Open Gas Exchange System Research Articles

A metabolic connection between nitrogenase activity and the synthesis of ureides in nodulated soybean

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.

Photosynthesis and transpiration of in vitro cultured asparagus plantlets

Nodal sections of asparagus ( Asparagus officinalis L.) were cultured and rooted on a modified Murashige and Skoog (MS) medium for 10 weeks. These plants were then acclimatized for 5 weeks. Photosynthesis and transpiration of in vitro plantlets, acclimatized plantlets and control seedlings were measured with an open gas exchange system. Rates of photosynthesis of in vitro-cultured plantlets were as high as those of seedlings grown in a greenhouse, while their rates of transpiration were much higher. Photosynthetic capacity of in vitro-cultured asparagus plantlets was sufficiently high to support autotrophic growth during the period of acclimatization. Photosynthesis of acclimatized plantlets was always considerably lower than that of in vitro plantlets except at high concentrations of CO 2. Evaportranspiration of acclimatized plantlets was comparable to that of seedlings. These results suggest that high water loss incurred by in vitro shoots imposes severe limitations on newly formed shoots in acclimatization thus reducing whole plant photosynthesis. Protecting in vitro plantlets from water stress is the most important factor to consider in order to ensure their survival during the acclimatization period.

Long-term respiration in relation to growth and maintenance processes of the aboveground parts of a hinoki forest tree

Over a three-year period, CO(2) exchange rates were measured continuously on the aboveground parts of a 13-year-old hinoki (Chamaecyparis obtusa (Sieb. et Zucc.) Endl.) tree in the field, using an open gas-exchange system tracking ambient temperature. The relationship between daily aerial respiratory consumption and daily gross production, which was analyzed monthly, closely resembled McCree's equation. The value of the coefficient of growth respiration ranged between 0.0 in winter and 0.16 in summer and increased with increasing monthly mean temperature. A clockwise loop was observed for monthly change in the relationship between the coefficient of growth respiration and temperature. Maintenance respiration could be formulated as a power function of aboveground dry weight. The exponent of the equation ranged from 0.3 to 1.1. A value of 1.1 in May and June, when trees were growing most actively, indicated that maintenance respiration was directly proportional to aboveground weight. In March, April, July, and August, maintenance respiration was not proportional to aboveground weight, but it was closely proportional to surface area. The exponent value exhibited seasonal change with a clockwise loop in response to monthly average temperature. During the dormant season, respiration was used only for maintenance purposes, whereas during the growing season both growth and maintenance respiration occurred. Annual growth and maintenance respiration increased with increasing tree age. The average annual contribution to total respiratory consumption was 21% for growth respiration and 79% for maintenance respiration.

CO2 Exchange in soil algal crusts occurring in the trachypogon savannas of the Orinoco Llanos, Venezuela

CO2 exchange between the atmosphere and soil algal crusts of the Trachypogon savannas of the Orinoco Llanos has been analyzed using an open gas exchange system. These savannas encompass a wide range of physiognomic types, from herbaceous communities to savanna woodlands. A maximum CO2 flux of 0.207 mg m-2 s-1 was measured in the crusts of the Guanipa savannas, while in the other examined crusts (0.035–0.105 mg m-2 s-1) the flux was similar to values reported for terrestrial algae. The CO2 flux data were statistically fitted to the photosynthetically active radiation by a logarithmic relationship, and the photosynthetic efficiencies of the crusts were compared. The activation energy calculated for the CO2 fixation indicates that limitations by diffusion and photochemical processes were excluded in the Guanipa crusts (above 12 kcal mole-1), whereas they were evident in the other crust studied. An optimum CO2 incorporation as a function of the crust water potential was established and carbon gain strategies were proposed on the basis of the results and characteristics of the habitats.

Long-term measurement of CO2 release from the aboveground parts of a hinoki forest tree in relation to air temperature

Nighttime respiration of the aboveground parts of a 12-year-old (as of 1986) hinoki (Chamaecyparis obtusa (Sieb. et Zucc.) Endl.) tree growing in the field was continuously measured over a three-year period, using an open gas exchange system which tracked ambient air temperature. The respiration rate in any month was described as an exponential function of air temperature. Daily respiration rate was lower in winter when the tree was dormant than in the summer. The daily respiration rate rose rapidly in spring to a maximum in June-July, decreased abruptly in autumn, and then decreased gradually to a minimum in January-February. The Qte for nighttime respiration was inversely related to air temperature. The highest Qla (2 3.0) was observed in January when the monthly mean nighttime air temperature was around 4.0 “C. The lowest Qla (~1.5) was observed in August when the monthly mean nighttime air temperature was about 23 “C. A Qla value of about 2.0 was observed in May and June when monthly mean nighttime air temperature ranged from 15 to 20 “C.

Effects of Nitrogen Nutrition on Nitrogen Partitioning between Chloroplasts and Mitochondria in Pea and Wheat

Nitrogen partitioning among proteins in chloroplasts and mitochondria was examined in pea (Pisum sativum L.) and wheat (Triticum aestivum L.) grown hydroponically with different nitrogen concentrations. In pea leaves, chloroplast nitrogen accounted for 75 to 80% of total leaf nitrogen. We routinely found that 8% of total ribulose-1,5-bisphosphate carboxylase/oxygenase adhered to thylakoids during preparation and could be removed with Triton X-100. With this precaution, the ratio of stroma nitrogen increased from 53 to 61% of total leaf nitrogen in response to the nitrogen supply, but thylakoid nitrogen remained almost constant around 20% of total. The changes in the activities of the stromal enzymes and electron transport in response to the nitrogen supply reflected the nitrogen partitioning into stroma and thylakoids. On the other hand, nitrogen partitioning into mitochondria was appreciably smaller than that in chloroplasts, and the ratio of nitrogen allocated to mitochondria decreased with increasing leaf-nitrogen content, ranging from 7 to 4% of total leaf nitrogen. The ratio of mitochondrial respiratory enzyme activities to leaf-nitrogen content also decreased with increasing leaf-nitrogen content. These differences in nitrogen partitioning between chloroplasts and mitochondria were reflected in differences in the rates of photosynthesis and dark respiration in wheat leaves measured with an open gas-exchange system. The response of photosynthesis to nitrogen supply was much greater than that of dark respiration, and the CO(2) compensation point decreased with increasing leaf-nitrogen content.

Effect of the Long-Term Elevation of CO2 Concentration in the Field on the Quantum Yield of Photosynthesis of the C3 Sedge, Scirpus olneyi

CO(2) concentration was elevated throughout 3 years around stands of the C(3) sedge Scirpus olneyi on a tidal marsh of the Chesapeake Bay. The hypothesis that tissues developed in an elevated CO(2) atmosphere will show an acclimatory decrease in photosynthetic capacity under light-limiting conditions was examined. The absorbed light quantum yield of CO(2) uptake (ø(abs) and the efficiency of photosystem II photochemistry were determined for plants which had developed in open top chambers with CO(2) concentrations in air of 680 micromoles per mole, and of 351 micromoles per mole as controls. An Ulbricht sphere cuvette incorporated into an open gas exchange system was used to determine ø(abs) and a portable chlorophyll fluorimeter was used to estimate the photochemical efficiency of photosystem II. When measured in an atmosphere with 10 millimoles per mole O(2) to suppress photorespiration, shoots showed a ø(abs) of 0.093 +/- 0.003, with no statistically significant difference between shoots grown in elevated or control CO(2) concentrations. Efficiency of photosystem II photochemistry was also unchanged by development in an elevated CO(2) atmosphere. Shoots grown and measured in 680 micromoles per mole of CO(2) in air showed a ø(abs) of 0.078 +/- 0.004 compared with 0.065 +/- 0.003 for leaves grown and measured in 351 micromoles per mole CO(2) in air; a highly significant increase. In accordance with the change in ø(abs), the light compensation point of photosynthesis decreased from 51 +/- 3 to 31 +/- 3 micro-moles per square meter per second for stems grown and measured in 351 and 680 micromoles per mole of CO(2) in air, respectively. The results suggest that even after 3 years of growth in elevated CO(2), there is no evidence of acclimation in capacity for photosynthesis under light-limited conditions which would counteract the stimulation of photosynthetic CO(2) uptake otherwise expected through decreased photorespiration.

Photosynthesis and root respiration ofPinus sylvestrisandpicea abiesseedlings after different root freezing and storage temperatures

Photosynthetic performance and root respiration were measured for seedlings of Scots pine and Norway spruce under constant conditions in an open gas exchange system in the laboratory. Measurements were carried out after root exposure to ‐20, ‐5 and 0°C and subsequent longtime storage in darkness at +1 or +4°C. Stomatal conductance in relation to net photosynthetic rates was also investigated after the same treatment of seedlings. Root respiration was low for seedlings whose root system had been exposed to ‐20°C, Scots pine showing lower rates than Norway spruce. This was probably an indication of root damage. At least for one provenance of Scots pine, respiration rates were higher for seedlings stored at +1 than at +4°C. Photosynthetic performance was also lowest for seedlings whose roots had been exposed to +20°C compared to higher temperatures, the difference being more clear‐cut for Norway spruce than for Scots pine. Storage at +1 gave slightly higher photosynthetic rates than at +4°C. There was a clos...

STRAWBERRY LEAF PHOTOSYNTHESIS ACCLIMATION TO TEMPERATURE

The commercial strawberry Frageria × ananassa and several clones of F. chiloensis and F. virginiana were preconditioned in growth chambers under the following conditions: 500 μmol m-7s-1 PAR and 20° day, 10° night or 30° day and 20° night. After at least 3 weeks of preconditioning at the two different temperature regimes, leaf photosynthetic rate (A) was determined for temperatures ranging from 10 to 35° in 2-3° increments with an open gas exchange system under laboratory conditions. The objective was to determine if F. virginiana and F. × ananassa can photosynthesize at higher temperatures than F. chiloensis, and if any of these would acclimate to higher temperatures. F. chiloensis did not acclimate to higher temperatures, and bad maximum A between 16 and 20°. F. virginiana did acclimate to higher temperatures, with maximum A for the low temp treatment between 18 and 24°, and for the high temp treatment between 24 and 30°. The commercial cultivars of `Earliglow' and `Redchief' acclimated to higher temperature and responded similar to F. virginiana.

An integrated portable apparatus for the simultaneous field measurement of photosynthetic CO2 and water vapour exchange, light absorption and chlorophyll fluorescence emission of attached leaves

Abstract. A portable apparatus has been constructed to measure simultaneously the quantum yield of CO2 assimilation, light absorption, chlorophyll fluorescence emission and water vapour exchange of attached intact leaves in the field. The core of the instrument is a light‐integrating spherical leaf chamber which includes ports for a light source, photosynthetically active radiation sensor, fluorescence probes and gas inlet and outlet manifolds. Measurement of the quantum flux inside the empty chamber and with a leaf present allows determination of leaf absorptance. An open gas exchange system is employed using an infra‐red analyser to measure leaf CO2 exchange. Using a DC white light source the quantum yield of CO2 assimilation based on absorbed light (φabs) may be determined rapidly in either ambient air or artificial gas mixtures. Inclusion of capacitance humidity probes into the gas inlet and outlet ports allows simultaneous determination of water vapour exchange and subsequent estimation of stomatal conductance to CO2 and intercellular CO2 concentration. Measurement of fluorescence emission by the sample leaf exposed to white light is achieved by a modulated fluorescence detection system. In addition to determination of the minimal, maximal and variable fluorescence levels, a further analysis allows the photochemical and non‐photochemical components of fluorescence quenching, to be estimated. The theory and design of this apparatus is described in detail. The use of the apparatus in the field is demonstrated through a study of the photosynthetic performance of a maize and bean crop during the growing season and by analysis of the photosynthetic performance of crops subjected to nitrogen‐stress and a herbicide treatment.

Effects of Gradual Increases in O2 Concentration on Nodule Activity in Soybean

The objectives of this study were to determine whether attached nodules of soybean (Glycine max L. Merr.) could adjust to gradual increases in rhizosphere pO(2) without nitrogenase inhibition and to determine whether the nitrogenase activity of the nodules is limited by pO(2) under ambient conditions. A computer-controlled gas blending apparatus was used to produce linear increases (ramps) in pO(2) around attached nodulated roots of soybean plants in an open gas exchange system. Nitrogenase activity (H(2) production in N(2):O(2) and Ar:O(2)) and respiration (CO(2) evolution) were monitored continuously as pO(2) was ramped from 20 to 30 kilopascals over periods of 0, 5, 10, 15, and 30 minutes. The 0, 5, and 10 minute ramps caused inhibitions of nitrogenase and respiration rates followed by recoveries of these rates to their initial values within 30 minutes. Distinct oscillations in nitrogenase activity and respiration were observed during the recovery period, and the possible basis for these oscillations is discussed. The 15 and 30 minute ramps did not inhibit nitrogenase activity, suggesting that such inhibition is not a factor in the regulation of nodule diffusion resistance. During the 30 minute ramp, a stimulation of nitrogenase activity was observed, indicating that an O(2)-based limitation to nitrogenase activity occurs in soybean nodules under ambient conditions.

NO 2 deposition to elements representative of a forest landscape

Measurements of NO 2 deposition to elements representative of a forest landscape (e.g. foliage, bole, soil) were conducted in an open gas exchange system. Deposition rates (nmol m −2 s −1) were calculated as the product of flow rate and the inlet/outlet concentration differential normalized for surface area and corrected for losses to chamber walls. Under daylight conditions and a mean NO 2 concentration of 33 nl ℓ −1, NO 2 deposition to foliage of forest tree species varied by more than an order of magnitude, ranging from 0.35 ( Pinus taeda L.) to 5.75 nmol m −2 s −1 ( Platanus occidentalis L.), and deposition of NO 2 to most broadleaf species was greater than that to conifers. Sequential light-dark measurements of NO 2 deposition indicated that the principal foliar site of deposition was the leaf interior in support of other observations of stomatal control over NO 2 deposition. Vegetation surfaces typically snowed greater conductance to NO 2 than did distilled water alone, and forest floor samples had a disproportionately high conductance to NO 2 when compared to bark or foliage surfaces. Laboratory data for several forest species were extrapolated to stand-level rates of NO 2 deposition using leaf area index as a scaling factor. Based on appropriate NO 2 concentrations, NO 2 deposition was calculated to provide annual N inputs between 0.08 and 1.9 kg ha −1 a −1 of NO 2 derived N(NO 2— N) for natural forests and up to 12kg NO 2-N ha −1 a −1 for forest canopies in urban environments.

Fluorescence Quenching and Gas Exchange in a Water Stressed C3 Plant, Digitalis lanata

A leaf cuvette has been adapted for use with a pulse-modulation fluorometer and an open gas exchange system. Leaf water potential (psi) was decreased by withholding watering from Digitalis lanata EHRH. plants. At different stages of water deficiency the photochemical (q(Q)) and nonphotochemical (q(E)) fluorescence quenching was determined during the transition between darkness and light-induced steady state photosynthesis of the attached leaves. In addition, the steady state CO(2) and H(2)O gas exchange was recorded. Following a decrease of leaf water potential with increasing water deficiency, the transition of photochemical quenching was almost unaffected, whereas nonphotochemical quenching increased. This is indicative of an enhanced thylakoid membrane energization during the transition and is interpreted as a partial inhibition of either the ATP generating or the ATP consuming reaction sequences. Complete reversion of the stress induced changes was achieved within 6 hours after rewatering. In contrast to the variations during transition, the final steady state values of q(Q) and q(E) remained unchanged over the entire stress range from -0.7 to -2.5 megapascals. From these results we conclude that, once established, electron transport via photosystem II and the transmembrane proton gradient remain unaffected by water stress. These data are indicative of a protective mechanism against photoinhibition during stress, when net CO(2) uptake is limited.

Ozone Uptake and Effects on Transpiration, Net Photosynthesis, and Dark Respiration in Scots Pine

Abstract Ozone uptake, transpiration, net photosynthesis, and dark respiration were studied in the field by using an open gas exchange system in a 20-year-old stand of Scots pine (Pinus sylvestris L.). A current shoot was treated with ozone concentrations ranging from 120 to 400 μg · m-3 during one month. During daytime there was a linear relationship between ozone concentration and ozone uptake, and the deposition rate varied between 0.05 and 0.13 cm · s-1. Ozone at the highest concentrations seemed to decrease transpiration somewhat during daytime. At night, ozone was taken up only at the highest concentration. Both transpiration and stomatal conductance increased at night when ozone concentration was 250 μg · m-3 and higher. There was no significant influence on the net photosynthetic performance during exposure to ozone. Dark respiration, however, increased throughout the experimental period, and the accumulated respiration was about 60% higher for the ozone-exposed shoot at the end of the experiment. For. Sci. 33(3):801-808.

Steady and Nonsteady State Gas Exchange Characteristics of Soybean Nodules in Relation to the Oxygen Diffusion Barrier

An open gas exchange system was used to monitor the nonsteady state and steady state changes in nitrogenase activity (H(2) evolution in N(2):O(2) and Ar:O(2)) and respiration (CO(2) evolution) in attached, excised, and sliced nodules of soybean (Glycine max L. Merr.) exposed to external pO(2) of 5 to 100%. In attached nodules, increases in external pO(2) in steps of 10 or 20% resulted in sharp declines in the rates of H(2) and CO(2) evolution. Recovery of these rates to values equal to or greater than their initial rates occurred within 10 to 60 minutes of exposure to the higher pO(2). Recovery was more rapid at higher initial pO(2) and in Ar:O(2) compared to N(2):O(2). Sequential 10% increments in pO(2) to 100% O(2) resulted in rates of H(2) evolution which were 1.4 to 1.7 times the steady state rate at 20% O(2) in Ar. This was attributed to a relief at high pO(2) from the 40% decline in nitrogenase activity that was induced by Ar at a pO(2) of 20%. Changes in nodule respiration rate could not account for the nodules' ability to adjust to high external pO(2), supporting the hypothesis that soybean nodules have a variable barrier to O(2) diffusion which responds slowly (within minutes) to changes in pO(2). Nodule excision and slicing resulted in 45 and 78% declines, respectively, in total specific nitrogenase activity at 20% O(2). In contrast with the result obtained with intact nodules, subsequent 10% increases in pO(2) in Ar:O(2) did not result in transient declines in H(2) evolution rates, but in the rapid attainment of new steady state rates. Also, distinct optima in nitrogenase activity were observed at about 60% O(2). These results were consistent with an increase in the diffusive resistance of the nodule cortex following nodule excision or nodule slicing. This work also shows the importance of using intact plants and continuous measurements of gas exchange in studies of O(2) diffusion and nitrogenase activity in legume nodules.

A BASIC computer program for calculation of photosynthesis, stomatal conductance, and related parameters in an open gas exchange system

Computer programs written in BASICA (IBM'S VERSION OF BASIC) language were developed for the calculation of the gas exchange parameters of CO2 assimilation, leaf conductance, stomatal conductance, residual conductance, intercellular CO2 concentration, transpiration, water use efficiency and transpiration ratio in an open system. Formulas are discussed in both an algebraic and in a BASIC computer program form. Calculations based on mole fractions of CO2 and water vapor are explained and both molar and mass fluxes are included in the program output to facilitate comparisons with data from the literature. Corrections are made in the program to account for under-estimation of CO2 assimilation due to the increase in flow rates out of sample chambers caused by simultaneous transpiration. A sample output is included to illustrate the formatting capability of the program.

Nitrogen Metabolism in Senescent Flag Leaves of Wheat (Triticum aestivum L.) in the Light

Nitrogen metabolism was examined in senescent flag leaves of 90- to 93-day-old wheat (Triticum aestivum L. cv Yecora 70) plants. CO(2) assimilation and the levels of protein, chlorophyll, and nitrogen in the leaves decreased with age. Glutamine synthetase activity decreased to one-eighth of the level in young flag leaves. Detached leaves were incubated (with the cut base) in (15)N-labeled NH(3), glutamate, or glycine in the light (1.8 millieinstein per square meter per second) at 25 degrees C in an open gas exchange system under normal atmospheric conditions for up to 135 minutes. The (15)N-enrichment of various amino acids derived from these (15)N-substrates were examined. The amido-N of glutamine was the first (15)N-labeled product in leaves incubated with (15)NH(4)Cl whereas serine, closely followed by the amido- and amino-N of glutamine, were the most highly (15)N-labeled products during incubation with [(15)N]glycine. In contrast, aspartate and alanine were the first (15)N-labeled products when [(15)N] glutamate was used. These results indicate that NH(3) was assimilated via glutamine synthetase and glutamate synthase activities and the photorespiratory nitrogen cycle remained functional in these senescent wheat flag leaves. In contrast, an involvement of glutamate dehydrogenase in the assimilation of ammonia could not be detected in these tissues.

Photoacoustic and fluorescence measurements of the chilling response and their relationship to carbon dioxide uptake in tomato plants

The response of tomato plants to various chilling treatments was studied using two approaches for the measurement of photosynthetic activity. One involved the use of a portable fluorometer for the measurement of in-vivo chlorophyll fluorescence, while the other employed a newly introduced photoacoustic system which allowed changes in oxygen evolution to be followed in a leaf disc. A strong correlation was found between results obtained by each system and those obtained by a conventional open gas-exchange system for the determination of CO2 uptake. Both systems of measurements could readily distinguish between the effects of chilling in the dark (at 3° C for 18 h) and chilling at high photon flux density (2000 μmol m(-2) s(-1) for 5h at 5° C). Chilling in the dark had practically no effect on the quantum yield of oxygen evolution, chlorophyll fluorescence or CO2 uptake, while chilling at excessively high photon flux density resulted in a sharp reduction (50-70%) in the quantum yields obtained. The results support the view that photosystem II cannot be the primary site of damage by chilling in the dark, although it is significantly affected by chilling at high light intensity.

Light Acclimation in Citrus Leaves. II. CO2 Assimilation and Light, Water, and Nitrogen Use Efficiency

Abstract Net CO2 assimilation (A) rates of ‘Duncan’ grapefruit (Citrus paradisi Macf.) and ‘Pineapple’ orange (C. sinensis L.) seedlings grown under 3 different photosynthetic photon flux densities (PPFD), were measured in an open gas exchange system under controlled environmental conditions. Apparent quantum yield (ø), mesophyll conductance to CO2 (Gm), leaf conductance to H2O vapor (G1), transpiration (E) and water use efficiency (WUE) also were examined. Leaves of both species grown under high PPFD (full sunlight) had the greatest maximum rates of A, but the low PPFD (90% shade) leaves had the highest ø. The WUE of low PPFD grapefruit leaves was less than that of the high PPFD leaves but increased within 2 weeks after being moved into full sunlight. Transferring seedlings from low to high PPFD decreased ø of grapefruit but not of orange leaves. Changes in A were more strongly correlated to Gm than to G1. Carbon dioxide assimilation rate was positively correlated to total leaf nitrogen content. Citrus leaf photosynthetic characteristics and resources use efficiency not only acclimate to the light regimes under which they expand and mature, but leaves are capable of acclimating to new light regimes, even after full maturation.

Low root temperatures and nitrogenase activity in soybean

A continuously flowing open gas exchange system was used to monitor the rates of CO2 evolution, C2H2 reduction, and H2 evolution from intact nodulated soybean ('Harosoy 63') roots, while root temperatures were dropped from the growing temperature (25 °C) to 10 °C at the rate of 5 °C/h. Four Rhizobium japonicum inoculants were used, three of which (type S (Nitragin Co.), USDA 16, USDA 35) displayed net H2 evolution while the fourth (USDA 110) did not evolve H2. Between 25 and 15 °C, values for Q10 in the H2-evolving symbioses ranged from 2.0 to 2.7 for CO2 evolution. 1.3 to 2.4 for C2H2 reduction, and 3.2 to 3.7 for H2 evolution. Nodulated roots of USDA 110 displayed the highest Q10 values for both CO2 evolution (Q10 = 2.9) and C2H2 reduction (Q10 = 15.2). The temperature profiles of these gas exchange measurements were used to calculate both the relative efficiency (RE = 1 − (H2 evolution/C2H2 reduction)) of N2 fixation and the ratio between CO2 evolution and C2H2 reduction at temperatures between 10 and 25 °C. The effect of short-term changes on the CO2/C2H2 ratio varied with symbiotic association. In the type-S symbiosis, long-term (96 h) treatments at low root temperature (9.5 °C) decreased the amount of CO2 evolved per C2H2 reduced to 35% of the value obtained at 25 °C. This study did not determine whether these observed changes were associated with N2 fixation or growth and maintenance of the nodulated root. In all H2-evolving symbioses, the RE increased from ca. 0.6 to 0.9 in response to a temperature drop from 25 to 10 °C. Studies of net H2 uptake and 3H2 exchange at 25 and 15 °C indicated that the observed changes in relative efficiency were due to variations in electron allocation by nitrogenase rather than a relative increase in uptake hydrogenase activity. The increase in RE above 0.75 suggested that lower temperatures may have altered the minimal nitrogenase electron allocation to less than one H2 per N2 fixed. This observed increase in RE with lower temperatures indicated that, in the symbioses studied, H2 evolution may have provided a buffer which permitted the maintenance of high levels of N2 fixation during short-term or diurnal fluctuations in soil temperature.