dates, but only marginally and no more than among sample dates in midsummer. N-addition greatly elevated N2O fluxes, indicating that although denitrifiers were present their activity during early spring was strongly limited by N-availability, but otherwise trace gas production was very low in early spring. The later thaw, warmer winter and colder spring soil temperatures resulting from deepened snow did not significantly alter N pools or rates in early spring. Together, our results indicate strong stability in microbial and soil solution C and N pool sizes in the early spring period just after snowmelt when soil temperatures are close to 0°C (−1.5 to +5°C). A review of annual temperature records from this and other sites suggests that soil freeze–thaw cycles are probably infrequent in mesic tundra in early spring. We suggest that future studies concerned with temperature controls on soil and microbial biogeochemistry should focus not on soil freeze–thaw cycles per se, but on the rapid and often stepped increases in soil temperature that occur under the thawing snowpack.

Changes in the sequence of crops grown on agricultural land are well known to enhance the yield of grain crops such as wheat. A survey of the literature gathered from around the world show mean yield benefits of up to 20% or more. Much is known about the principal mechanisms responsible for these benefits, including effects on disease control, improved nitrogen nutrition and water supply, although researchers continue to be challenged by inexplicable “rotation effects” that have yet to be documented or fully understood. This review summarizes our current understanding of the ‘better-known’ mechanisms of crop rotation, and discusses other mechanisms (e.g. changes in rhizosphere biology, allelopathy or soil structure) that may help to account fully for the rotation benefits that have been observed by agricultural producers for more than 2000 years. Where possible we emphasise new techniques employed to investigate these less well-understood aspects of the “rotation effect”. At the farm level, the inability to capitalize on the benefits of break crops may owe more to economics, the availability of suitable break crops and the complexity of the crop response. Computer-based decision support tools have been developed to assist growers to apply the information gathered from scientific studies, although efforts to integrate this information at whole-farm scales are embryonic.

In response to changes in phloem supply, adenylate demand, and oxygen status, legume nodules are known to exercise rapid (seconds to hours) physiological control over their permeability to oxygen diffusion. Diffusion models have attributed this permeability control to the reversible flow of water into or out of intercellular spaces. To test hypotheses on the mechanism of diffusion barrier control, nodulated soybean (Glycine max L. Merr.) plants were exposed to a range of treatments known to alter nodule O2 permeability (i.e. 10% O2, 30% O2, Ar:O2 exposure, and stem girdling) before the nodules were rapidly frozen, freeze dried, and dissected into cortex and central zone (CZ) fractions that were assayed for K, Mg, and Ca ion concentrations. Treatments known to decrease nodule permeability (30% O2, Ar:O2 exposure, and stem girdling) were consistently associated with an increase in the ratio of [K+] in cortex to [K+] in the CZ tissue, whereas the 10% O2 treatment, known to increase nodule permeability, was associated with a decrease in the [K+]cortex:[K+]CZ. When these findings were considered in the light of previous results, a proposed mechanism was developed for the adenylate-coupled movement of ions and water into and out of infected cells as a possible mechanism for diffusion barrier control in legume nodules.

Soybean (Glycine max L. Merr) plants grown under control (360 µmol mol−1) or elevated CO2 concentration (800 µmol mol−1) from 33 to 42 d after sowing were assayed for various components of in vivo nitrogenase activity to test the hypothesis that increasing carbohydrate supply to nodules would increase the potential (i.e. O2 saturated) nitrogenase activity and impose a more severe O2 limitation on both nodule metabolism and total nitrogenase activity. Within 51 h of elevated CO2 treatment, significant increases relative to control plants were seen in total nitrogenase activity expressed per plant. After 6 d of elevated CO2, the total nitrogenase activity per plant was 18% higher than that in control. This was attributed to an initial increase in nodule size, and a subsequent increase in nodule number following plant exposure to elevated CO2. However, after 9 d of elevated CO2, the potential and total nitrogenase activities per gram nodule dry weight were lower, not higher than corresponding values in plants in the control treatment. These results did not support the hypothesis. It was concluded that the metabolic capacity of the control nodules were not limited by carbohydrate supply, at least at the assay temperatures employed here.

Central infected zone tissue of soybean (Glycine max L. Merr.) nodules was fractionated into separate subcellular compartments using density gradient centrifugation in nonaqueous solvents to better understand how exposure to Ar:O2 (80:20%, v/v) atmosphere affects C and N metabolism, and to explore a potential role for adenylates in regulating O2 diffusion. When nodules were switched from air to Ar:O2, adenylate energy charge (AEC) in the plant cytosol rose from 0.63 ± 0.02 to 0.73 ± 0.02 within 7 min and to 0.80 ± 0.01 by 60 min. In contrast, AEC of the mitochondrial compartment of this central zone tissue remained high (0.80 ± 0.02 to 0.81 ± 0.02) following Ar treatment while that of the bacteroid compartment was unchanged, at 0.73 ± 0.02, after 7 min, but declined to 0.57 ± 0.03 after 60 min. These results were consistent with a simulation model that predicted Ar:O2 exposure would first reduce ATP demand for ammonia assimilation and rapidly increase cytosolic AEC, before the Ar:O2-induced decline mediated by a decrease in nodule O2 permeability reduces bacteroid AEC. The possibility that adenylates play a key, integrating role in regulating nodule permeability to oxygen diffusion is discussed.

Hydrogen gas (H2) is an obligate byproduct of the N2-fixing enzyme, nitrogenase, claiming about 5–6% of the crops’ net photosynthesis but most, if not all of the H2 lost from nodules is oxidized by the soil surrounding the root system of the plant. When soils not recently used to support the growth of legumes were exposed to H2 gas at a rate and duration similar to that of soil adjacent to legume nodules, the fertility of the soil was enhanced in comparison with soil treated with air. Under growth-chamber and field conditions, H2-treated soils improved the growth performance of spring wheat, canola, barley and soybean (non-symbiotic) when compared with untreated soils or with soils pretreated with air. The dry weights of 4- to 7-week-old plants were 15–48% greater in the H2-treated soil, and in barley and spring wheat, tiller number of 7-week-old plants were 36 and 48% greater in the H2-treated soils. These findings may contribute to an explanation for the persistence of H2 evolving associations in agricultural legume symbioses selected for high yields (Uratsu et al., Crop Science 22, 600–602, 1982) and suggest that it may be possible to isolate, identify and culture the micro-organisms that are responsible for at least some of the benefits of legumes in crop rotation.

Energy systems are under pressure to transform in order to address concerns about climate change. The modeling and visualization of energy systems can play an important role in communicating the costs, benefits and tradeoffs of energy systems choices. We introduce a visualization tool that provides an interface for exploring time-varying, multiattribute and spatial properties of a particular energy system. The tool integrates several visualization techniques to facilitate exploration of a particular energy system. These techniques include flow diagram representation to show energy flow, 3D interaction with flow diagrams for expanding viewable data attributes such as emissions and an interactive map integrated with flow diagrams for simultaneous exploration of spatial and abstract information. We also perform level of detail exploration on flow diagrams and use smooth animation across the visualizations to represent time-varying data. As a means of preliminary evaluation, we have included feedback on this tool from our energy system collaborators.

In oil sands mining extraction, water is an essential component; however, the processed water becomes contaminated through contact with the bitumen at high temperature, and a portion of it cannot be recycled and ends up in tailing ponds. The removal of naphthenic acids (NAs) from tailing pond water is crucial, as they are corrosive and toxic and provide a substrate for microbial activity that can give rise to methane, which is a potent greenhouse gas. In this study, the conversion of sawdust into an activated carbon (AC) that could be used to remove NAs from tailings water was studied. After producing biochar from sawdust by a slow-pyrolysis process, the biochar was physically activated using carbon dioxide (CO2) over a range of temperatures or prior to producing biochar, and the sawdust was chemically activated using phosphoric acid (H3PO4). The physically activated carbon had a lower surface area per gram than the chemically activated carbon. The physically produced ACs had a lower surface area per gram than chemically produced AC. In the adsorption tests with NAs, up to 35 mg of NAs was removed from the water per gram of AC. The chemically treated ACs showed better uptake, which can be attributed to its higher surface area and increased mesopore size when compared with the physically treated AC. Both the chemically produced and physically produced AC provided better uptake than the commercially AC.

Simultaneous measurements of CO2 (CER) and O2 (OER) exchange in roots and shoots of vegetative white lupin (Lupinus albus) were used to calculate the flow of reducing power to the synthesis of biomass that was more reduced per unit of carbon than carbohydrate. On a whole-plant basis, the diverted reductant utilization rate (DRUR which is: 4 × [CER + OER]) of shoot tissue was consistently higher than that of roots, and values obtained in the light were greater than those in the dark. An analysis of the biomass being synthesized over a 24-h period provided an estimate of whole-plant DRUR (3.5 mmol e−plant−1 d−1), which was similar to that measured by gas exchange (3.2 mmol e− plant−1d−1). Given that nitrate reduction to ammonia makes up about 74% of whole-plant DRUR, root nitrate reduction in white lupin was estimated to account for less than 43% of whole-plant nitrate reduction. The approach developed here should offer a powerful tool for the noninvasive study of metabolic regulation in intact plants or plant organs.

Regulation of the intracellular concentration of substrates is essential for the maintenance of a stable cellular environment. Diffusion and reaction processes supply and consume substrates within cells and determine their steady-state concentrations. To realistically represent these processes by computer simulation they must be modeled in three dimensions. Yet three-dimensional models are inherently computing intensive. This study describes a method, which substantially simplifies the modeling of diffusion into a polyhedral body (a cube), that was used as a model representation of a cell. The method is applied to a case study of oxygen diffusion into nitrogen-fixing, rhizobia-infected cells in legume nodules. The method involved generating a one-dimensional representation of the three-dimensional problem to provide a “surface area profile” of three-dimensional diffusion. The one-dimensional models were significantly easier to program, several orders of magnitude faster to solve and in this study were validated by assessing their results against those of comparable three-dimensional models of diffusion into the same body. The results show the one-dimensional method to be a close approximation of a three-dimensional source–sink problem with systematic differences below 10% for fractional oxygenation of leghemoglobin, cell respiration and nitrogenase activity. Larger differences between models (up to 45%) in the predicted average and innermost O 2 concentrations had no effects on the physiological conclusions of the study, but were attributed to the poorer resolution of the three- than the one-dimensional model, and to an inherent simplification in the derivation of the one-dimensional surface area profiles. The one-dimensional modeling approach was found to be a simple, yet powerful tool for the study of diffusion and reaction in biological systems.

Previous simulation models for the diffusion and reaction of oxygen in legume nodules were based on infected cells and neglected adjacent uninfected cells. This study uses a three-dimensional model of the central zone of legume nodules made up of the two cell types represented by a geometrically defined, space-filling, binary combination of polyhedra, each with bevelled edges to allow for a network of intercellular gas spaces. The model predicted a distinctively compartmentalized distribution of [O2] between uninfected and infected cells; with high O2 concentrations for an uninfected cell being consistent with, and necessary for, efficient operation of uricase and ureide synthesis and low O2 concentrations across most of the infected cell providing a suitable environment for N2-fixation. Compartmentalization of O2 also predicted significant O2 fluxes between cell types, compromising maintenance of low [O2] in infected cells, as well as high [O2] in uninfected cells. The results predict that there might be significant resistance to O2 diffusion across the cell : cell interface due to the plasmalemma and cell walls.

In leaves, the light reactions of photosynthesis support fatty acid synthesis but disagreement exists as to whether this occurs in green oilseeds. To address this question, simultaneous measurements of the rates of CO2 and O2 exchange (CER and OER, respectively) were made in soybean (Glycine max L.) fruits. The imbalance between CER and OER was used to estimate the diverted reductant utilization rate (DRUR) in the equation: DRUR = 4 × (OER + CER). This yielded a quantitative measure of the rate of synthesis of biomass that is more reduced per unit carbon than glucose (in photosynthesizing tissues) or than the substrates of metabolism (in respiring tissues). The DRUR increased by about 2.2-fold when fruits were illuminated due to a greater increase in OER than decrease in CER. This characteristic was shown to be a property of the seed (not the pod wall), to be present in fruits at all developmental stages, and to reach a maximal response at relatively low light. When seeds were provided with 13CO2, light reduced12CO2 production but had little effect on13CO2 fixation. When they were provided with18O2, light stimulated16O2 production but had no effect on18O2 uptake. Together, these findings indicate that light stimulates fatty acid synthesis in photosynthetic oilseeds, probably by providing both ATP and carbon skeletons.

In legume nodules the [O2] in the infected cells limits respiration and nitrogenase activity, becoming more severe if nodules are exposed to subambient O2 levels. To identify the site of O2 limitation, adenylate pools were measured in soybean (Glycine max) nodules that were frozen in liquid N2 before being ground, lyophilized, sonicated, and separated on density gradients of nonaqueous solvents (heptane/tetrachloroethylene) to yield fractions enriched in bacteroid or plant components. In nodules maintained in air, the adenylate energy charge (AEC = [ATP + 0.5 ADP]/[ATP + ADP + AMP]) was lower in the plant compartment (0.65 ± 0.04) than in the bacteroids (0.76 ± 0.095), but did not change when the nodulated root system was exposed to 10% O2. In contrast, 10% O2decreased the bacteroid AEC to 0.56 ± 0.06, leading to the conclusion that they are the primary site of O2 limitation in nodules. To account for the low but unchanged AEC in the plant compartment and for the evidence that mitochondria are localized in O2-enriched microenvironments adjacent to intercellular spaces, we propose that steep adenylate gradients may exist between the site of ATP synthesis (and ADP use) in the mitochondria and the extra-mitochondrial sites of ATP use (and ADP production) throughout the large, infected cells.

Early nodulin 2 (ENOD2) transcripts and protein are specifically found in the inner cortex of legume nodules, a location that coincides with the site of a barrier to O2 diffusion. The extracellular glycoprotein that binds the monoclonal antibody MAC236 has also been localized to this site. Thus, it has been proposed that these proteins function in the regulation of nodule permeability to O2diffusion. It would then be expected that the levels of ENOD2 mRNA/protein and MAC236 antigen would differ in nodules with different permeabilities to O2. We examined the expression of ENOD2 and other nodule-expressed genes in Rhizobium meliloti-induced alfalfa nodules grown under 8, 20, or 50% O2. Although there was a change in the amount of MAC236 glycoprotein, the levels of ENOD2 mRNA and protein did not differ significantly among nodules grown at the different [O2], suggesting that neither ENOD2 transcription nor synthesis is involved in the long-term regulation of nodule permeability. Moreover, although nodules from all treatments reduced their permeability to O2 as the partial pressure of O2(pO2) was increased to 100%, the levels of extractable ENOD2 and MAC236 proteins did not differ from those measured at the growth pO2, further suggesting that if these proteins are involved in a short-term regulation of the diffusion barrier, they must be involved in a way that does not require increased transcription or protein synthesis.

The Natural Sciences and Engineering Research Council of Canada (NSERC) recently announced a plan to undertake a "reallocations exercise" designed to provide support for new initiatives and emerging fields of research in a period of fiscal restraint. NSERC's Grant Selection Committees (GSC) were instructed to establish committees charged with preparing reports that respond to the question "Why is it important for Canada that your research community should receive some of the funds available for reallocation?" The Plant Biology and Food Science Grant Selection Committee (GSC03) responded by striking a committee representing the breadth of research it supports. Extensive input was obtained from the scientific community through a web site and electronic discussion group. A discussion document was then prepared and distributed to 34 "consultants" representing NSERC-funded researchers and leaders in the agricultural, food, and forest industries. After refining the report, a second draft was circulated to more than 90 scientists for further review. This process provided us with a collective "vision" of our discipline that focuses on the importance of fundamental research, guided by excellence, innovation through multidisciplinary approaches, international leadership potential, and relevance to Canada and Canadians. To achieve this vision, NSERC must introduce a strategy to attract, train, and retain our best young minds, establish a broad, realistic funding base, and create opportunities for more interaction among disciplines. Six specific recommendations were put forward to achieve these goals with a total request for $15.3 million in reallocated and new money. If funded, this initiative will provide GSC03 researchers with the support required to generate the ideas, communicate the insights, develop the skills, and educate the personnel that will be essential for Canada's participation in the biotechnology revolution that is transforming global agriculture, food, and forest industries. Moreover, fundamental knowledge of the interaction between plants and their environment will also help Canada play a leading role in the effective, responsible stewardship of planetary resources in the 21st century.Key words: Natural Sciences and Engineering Research Council of Canada, reallocations exercise, plant biology, food science, biotechnology, fundamental research.

The impact of nitrate (5–15 mM, 2 to 7 days) on nitrogenase activity and nodule-oxygen limitation was investigated in nodulated, 21-day-old plants of a near-isogenic nitrate reductase-deficient pea mutant (A3171) and its wild-type parent (Pisum sativum L. cv. Juneau). Within 2 days, 10 or 15 mM nitrate, but not 5 mM nitrate, inhibited the apparent nitrogenase activity (measured as in situ hydrogen evolution from nodules of intact plants) of wild-type plants; none of these nitrate levels inhibited the apparent nitrogenase activity of A3171 plants. Nodule-oxygen limitation, measured as the ratio of total nitrogenase activity to potential nitrogenase activity, was increased in both wild-type and A3171 plants by all nitrate treatments. By 3 to 4 days the apparent nitrogenase activity of A3171 and wild-type plants supplied with 5 mM nitrate declined to 53 to 69% of control plants not receiving nitrate. By 6 to 7 days the apparent nitrogenase activity of A3171 plants was similar to the control value whereas that of the wild-type plants continued to decline. From 3 to 7 days, no significant differences in nodule-oxygen limitation were observed between the nitrate (5 mM) and control treatments. The results are interpreted as evidence for separate mechanisms in the initial (O2 limitation) and longer-term (nitrate metabolism) effects of nitrate on nitrogen fixation by effectively nodulated pea.

Soybean [Glycine max (L.) Merr] is a subtropical legume that is at the northern limit of its range in the cool temperate zone of North America. Low temperature stress inhibits N-2 fixation and NO3 metabolism in soybean. A controlled environment experiment was conducted to test the effects of low temperature stress (16/9 degrees C day/night versus 26/19 degrees C) on N-2 fixation and NO3 metabolism of one American, one northern Chinese, one Polish, one Swedish and two Canadian soybean cultivars. The objectives of this work were to measure variability in N-2 fixation, NO3 use, and plant growth under low and optimal temperature regimes among these diverse genotypes, and to determine the physiological bases for these differences. The results showed that: (1) the relative ranking of cultivars for growth and accumulation is different between nitrogen sources under optimal or suboptimal temperatures; (2) the cultivars which are better able to fix N-2 usually have more nodule mass; (3) the plants which fix more N-2 allocate less photosynthate to the plant root system, and (4) there was no negative relationship between N:! fixation or NO, utilization among the cultivars tested. (C) 1997 Elsevier Science B.V.

The aim of the present study was to test the hypothesis that the N content or the composition of the phloem sap that supplies nodulated roots may play a role in the feedback regulation of nitrogenase activity by increasing nodule resistance to O2 diffusion. Treating shoots of lupin (Lupinus albus cv Manitoba) or soybean (Glycine max L. Merr. cv Maple Arrow) with 100 [mu]L L-1 NH3 caused a 1.3-fold (lupin) and 2.6-fold (soybean) increase in the total N content of phloem sap without altering its C content. The increase in phloem N was due primarily to a 4.8-fold (lupin) and 10.5-fold (soybean) increase in the concentration of glutamine N. In addition, there was a decline in both the apparent nitrogenase activity and total nitrogenase activity that began within 4 h and reached about 54% of its initial activity within 6 h of the start of the NH3 treatment. However, the potential nitrogenase activity values in the treated plants were not significantly different from those of the control plants. These results provide evidence that changes in the N composition of the phloem sap, particularly the glutamine content, may increase nodule resistance to O2 diffusion and, thereby, down-regulate nodule metabolism and nitrogenase activity by controlling the supply of O2 to the bacteria-infected cells.

In soybeans (Glycine max L. Merr.), high levels of soil nitrate inhibit N2 fixation, and nitrate-tolerant symbioses have been identified within a chemically mutagenized line of cv. Bragg denoted nts382 and within the line K466, a genotype representative of a number of Korean soybean cultivars. The genotypes nts382 and K466 were examined to see if they could be used as a model system for studying the mechanism responsible for the short-term (i.e. 3-day) inhibition of specific nitrogenase activity, especially the mechanism behind the greater O 2 limitation of nodule metabolism that is characteristic of nitrate inhibition of N2 fixation in soybean. In nts382, total nitrogenase activity (TNA = H 2 production in Ar:O 2 ) was inhibited to a lesser degree (48% of control) relative to Bragg (30% of control), and the nitrate-treated symbioses showed less of an 02 limitation of nodule metabolism in nts382 than in Bragg. However, the relative proportion of O 2 limitation to the total nitrate inhibition was similar (40 and 41%) in nts382 and Bragg, respectively. Therefore, the nts382 symbioses may be useful in elucidating the general mechanism for down-regulation of nitrogenase activity in soybean, but would not be a useful model system for studying the control of O 2 -limited metabolism following nitrate exposure. The effects of nitrate on TNA and on the degree of O2 limitation of nodule metabolism were the same in K466 and a reference cultivar Maple Arrow. Consequently, the tolerance of K466 to nitrate reported previously was attributed to the ability of this symbiosis to maintain nodule biomass in the presence of nitrate, not to any ability to maintain specific nitrogenase activity in the presence of nitrate.

Barry J. Shelp, Craig S. Walton, Wayne A. Snedden, Lucie G. Tuin, Ivan J. Oresnik and David B. LayzellShelp, BJ., Walton.C.S..Snedden,W.A..Tuin.L.G.,Oresnik,I.J.andLayzell.D.B.1995. Gaba shunt in developing soybean seeds is associated with hypoxia. - Physiol.Plant. 94: 219-228.In the present study we investigated the proposal that Ihe )'-aminobutyrate (Gaba)shunt in developing soybean (Glycine max [L.] Merr.) seeds is associated with hypoxia.The ontogeny and pH profile of enzymes associated with glutamate metabolism(glutamate decarboxylase [EC 4.1.1.15]. Gaba transaminase [EC 2.6.1.19], succinicsemialdehyde dehydrogenase [EC 1.2.1.16], glutamate dehydrogenase {EC 1.4.U],glutamate:oxaloacetate transaminase [EC 2.6.1.1], glutamate:pyruvate transaminase[EC 2.6.1.2] and 2-cxoglutarate dehydrogenase complex [EC 1.2.4.2]) and hypoxia(aicohol dehydrogenase [ADH, EC 1.1.1.1] and pyruvate decarboxylase [PDC, EC4.1.1.1]) were determined in cotyledons, nucellus and seed-coat tissues. Gaba-shuntenzymes were ubiquitous in the developing seed. Activities of enzymes catalyzingglutamate-C entry into the Krebs cycle via 2-oxoglutarate were generally greater thanthose of Gaba-shunt enzymes. In cotyledons, the activity of ADH increased throughoutseed development (up to 72 days after anthesis [DAA]), whereas PDC was static duringearly development, then increased. In contrast, the activities of ADH and PDC inmaternal tissues (nucellus and seed coat) were initially high, then declined dramaticallyafter .37 DAA. The adenylate energy charge (AEC) = ([ATP] + 0.5 [ADP])/([ATP] + ]ADP] + [AMP]) of soybean seeds from fruits (37 DAA) frozen in situ waslow (0.67±0.01) compared to the AEC of adjacent pod tissue (0.82±0.04) andcotyledons exposed to air (0.84±0.01). A 60-min time-course study showed that therate of ]U-'^C]-glutamate catabolism by an intact excised cotyledon at 37 DAA wasmarkedly lower at 8 and 0% O: than at 21%; the pool size of ['•'C]-Gaba wasunaffected. The data indicated that: (i) Gaba-shunt activity is not a response to limitedglutamate deamination/transamination: (2) the soybean seed is hypoxic; and (3) therelative partitioning of glutamate-C through glutamate decarboxylase is increased byhypoxia.Key words - Adenylate energy charge, Gaba shunt, glutamate decarboxylase, Glycinemax. hypoxia. Krebs cycle, stress.B.J. Shelp icorresponding author). C.S. Walton, W.A. Snedden andL. G. Tuin. Dept ofHorticultural Science and Interdepartmental Plant Physiology Program. Univ. ofGiielph. Guelph, ON. Canada N1G2W1; I.J. Oresnik and D.B. Layzell. Dept ofBiology. Queen's Univ., Kingston. ON. Canada K7L3N6.nucellus, which separates the seed coat from the embryo,Introanction Balance sheets for the utilization of amino compounds inDeveloping seeds must receive their nutrients for growth maturing fruits of cowpea (Peoples et al. 1985) and inand development from the mother plant (Pate 1984, developing seeds of soybean (Micallef and Shelp 1989a)Thome 1985), The major form of N delivered to a soy- indicate that many other amino acids present in seedbean embryo is generally helieved to he glutamine (Rain- protein are synthesized in situ. Thus, glutamate and/orbird et al, 1984), hut Hsu et al, (1984) suggested that glutamine are probably involved in a number of bio-glutamine is quantitatively converted to glutamate in the synthetic reactions leading to the synthesis of aminoReceived 7 September, 1994; revised 17 February, 1995

In legume nodules, treatments such as detopping or nitrate fertilization inhibit nodule metabolism and N2 fixation by decreasing the nodule's permeability to O2 diffusion, thereby decreasing the infected cell O2 concentration (Oi) and increasing the degree to which nodule metabolism is limited by O2 availability. In the present study we used nodule oximetry to assess and compare the role of O2 limitation in soybean (Glycine max L. Merr) nodules inhibited by either drought or detopping. Compared to detopping, drought caused only minor decreases in Oi, and when the external O2 concentration was increased to raise Oi, the infected cell respiration rate in the drought-stressed plants was not stimulated as much as it was in the nodules of the detopped plants. Unlike those in detopped plants, nodules exposed to moderate drought stress displayed an O2-sufficient respiration rate that was significantly lower than that in control nodules. Despite possible side effects of oximetry in altering nodule metabolism, these results provided direct evidence that, compared to detopping, O2 limitation plays a minor role in the inhibition of nodule metabolism during drought stress and changes in nodule permeability are the effect, not the cause, of a drought-induced inhibition of nodule metabolism and the O2-suffiecient rate of respiration.

Nodulated soybeans (Glycine max L. Merr, cv. Maple Arrow) were exposed to various physiological and environmental treatments to determine the relationship between nodule adenylate pools and the degree of O2 limitation of nitrogenase. Adenylate energy charge (AEC = [ATP + 0.5 ADP]/[ATP + ADP + AMP]) and ATP/ADP ratios declined under conditions of decreased (10%) external pO2 but increased in nodules exposed to elevated (30%) external pO2. Nitrogenase activity was inhibited by both pO2 treatments, but recovered towards initial levels within 45 min. AEC also returned to initial levels during this period. To account for these and related data in the literature, it was hypothesized that 1) legume nodules regulate infected cell O2 concentration (Oi) to maintain adenylate pools at levels which limit respiratory metabolism: 2) treatments which decrease Oi alter the adenylate pools and further limit nodule metabolism; 3) treatments which increase Oi to levels in excess of a narrow range alter the adenylate pools and activate biochemical pathways which are not conducive to nitrogenase activity. In a preliminary test of these hypotheses, changes in AEC and ATP/ADP ratio were studied in nodules in which nitrogenase activity was inhibited by stem girdling, nitrate fertilization and exposure to an Ar:O2 atmosphere. All three treatments caused an increased O2 limitation of nodule respiration and nitrogenase activity. However, decreases in AEC were observed only in the stem girdling and nitrate fertilization treatment: Ar:O2 exposure had no effect on whole nodule AEC. While this result challenged the hypotheses suggesting a central role for adenylates in the regulation of O2-limited metabolism, it was noted that the Ar:O2 treatment would differ from the other treatments in that it would have a specific effect on the ATP demands for NH3 assimilation in the plant fraction. Since AEC and ATP/ADP ratio would be affected by both the rate of ATP synthesis (potentially an O2-limited process) and the demand for ATP, changes in these parameters in the whole nodule may not be a reliable indicator of adenylate-mediated O2 limitation. Futher studies are needed to examine in vivo changes in adenylate pools in the plant and bacteroid fractions in nodules which vary in their degree of O2-limited metabolism.

Two different simulation models were constructed to describe O2 diffusion into the bacteria-infected cells of legume nodules: one based on a central zone of uniform spherical cells and the other on a central zone of packed, uniform cubical cells with air spaces along the edges. The cubical model more closely approximated the geometry and gas diffusion characteristics of infected cells than did the spherical model. The models relied on set values for the innermost O2 concentration in the infected cell (1-20 nM) and predicted values for the free O2 and oxygenated leghemoglobin gradients toward the cell:space interface. The cubical model but not the spherical model predicted saturation of leghemoglobin (Lb) oxygenation at or within a few micrometers of the gas-filled intercellular space and predicted that the space concentration could be as high as 1.3% O2 when the fractional oxygenation of Lb and respiration rate within the infected cell were typical of that which has been measured in vivo. In the model, the higher the space O2 concentration, the greater the saturation of Lb by O2 and the greater the collapse of Lb-facilitated diffusion near the cell:space interface. This was predicted to result in a greater resistance to O2 diffusion from the space to the bacteroids, thereby providing an intrinsic, homeostatic mechanism for controlling the rate of O2 influx into infected cells. Changes in the physiological features of the simulated cubical infected cell, such as the proportion of the cell as cytosol, the surface area of the cell exposed to a space, the maximum rate of cellular respiration, or the concentration of Lb in the cytoplasm, significantly altered the extent to which the infected cell would be able to regulate its diffusive resistance. These results demonstrate the possibility of a Lb-based mechanism for controlling the O2 concentration within the infected cells. If such a mechanism exists in legume nodules, it would give the infected cell an ability to exercise fine control over its internal environment, a process that could complement a physical diffusion barrier that may exist in the inner cortex or elsewhere in the nodule and provide coarse control over O2 diffusion.

Regulation of nodule permeability in response to short-term changes in environmental and physiological conditions is thought to occur by occlusion of intercellular spacesin the nodule inner cortex. To test this hypothesis, the permeability of legume nodules was altered by adapting them to either 20 or 80% O 2 over a 2.5-h period. The nodules were then rapidly frozen, cryo-planed and examined under cryo-scanning electron microscopy for differences in the number, area or shape factor of intercellular spaces

Adenylates (ATP, ADP, and AMP) may play a central role in the regulation of the O2-limited C and N metabolism of soybean nodules. To be able to interpret measurements of adenylate levels in whole nodules and to appreciate the significance of observed changes in adenylates associated with changes in O2-limited metabolism, methods were developed for measuring in vivo levels of adenylate pools in the cortex, plant central zone, and bacteroid fractions of soybean (Glycine max L. Merr cv Maple Arrow x Bradyrhizobium japonicum strain USDA 16) nodules. Intact nodulated roots were either frozen in situ by flushing with prechilled Freon-113(-156[deg]C) or by rapidly (

The ability of legume nodules to regulate their permeability to gas diffusion has been attributed to physiological control over the size and distribution of gas-filed intercellular spaces within the nodule cortex. To examine the size and distribution of intercellular spaces and to determine whether they were filled with gas (high diffusion permeability) or liquid (low diffusion permeability), whole nodules were frozen in liquid nitrogen slush (-210°C), and then either cryo-fractured or cryo-planed before being examined by cold-stage scanning electron microscopy (SEM). The cryo-planed tissue was found to have many advantages over cryo-fractured nodules in providing images which were easier to interpret and quantify. Intercellular spaces throughout the nodule were examined in both tangential and medial planed faces. Since no differences were observed between views in either the size or shape of the open intercellular spaces, it was concluded that the intercellular spaces of nodules were not radially oriented as assumed in many mathematical models of gas diffusion. The inner cortex region in the nodules had the smallest intercellular spaces compared to other zones, and less than 10% of the intercellular spaces were occluded with any type of material in the central zone regions. Vacuum infiltration of nodules with salt solutions and subsequent cryo-planing for SEM examination showed that open and water-filled intercellular spaces could be differentiated. The potential is discussed for using this method to study the mechanism of diffusion barrier regulation in legume nodules.

Measurements of the short-term response of nodulated roots of soybean (Glycine max L. Merr, cv. Harosoy: Bradyrhizobium japonicum USDA 16) to rapid changes in surrounding pO2 indicate that their ability to reversibly adjust gaseous diffusive resistance is retained whether plants are cultured in rhizospheres of very low (2.8%) or very high (61.2%) pO2. Thus the capacity for reversible short-term diffusion adjustment is additional to structural changes in the fixed diffusional barriers of nodules which allow their continued fixation of N2 in unfavourably high or low external pO2. Anatomical evidence, involving quantitative measurement of intercellular spaces in the cortical tissues using electron microscopy of thin sections, indicates that the major fixed diffusional barrier is a boundary layer of cells in the inner cortex which may be as small as one cell thick in nodules from 2.8% O2 to 5 or 6 cells thick, and almost completely devoid of intercellular spaces, in those from 61.2% O2. The data are interpreted to indicate that the variable diffusion harrier is distinct from the boundary layer and is most likely to be a property of cells and/or intercellular spaces inside the boundary layer of the nodule cortex.

Large-scale production of renewable synthetic natural gas from biomass (bioSNG) in Canada was assessed for its ability to mitigate energy security and climate change risks. The land area within 100 km of Canada's network of natural gas pipelines was estimated to be capable of producing 67-210 Mt of dry lignocellulosic biomass per year with minimal adverse impacts on food and fiber production. Biomass gasification and subsequent methanation and upgrading were estimated to yield 16,000-61,000 Mm(3) of pipeline-quality gas (equivalent to 16-63% of Canada's current gas use). Life-cycle greenhouse gas emissions of bioSNG-based electricity were calculated to be only 8.2-10% of the emissions from coal-fired power. Although predicted production costs ($17-21 GJ(-1)) were much higher than current energy prices, a value for low-carbon energy would narrow the price differential. A bioSNG sector could infuse Canada's rural economy with $41-130 billion of investments and create 410,000-1,300,000 jobs while developing a nation-wide low-carbon energy system.

The gas exchange characteristics of intact attached nodulated roots of pea (Pisum sativum cv. Finale X) and lupin (Lupinus albus cv. Ultra) were studied under a number of environmental conditions to determine whether or not the nodules regulate resistance to oxygen diffusion. Nitrogenase activity (H2 evolution) in both species was inhibited by an increase in rhizosphere pO2 from 20% to 30%, but recovered within 30 min without a significant increase in nodulated root respiration (CO2 evolution). These data suggest that the nodules possess a variable barrier to O2 diffusion. Also, nitrogenase activity in both species declined when the roots were either exposed to an atmosphere of Ar:O2 or when the shoots of the plants were excised. These declines could be reversed by elevating rhizosphere pO2, indicating that the inhibition of nitrogenase activity resulted from an increase in gas diffusion resistance and consequent O2-limitation of nitrogenase-linked respiration. These results indicate that nodules of pea and lupin regulate their internal O2 concentration in a manner similar to nodules of soybean, despite the distinct morphological and biochemical differences that exist between the nodules of the 3 species. Experiments in which total nitrogenase activity (TNA = H2 production in Ar:O2) in pea and lupin nodules was monitored while rhizosphere pO2 was increased gradually to 100%, showed that the resistance of the nodules to O2 diffusion maintains nitrogenase activity at about 80% of its potential activity (PNA) under normal atmospheric conditions. The O2-limitation coefficient of nitrogenase (OLCN= TNA/PNA) declined significantly with prolonged exposure to Ar:O2 or with shoot excision. Together, these results indicate a significant degree of O2-limitation of nitrogenase activity in pea and lupin nodules, and that yields may be increased by realizing full potential activity.

Gas exchange measurements and noninvasive leghemoglobin (Lb) spectrophotometry (nodule oximetry) were used to monitor nodule responses to shoot removal in alfalfa (Medicago sativa L. cv Weevlchek) and birdsfoot trefoil (Lotus corniculatus L. cv Fergus). In each species, total nitrogenase activity, measured as H(2) evolution in Ar:O(2) (80:20), decreased to50% of the initial rate within 1 hour after detopping, and net CO(2) production decreased to about 65% of the initial value. In a separate experiment in which nodule oximetry was used, nodule O(2) permeability decreased 50% within 5 hours in each species. A similar decrease in the O(2)-saturated respiration rate (V(max)) for the nodule central zone occurred within 5 hours in birdsfoot trefoil, but only after 24 hours in alfalfa. Lb concentration, also measured by oximetry, decreased after 48 to 72 hours. The decrease in permeability preceded the decrease in V(max) in each species. V(max) may depend mainly on carbohydrate availability in the nodule. If so, then the decrease in permeability could not have been triggered by decreasing carbohydrate availability. Both oximetry and gas exchange data were consistent with the hypothesis that, for the cultivars tested, carbohydrate availability decreased more rapidly in birdsfoot trefoil than in alfalfa nodules. Fractional Lb oxygenation (initially about 0.15) decreased during the first 24 hours after detopping but subsequently increased to0.65 for a majority of nodules of each species. This increase could lead to O(2) inactivation of nitrogenase.

Application of allopurinol (AP; 1H-pyrazolo-[3,5-d]pyrimidine-4-o1) to intact nodulated roots of ureide-forming legumes causes rapid inhibition of NAD:xanthine dehydrogenase (XDH: EC 1.2.1.37), cessation of ureide synthesis and, subsequently, severe nitrogen deficiency (Atkins et al. 1988. Plant Physiology 88: 1229–1234). Nitrogen deficiency is a result of inhibited nitrogenase (EC 1.7.99.2) activity. Using an open gas exchange system to measure H2 and CO2 evolution, short term effects of AP application were examined in a Hup− soybean symbiosis [Glycine max (L.) Merr. cv. Harosoy: USDA 16]. The onset of inhibition of nitrogenase was detected after ca 2 h exposure of the roots to AP. At the same time xanthine began to accumulate and ureide levels declined in nodules as a result of inhibition of XDH. The decline in H2 evolution following AP application was not due to altered electron allocation between N2 and H+ by nitrogenease but was coincident with increased gaseous diffusive resistance of nodules and a decline in intracellular oxygen concentration. A possible scheme for the intermediary metabolism of soybean nodules which might account for a direct connection between nitrogenase activity and ureide synthesis is proposed. The suggested mechanism envisages coupling production of reducing power by cytosolic enzymes of purine oxidation to synthesis of dicarboxylic acid substrates (malate and succinate) required for bacteroid respiration.

When intact nodulated roots of soybean ( Glycine max L. Merr. nodulated with Bradyrhizobium japonicum strain USDA 16) were exposed to an atmosphere lacking N 2 gas (Ar:O 2 80:20), total nitrogenase activity (measured as H 2 evolution) and respiration (CO 2 evolution) declined with time of exposure. In Ar-inhibited nodules, when the O 2 concentration in the rhizosphere was increased in a linear `ramp9 of 2.7% per minute, 93% of the original H 2 evolution and 99% of the CO 2 evolution could be recovered. The internal nodule O 2 concentration (estimated from leghemoglobin oxygenation) declined to 56% of its initial value after 60 minutes of Ar:O 2 exposure and could be partially recovered by the linear increases in O 2 concentration. Nodule gas permeability, as estimated from the lag in ethylene production following exposure of nodules to acetylene, decreased to 26% of its initial value during the Ar-induced decline. Collectively, the results provide direct evidence that the Ar-induced decline results from decreased nodule gas permeability and indicate that the decline in permeability, rather than being immediate, occurs gradually over the period of Ar:O 2 exposure.

Physiological regulation of nodule gas permeability has a central role in the response of legumes to such diverse factors as drought, defoliation, and soil nitrate. A new method for quantifying nodule respiration and O(2) permeability, based on noninvasive spectrophotometry of leghemoglobin, was evaluated using intact, attached nodules of Lotus corniculatus. First, the relationship between nodule respiration (O(2) consumption) rate and internal O(2) concentration was determined from the rate of decrease in fractional oxygenation of leghemoglobin (FOL) under N(2). The rate of increase of FOL under 100% O(2) was then used to calculate nodule O(2) permeability, after correcting for respiration. Inactivation of nitrogenase by exposure to 100% O(2) for 15 minutes led to decreases in both permeability and O(2)-saturated respiration (V(max)), but the brief (15 seconds) exposures to 100% O(2) required by the assay itself had little effect on either parameter. A gradual increase in external O(2) concentration from 20 to 40% resulted in a reversible decrease in permeability, but no change in V(max). The new method is likely to be useful for research on nodule physiology and might also be applicable to agronomic research and crop improvement programs.

(...) a noninvasive method (based on spectrophotometry of leghemglobin) was used to measure O 2 concentration within intact, attached nodules of soybean [Glycine max (L.) Merr.]. A detopping (shoot removal) treatment, which interrupted phloem supply to nodules, resulted in lower internal O 2 concentration relative to controls suggesting that nodule respiration was directly limited by O 2 rather than carbohydrate. When the gas stream around a nodule was switched from pure N 2 to pure O 2 , internal O 2 concentration increased more rapidly in control nodules than in nodules attached to detopped plants (...)