An apparent shoot rest period was induced in the 2nd month of growth of alfalfa (Medicago sativa ... more An apparent shoot rest period was induced in the 2nd month of growth of alfalfa (Medicago sativa L.) seedlings by a drop in growth temperature from 25:20 °C to 10:7 °C. After prolonged chilling the shoots were replaced by new shoots. Temperature profiles of nodulated root respiration and nitrogenase activity (acetylene reduction and H2 evolution) were measured simultaneously in experiments with a flow-through gassing system during 3 months of cold treatment. Net photosynthesis of whole plants was measured in a closed system. More than half the total initial nitrogenase activity and relative efficiency (RE) were lost during the rest period and recovered during regrowth. Acetylene reduction by chilled plants was insensitive to temperature in the 5 – 15 °C range, unlike the temperature dependence of respiration and H2 evolution in air. In all temperature profiles of RE the RE was highest at 5 – 10 °C. The RE was minimum 10 – 15 °C during the rest period. The optimum temperature for whole plant net photosynthesis also declined to 10 – 15 °C during chilling and it later flattened out in the cold-acclimated regrowth. Possible mechanisms are discussed.
Floral development in Caryopteris × clandonensis A. Simmonds (C. incana (Houtt.) Miq. × C. mongho... more Floral development in Caryopteris × clandonensis A. Simmonds (C. incana (Houtt.) Miq. × C. mongholica Bunge) from cyme initiation through to anthesis is described, with emphasis on the development of the anthers. The later stages of development and anthesis are completed only on mature plants under short days (SD, 8 h light per day). Under long days (LD, 20 h light per day) senescence occurs in the early stages of anther wall differentiation.
Simultaneous measurements of CO 2 (CER) and O 2 (OER) exchange in roots and shoots of vegetative ... more Simultaneous measurements of CO 2 (CER) and O 2 (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 ϫ [CER ϩ 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 Ϫ plant Ϫ1 d Ϫ1), which was similar to that measured by gas exchange (3.2 mmol e Ϫ plant Ϫ1 d Ϫ1). 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.
Current plant science and biotechnology in agriculture, 1998
Nitrogen-fixing legume nodules live on a metabolic knife-edge. The nitrogenase (N2ase) enzyme is ... more Nitrogen-fixing legume nodules live on a metabolic knife-edge. The nitrogenase (N2ase) enzyme is extremely sensitive to irreversible damage by O2, yet the enzyme, the bacteria that synthesize it, and the infected cell of the host require ATP generated through oxidative phosphorylation within the plant mitochondria or the bacteria. This review covers current understanding of how legume nodules maintain precise control over intercellular O2 despite changes in environmental or physiological conditions.
We develop and illustrate a method for reconciling index decomposition analysis of energy intensi... more We develop and illustrate a method for reconciling index decomposition analysis of energy intensity with physically based, sector-specific energy efficiency indicators. Decomposition analysis of individual sector intensity contributions to total energy intensity is nested within the higher-order decomposition analysis of E/GDP such that the contribution of energy efficiency gains to changes in total energy intensity can be determined. Energy, economic and physical activity data for Canada for the period 1995-2010 are used to illustrate the method. Intrasector structural factors were found to be both positive and negative and to be significant contributors to energy intensities in both the business and household sectors. In aggregate, intrasectoral structural change offset energy efficiency gains and put upward pressure on (E/GDP) between 1995 and 2010 but was three times smaller than the offsetting decline in E/GDP due to intersectoral structural change. The method can be used for assessing the contribution of energy efficiency to sector energy intensities; for placing energy efficiency policies in the larger context of the other factors that determine an economy's energy intensity and greenhouse gas emissions; for identifying nonefficiency policy targets for improving energy productivity; and for increasing the sophistication of forecasting and scenario analysis of future levels and patterns of fuel and electricity consumption and related greenhouse gas emissions.
To assess the role of O2 in the regulation of nodule metabolism following a decrease or an increa... more To assess the role of O2 in the regulation of nodule metabolism following a decrease or an increase in temperature, the fractional oxygenation of leghemoglobin (FOL) was measured in soybean (Glycine max L. Merr.) nodules during rapid and gradual changes in temperature from 20[deg]C to either 15 or 25[deg]C. The affinity of leghemoglobin for O2 was also measured at each temperature and the values were used to calculate the infected cell O2 concentration (Oi). After nodules were transferred to 15[deg]C, FOL and Oi increased and adenylate energy charge (AEC = [ATP + 0.5ADP]/[ATP + ADP + AMP]) increased from 0.70 to 0.78. The temperature increase was associated with a decrease in FOL and Oi. We concluded that changes in nodule temperature alter the respiratory demand of the nodules for O2, resulting in a change in Oi and a shift in the balance between ATP consumption and ATP production within the nodule tissue.
. Positron emission tomography (PET) has been utilized to obtain dynamic images of long distance ... more . Positron emission tomography (PET) has been utilized to obtain dynamic images of long distance nutrient translocation in plants. Positron emitting 18F, produced by a Van de Graaff accelerator using the reaction 18O(p,n)18F, was fed in solution to excised stems of Glycine max positioned vertically in a large‐aperture PET detector system. Images of tracer activity were recorded with a time resolution of 0.5 min and a spatial resolution of 4 mm. Maximum tracer activities at stem sites were obtained within 3 min of the pulse feed. A model is presented enabling evaluation of regional values for tracer flow, tracer binding, flow speed and flow volume. Analysis of data for one stem position yielded a flow volume of 2.1mm3 min−1 and a flow speed of 36cm min−1. Comparison with the distribution of 14C‐inulin, which was simultaneously fed to the cut stems, indicates the 18F is suitable for use as an apoplastic tracer; 92% of the tracer activity accumulated in the leaves. The fraction of 18F that remained bound was most concentrated at stem nodal regions, an observation consistent with the existence of transfer cells at these sites. Advantages and limitations of PET applied to plant physiological investigations are discussed.
Large-scale production of renewable synthetic natural gas from biomass (bioSNG) in Canada was ass... more 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 nationwide low-carbon energy system.
After 40 days of growth at 25''C, Lotus pedunculatus eav,, cv. Maku plants infected with Rhizobiu... more After 40 days of growth at 25''C, Lotus pedunculatus eav,, cv. Maku plants infected with Rhizobium loti strain NZP2037 displayed similar relative growth rates but had twiee the nodule mass and only one third the whole plant dry weight of plants infected with Bradyrhizobium sp. {Lotus) strain CC814s. In the NZP2037 symbiosis, the rate of CO; evolution (per g dry weight of nodulated root) was 1.6 times as high as that in the CC814s symbiosis while the rate of QH; reduction (per g dry weight of nodule) was only 48% of that in the CC814s symbiosis. Studies of the effect of short term temperature changes on the gas exchange characteristics (CO, and H^ evolution, CjH, reduction) of these symbioses revealed wide differences in the optima for C^H, reduction. Nodules infected with NZP2037 displayed maximal QH, reduction rates [157 l^mol (g dry weight nodule)"' h"'] at 12°C, whereas nodules infected with CC814s were optimal at 30°C [208 nmoi (g dry weight nodnie)"' h'']. These short term studies suggested that differences in temperature optima for N; may have partially accounted for the poorer effectivity, at 25°C, of strain NZP2037 when compared with strain CC-814s. The relative efficiency [RE = 1-(H^evolution/C^H, reduction)] of N^ fixation varied widely with temperature in the two symbioses, but there was a general trend toward higher RE with lower temperatures. The ratio of CO; evolution: QH^ reduetion (mol/mol) in nodulated roots infected with CC814s was constant (ca 10 COVC^H,) between 5°C and 30°C, whereas in plants infected with NZP2037 it reached a minimal value of 3.3 CO^/QH; at lO^C and was 19 COj/CjHj at the growing temperature (25°C), Additional key words-Bradyrhizobium sp. (Lotus); Rhizobium loti.
Steady state cultures of Anabaena Jlos-aquae were established over a wide range of phosphate-limi... more Steady state cultures of Anabaena Jlos-aquae were established over a wide range of phosphate-limited growth rates while N was supplied as either NH3, N03-, or N2 gas. At growth rates greater than 0.03 per hour, rates of gross and net carbon fixation were similar on all N sources. However, at lower growth rates (<0.03 per hour) in the N03 and N2 cultures, gross photosynthesis greatly exceeded net photosynthesis. The increase in photosynthetic 02 evolution with growth rate was greatest when N requirements were met by N03and least when met by NH3. These results were combined with previously reported measurements of cellular chemical composition, N assimilation, and acetylene reduction (Layzell, Turpin, Elrifi 1985 Plant Physiol 78: 739-745) to construct empirical models of carbon and energy flow for cultures grown at 30, 60, and 100% of their maximal growth rate on all N sources. The models suggested that over this growth range, 89 to 100% of photodriven electrons were allocated to biomass production in the NH3 cells, whereas only 49 to 74% and 54 to 90% were partitioned to biomass in the NO3-and Nrgrown cells, respectively. The models were used to estimate the relative contribution of active, maintenance, and establishment costs associated with N03and N2 assimilation over the entire range of growth rates. The models showed that the relative contribution of the component costs of N assimilation were growth rate dependent. At higher growth rates, the major costs for N03assimilation were the active costs, while in N2-fixing cultures the major energetic requirements were those associated with heterocyst establishment and maintenance. It was concluded that compared with N03assimilation, N2 fixation was energetically unfavorable due to the costs of heterocyst establishment and maintenance, rather than the active costs of N2 assimilation.
The aim of the present study was to test the hypothesis that the N content or the composition of ... more 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 O, diffusion. Treating shoots of lupin (Lupinos albus cv Manitoba) or soybean (Glycine max L. Merr. cv Maple Arrow) with 100 p l L-' NH, 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 5 4 % of its initial activity within 6 h of the start of the NH, 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 O, diffusion and, thereby, downregulate nodule metabolism and nitrogenase activity by controlling the supply of O, to the bacteria-infected cells.
Annual review of plant physiology and plant molecular biology, Jun 1, 1993
Measuring the rates at which legume nodules exchange various gases with their environment allows ... more Measuring the rates at which legume nodules exchange various gases with their environment allows a great deal of information to be obtained concerning the factors that regulate and limit N2 fixation. However, studies over the past 10 years have shown that ...
Electron flow from oxidation of excess H2 released by root nodules was shown to contribute to mic... more Electron flow from oxidation of excess H2 released by root nodules was shown to contribute to microbial CO2 fixation in soybean crops. This discovery has important implications for carbon storage in soils used to grow legumes; however, further research is needed to understand the fate and turnover time of this H2-coupled CO2 fixation. Isotopic labeling of soil through incubation with 13CO2 was used to elucidate movement of sequestered carbon into soil carbon pools. Measurement of isotopic shifts was determined using Isotope Ratio Mass Spectrometry. Preliminary experiments have confirmed CO2 uptake through an isotopic shift (Delta13C -20.4 to -14.5 0/00) in 24 hour incubated soils labeled with 13CO2 (1% v/v, 99.5 Atom%) under elevated H2 concentration (6000 ppm). Other incubation experiments have confirmed the biotic nature of observed CO2 uptake by comparing isotopic shifts in oven dried and autoclaved soils to moist soil. Under an elevated H2 atmosphere, no significant isotopic shift was observed in dry and autoclaved soils whereas moist soil showed an isotopic shift of Delta13C -21.9 to 11.4 0/00 over 48 hours. Future experiments will involve longer incubations (7 days) and will be aimed at determining isotopic shifts within soil carbon pools. Samples will be incubated and fractionated into microbial biomass, light fraction carbon, and acid stable carbon and subsequent isotopic analysis will be carried out. This will help determine the distribution of H2- coupled fixed CO2 within soil carbon pools and the turnover time of sequestered carbon. This and further research may lead to modification of greenhouse gas coefficients for leguminous crops that includes a CO2 fixation component.
ABSTRACT Human activities have had a major impact on the global carbon and nitrogen cycles. In co... more ABSTRACT Human activities have had a major impact on the global carbon and nitrogen cycles. In contrast to studies of CO2 exchange, effective tools for the non-invasive measurements of N cycle transformations have not been developed and there is much to learn about the control of, and complex interactions associated with processes such as nitrification, denitrification and nitrate assimilation. Since N cycle transformations involve redox reactions, a theoretical model was developed to explore the potential for using the imbalance in measured CO2 and O2 exchange of biological systems as an indirect, non-invasive measure of nitrogen cycle transformations. For every mole of NH3 nitrified to NO3-, the model predicted that there would be 2 more moles of O2 taken up than CO2 produced by the same system that was not undergoing nitrification. To test this hypothesis, laboratory and field studies where carried out in which control and NH3-treated soils were continuously assayed for CO2 (Licor IRGA) and O2 (Qubit Systems Differential O2 Analyzer) exchange. The laboratory studies clearly showed that for about 3 weeks following soil treatment with NH3 (8.2 umol NH3/gDW soil, equivalent to 200 kg N/ha), CO2 production was suppressed by about 18%, and O2 uptake was stimulated by up to 251% (average of 153%) relative to control. Similar results were obtained with chamber studies in the field. The lab measurements were combined with the model to predict the rate of nitrification and then compare this with measurements of NH3 depletion and NO3- accumulation. Despite the large imbalance that was observed in CO2 and O2 exchange, the gas exchange measurements could only account for 50% of the reductant that was associated with the actual nitrification rate. Further work is needed to address the destination of the other reductant, but it may include mineral element reduction or microbial synthesis of highly reduced C compounds. Nevertheless, if the relationship between nitrification rates and soil CO2-O2 exchange is similar in other soils, it may be possible to use the imbalance of CO2 and O2 exchange as a non-invasive measure of nitrification. This assay should also allow for a better understanding of C and N dynamics and interactions in soils.
An apparent shoot rest period was induced in the 2nd month of growth of alfalfa (Medicago sativa ... more An apparent shoot rest period was induced in the 2nd month of growth of alfalfa (Medicago sativa L.) seedlings by a drop in growth temperature from 25:20 °C to 10:7 °C. After prolonged chilling the shoots were replaced by new shoots. Temperature profiles of nodulated root respiration and nitrogenase activity (acetylene reduction and H2 evolution) were measured simultaneously in experiments with a flow-through gassing system during 3 months of cold treatment. Net photosynthesis of whole plants was measured in a closed system. More than half the total initial nitrogenase activity and relative efficiency (RE) were lost during the rest period and recovered during regrowth. Acetylene reduction by chilled plants was insensitive to temperature in the 5 – 15 °C range, unlike the temperature dependence of respiration and H2 evolution in air. In all temperature profiles of RE the RE was highest at 5 – 10 °C. The RE was minimum 10 – 15 °C during the rest period. The optimum temperature for whole plant net photosynthesis also declined to 10 – 15 °C during chilling and it later flattened out in the cold-acclimated regrowth. Possible mechanisms are discussed.
Floral development in Caryopteris × clandonensis A. Simmonds (C. incana (Houtt.) Miq. × C. mongho... more Floral development in Caryopteris × clandonensis A. Simmonds (C. incana (Houtt.) Miq. × C. mongholica Bunge) from cyme initiation through to anthesis is described, with emphasis on the development of the anthers. The later stages of development and anthesis are completed only on mature plants under short days (SD, 8 h light per day). Under long days (LD, 20 h light per day) senescence occurs in the early stages of anther wall differentiation.
Simultaneous measurements of CO 2 (CER) and O 2 (OER) exchange in roots and shoots of vegetative ... more Simultaneous measurements of CO 2 (CER) and O 2 (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 ϫ [CER ϩ 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 Ϫ plant Ϫ1 d Ϫ1), which was similar to that measured by gas exchange (3.2 mmol e Ϫ plant Ϫ1 d Ϫ1). 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.
Current plant science and biotechnology in agriculture, 1998
Nitrogen-fixing legume nodules live on a metabolic knife-edge. The nitrogenase (N2ase) enzyme is ... more Nitrogen-fixing legume nodules live on a metabolic knife-edge. The nitrogenase (N2ase) enzyme is extremely sensitive to irreversible damage by O2, yet the enzyme, the bacteria that synthesize it, and the infected cell of the host require ATP generated through oxidative phosphorylation within the plant mitochondria or the bacteria. This review covers current understanding of how legume nodules maintain precise control over intercellular O2 despite changes in environmental or physiological conditions.
We develop and illustrate a method for reconciling index decomposition analysis of energy intensi... more We develop and illustrate a method for reconciling index decomposition analysis of energy intensity with physically based, sector-specific energy efficiency indicators. Decomposition analysis of individual sector intensity contributions to total energy intensity is nested within the higher-order decomposition analysis of E/GDP such that the contribution of energy efficiency gains to changes in total energy intensity can be determined. Energy, economic and physical activity data for Canada for the period 1995-2010 are used to illustrate the method. Intrasector structural factors were found to be both positive and negative and to be significant contributors to energy intensities in both the business and household sectors. In aggregate, intrasectoral structural change offset energy efficiency gains and put upward pressure on (E/GDP) between 1995 and 2010 but was three times smaller than the offsetting decline in E/GDP due to intersectoral structural change. The method can be used for assessing the contribution of energy efficiency to sector energy intensities; for placing energy efficiency policies in the larger context of the other factors that determine an economy's energy intensity and greenhouse gas emissions; for identifying nonefficiency policy targets for improving energy productivity; and for increasing the sophistication of forecasting and scenario analysis of future levels and patterns of fuel and electricity consumption and related greenhouse gas emissions.
To assess the role of O2 in the regulation of nodule metabolism following a decrease or an increa... more To assess the role of O2 in the regulation of nodule metabolism following a decrease or an increase in temperature, the fractional oxygenation of leghemoglobin (FOL) was measured in soybean (Glycine max L. Merr.) nodules during rapid and gradual changes in temperature from 20[deg]C to either 15 or 25[deg]C. The affinity of leghemoglobin for O2 was also measured at each temperature and the values were used to calculate the infected cell O2 concentration (Oi). After nodules were transferred to 15[deg]C, FOL and Oi increased and adenylate energy charge (AEC = [ATP + 0.5ADP]/[ATP + ADP + AMP]) increased from 0.70 to 0.78. The temperature increase was associated with a decrease in FOL and Oi. We concluded that changes in nodule temperature alter the respiratory demand of the nodules for O2, resulting in a change in Oi and a shift in the balance between ATP consumption and ATP production within the nodule tissue.
. Positron emission tomography (PET) has been utilized to obtain dynamic images of long distance ... more . Positron emission tomography (PET) has been utilized to obtain dynamic images of long distance nutrient translocation in plants. Positron emitting 18F, produced by a Van de Graaff accelerator using the reaction 18O(p,n)18F, was fed in solution to excised stems of Glycine max positioned vertically in a large‐aperture PET detector system. Images of tracer activity were recorded with a time resolution of 0.5 min and a spatial resolution of 4 mm. Maximum tracer activities at stem sites were obtained within 3 min of the pulse feed. A model is presented enabling evaluation of regional values for tracer flow, tracer binding, flow speed and flow volume. Analysis of data for one stem position yielded a flow volume of 2.1mm3 min−1 and a flow speed of 36cm min−1. Comparison with the distribution of 14C‐inulin, which was simultaneously fed to the cut stems, indicates the 18F is suitable for use as an apoplastic tracer; 92% of the tracer activity accumulated in the leaves. The fraction of 18F that remained bound was most concentrated at stem nodal regions, an observation consistent with the existence of transfer cells at these sites. Advantages and limitations of PET applied to plant physiological investigations are discussed.
Large-scale production of renewable synthetic natural gas from biomass (bioSNG) in Canada was ass... more 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 nationwide low-carbon energy system.
After 40 days of growth at 25''C, Lotus pedunculatus eav,, cv. Maku plants infected with Rhizobiu... more After 40 days of growth at 25''C, Lotus pedunculatus eav,, cv. Maku plants infected with Rhizobium loti strain NZP2037 displayed similar relative growth rates but had twiee the nodule mass and only one third the whole plant dry weight of plants infected with Bradyrhizobium sp. {Lotus) strain CC814s. In the NZP2037 symbiosis, the rate of CO; evolution (per g dry weight of nodulated root) was 1.6 times as high as that in the CC814s symbiosis while the rate of QH; reduction (per g dry weight of nodule) was only 48% of that in the CC814s symbiosis. Studies of the effect of short term temperature changes on the gas exchange characteristics (CO, and H^ evolution, CjH, reduction) of these symbioses revealed wide differences in the optima for C^H, reduction. Nodules infected with NZP2037 displayed maximal QH, reduction rates [157 l^mol (g dry weight nodule)"' h"'] at 12°C, whereas nodules infected with CC814s were optimal at 30°C [208 nmoi (g dry weight nodnie)"' h'']. These short term studies suggested that differences in temperature optima for N; may have partially accounted for the poorer effectivity, at 25°C, of strain NZP2037 when compared with strain CC-814s. The relative efficiency [RE = 1-(H^evolution/C^H, reduction)] of N^ fixation varied widely with temperature in the two symbioses, but there was a general trend toward higher RE with lower temperatures. The ratio of CO; evolution: QH^ reduetion (mol/mol) in nodulated roots infected with CC814s was constant (ca 10 COVC^H,) between 5°C and 30°C, whereas in plants infected with NZP2037 it reached a minimal value of 3.3 CO^/QH; at lO^C and was 19 COj/CjHj at the growing temperature (25°C), Additional key words-Bradyrhizobium sp. (Lotus); Rhizobium loti.
Steady state cultures of Anabaena Jlos-aquae were established over a wide range of phosphate-limi... more Steady state cultures of Anabaena Jlos-aquae were established over a wide range of phosphate-limited growth rates while N was supplied as either NH3, N03-, or N2 gas. At growth rates greater than 0.03 per hour, rates of gross and net carbon fixation were similar on all N sources. However, at lower growth rates (<0.03 per hour) in the N03 and N2 cultures, gross photosynthesis greatly exceeded net photosynthesis. The increase in photosynthetic 02 evolution with growth rate was greatest when N requirements were met by N03and least when met by NH3. These results were combined with previously reported measurements of cellular chemical composition, N assimilation, and acetylene reduction (Layzell, Turpin, Elrifi 1985 Plant Physiol 78: 739-745) to construct empirical models of carbon and energy flow for cultures grown at 30, 60, and 100% of their maximal growth rate on all N sources. The models suggested that over this growth range, 89 to 100% of photodriven electrons were allocated to biomass production in the NH3 cells, whereas only 49 to 74% and 54 to 90% were partitioned to biomass in the NO3-and Nrgrown cells, respectively. The models were used to estimate the relative contribution of active, maintenance, and establishment costs associated with N03and N2 assimilation over the entire range of growth rates. The models showed that the relative contribution of the component costs of N assimilation were growth rate dependent. At higher growth rates, the major costs for N03assimilation were the active costs, while in N2-fixing cultures the major energetic requirements were those associated with heterocyst establishment and maintenance. It was concluded that compared with N03assimilation, N2 fixation was energetically unfavorable due to the costs of heterocyst establishment and maintenance, rather than the active costs of N2 assimilation.
The aim of the present study was to test the hypothesis that the N content or the composition of ... more 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 O, diffusion. Treating shoots of lupin (Lupinos albus cv Manitoba) or soybean (Glycine max L. Merr. cv Maple Arrow) with 100 p l L-' NH, 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 5 4 % of its initial activity within 6 h of the start of the NH, 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 O, diffusion and, thereby, downregulate nodule metabolism and nitrogenase activity by controlling the supply of O, to the bacteria-infected cells.
Annual review of plant physiology and plant molecular biology, Jun 1, 1993
Measuring the rates at which legume nodules exchange various gases with their environment allows ... more Measuring the rates at which legume nodules exchange various gases with their environment allows a great deal of information to be obtained concerning the factors that regulate and limit N2 fixation. However, studies over the past 10 years have shown that ...
Electron flow from oxidation of excess H2 released by root nodules was shown to contribute to mic... more Electron flow from oxidation of excess H2 released by root nodules was shown to contribute to microbial CO2 fixation in soybean crops. This discovery has important implications for carbon storage in soils used to grow legumes; however, further research is needed to understand the fate and turnover time of this H2-coupled CO2 fixation. Isotopic labeling of soil through incubation with 13CO2 was used to elucidate movement of sequestered carbon into soil carbon pools. Measurement of isotopic shifts was determined using Isotope Ratio Mass Spectrometry. Preliminary experiments have confirmed CO2 uptake through an isotopic shift (Delta13C -20.4 to -14.5 0/00) in 24 hour incubated soils labeled with 13CO2 (1% v/v, 99.5 Atom%) under elevated H2 concentration (6000 ppm). Other incubation experiments have confirmed the biotic nature of observed CO2 uptake by comparing isotopic shifts in oven dried and autoclaved soils to moist soil. Under an elevated H2 atmosphere, no significant isotopic shift was observed in dry and autoclaved soils whereas moist soil showed an isotopic shift of Delta13C -21.9 to 11.4 0/00 over 48 hours. Future experiments will involve longer incubations (7 days) and will be aimed at determining isotopic shifts within soil carbon pools. Samples will be incubated and fractionated into microbial biomass, light fraction carbon, and acid stable carbon and subsequent isotopic analysis will be carried out. This will help determine the distribution of H2- coupled fixed CO2 within soil carbon pools and the turnover time of sequestered carbon. This and further research may lead to modification of greenhouse gas coefficients for leguminous crops that includes a CO2 fixation component.
ABSTRACT Human activities have had a major impact on the global carbon and nitrogen cycles. In co... more ABSTRACT Human activities have had a major impact on the global carbon and nitrogen cycles. In contrast to studies of CO2 exchange, effective tools for the non-invasive measurements of N cycle transformations have not been developed and there is much to learn about the control of, and complex interactions associated with processes such as nitrification, denitrification and nitrate assimilation. Since N cycle transformations involve redox reactions, a theoretical model was developed to explore the potential for using the imbalance in measured CO2 and O2 exchange of biological systems as an indirect, non-invasive measure of nitrogen cycle transformations. For every mole of NH3 nitrified to NO3-, the model predicted that there would be 2 more moles of O2 taken up than CO2 produced by the same system that was not undergoing nitrification. To test this hypothesis, laboratory and field studies where carried out in which control and NH3-treated soils were continuously assayed for CO2 (Licor IRGA) and O2 (Qubit Systems Differential O2 Analyzer) exchange. The laboratory studies clearly showed that for about 3 weeks following soil treatment with NH3 (8.2 umol NH3/gDW soil, equivalent to 200 kg N/ha), CO2 production was suppressed by about 18%, and O2 uptake was stimulated by up to 251% (average of 153%) relative to control. Similar results were obtained with chamber studies in the field. The lab measurements were combined with the model to predict the rate of nitrification and then compare this with measurements of NH3 depletion and NO3- accumulation. Despite the large imbalance that was observed in CO2 and O2 exchange, the gas exchange measurements could only account for 50% of the reductant that was associated with the actual nitrification rate. Further work is needed to address the destination of the other reductant, but it may include mineral element reduction or microbial synthesis of highly reduced C compounds. Nevertheless, if the relationship between nitrification rates and soil CO2-O2 exchange is similar in other soils, it may be possible to use the imbalance of CO2 and O2 exchange as a non-invasive measure of nitrification. This assay should also allow for a better understanding of C and N dynamics and interactions in soils.
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Papers by David Layzell