High N2 Fixation Rates Research Articles

Nitrogen fixation capacity and nodule occupancy by Bradyrhizobium japonicum and B. elkanii strains

In a previous study soybean Bradyrhizobium strains, used in Brazilian studies and inoculants over the last 30 years, and strains adapted to the Brazilian Cerrados, a region frequently submitted to environmental and nutritional stresses, were analyzed for 32 morphological and physiological parameters in vivo and in vitro. A cluster analysis allowed the subdivision of these strains into species Bradyrhizobium japonicum, Bradyrhizobium elkanii and a mixed genotype. In this study, the bacteria were analyzed for nodulation, N2 fixation capacity, nodule occupancy and the ability to increase yield. The goal was to find a relationship between the strain groups and the symbiotic performance. Two strains of Brazilian B. japonicum showed higher rates of N2 fixation and nodule efficiency (mg of N mg–1 of nodules) under axenic conditions. These strains also showed greater yield increases in field experiments when compared to B. elkanii strains. However, no differences were detected between B. japonicum and B. elkanii strains when comparing nodule occupancy capacity. The adapted strains belonging to the serogroup B. elkanii SEMIA 566, most clustered in a mixed genotype, were more competitive than the parental strain, and some showed a higher capacity of N2 fixation. Some of the adapted strains, such as S-370 and S-372, have shown similar N2 fixation rates and nodulation competitiveness to two Brazilian strains of B. japonicum. This similarity demonstrates the possibility of enhancing N2 fixing ability, after local adaptation, even within B. elkanii species. Differences in the DNA profiles were also detected between the parental SEMIA 566 and the adapted strains by analyses with the ERIC and REP-PCR techniques. Consequently, genetic, morphological and physiological changes can be a result of adaptation of rhizobia to the soil. This variability can be used to select strains capable of increasing the contribution of N2 fixation to soybean nutrition.

Influence of N2-fixing Trifolium on plant species composition and biomass production in alpine tundra.

Alpine Trifolium species have high rates of symbiotic N2-fixation which may influence the abundance and growth of plant species growing near them. The potential for facilitative effects on plant abundance and growth in dry meadow alpine tundra of Niwot Ridge, Colo., characterized by low resource availability, was investigated by measuring soil N, aboveground biomass production, and plant species composition in patches of Trifolium dasyphyllum and surrounding tundra. Extractable inorganic N was more than twofold greater and extractable P was 27% lower in Trifolium patches than in surrounding tundra. Aboveground production was twofold greater in Trifolium patches than in surrounding tundra. However, the difference was largely due to the production of T. dasyphyllum relative to the non-Trifolium component of biomass, which was not different between the Trifolium patches and surrounding tundra. In the Trifolium patches, the proportion of graminoid biomass was lower while the proportion of forb biomass was higher relative to surrounding tundra. Although the abundance of some species was positively associated with the presence of Trifolium, other species were less abundant, possibly due to increased competition for P and differential abilities of alpine species to respond to increased N availability. Trifolium may exert both facilitative and inhibitive effects on dry meadow alpine species and, in the process, substantially influence the spatial heterogeneity in community structure and primary production.

RAPID N2FIXATION IN PINES? RESULTS OF A MAINE FIELD STUDY

A recent study of the nitrogen budgets of pitch pines (Pinus rigida Mill.) and red pines (P. resinosa Ait.) growing in model ecosystems in New Hampshire (USA) reported very high nitrogen gains. Thus N2 fixation associated with the rhizosphere of pines may contribute to their success in nitrogen-limited ecosystems. We used the acetylene-reduction assay with carbon monoxide controls to determine whether high rates of N2 fixation are associated with pine forests in Maine. We measured nitrogenase activity in cores of the forest floor and the mineral soil beneath for five pitch pine, seven red pine, and six eastern white pine (P. strobus L.) stands. The stands represented 8 of the 15 biophysical regions of Maine (USA) and included a variety of age classes. We found only very low rates of nitrogenase activity, which varied seasonally with the highest values during the warmer months. The highest mean nitrogenase activity observed was 449 nmol C2H4·m−2·h−1. We estimated that <6 mg N/m2 was fixed annually by non-symbiotic diazotrophs in the soil horizons sampled. These rates are much smaller than atmospheric N inputs in Maine (300–450 mg N·m−2·yr−1) and consequently have little impact on the N budgets of the pines. Based on our results and the literature, it is likely that only low rates of N2 fixation are associated with pines.

Comparison between parental and variant soybeanBradyrhizobium strains with regard to the production of lipo-chitin nodulation signals, early stages of root infection, nodule occupancy, and N2 fixation rates

Soybean is the most important leguminous crop in Brazil and the nitrogen required for plant growth is supplied byBradyrhizobium bacteria through the symbiotic relation established by the inoculation process. Since 1992, two new strains, CPAC 7 and CPAC 15, which have been shown to increase yields in several field experiments, have been recommended in Brazilian commercial inoculants. CPAC 15 is a natural variant of theB. elkanii SEMIA 566 strain, and was isolated after several years of adaptation to a Brazilian Cerrado soil, while CPAC 7 is a variant ofB. japonicum strain CB 1809, selected under laboratory conditions for higher nodulation and yield. The comparison between parental and variant strains, under greenhouse conditions, showed that both CPAC 15 and CPAC 7 increased N2 fixation rates in relation to the parental strains. The better performance of CPAC 15 was related to an increase in nodule efficiency (mg N2 fixed mg-1 nodule) while with CPAC 7 the higher N2 fixation rates were due to increased nodulation. Both CPAC 15 and CPAC 7 increased nodule occupancy, when co-inoculated at a ratio of 1:1 withB. elkanii 29w, in relation to their parental strains. Variant strains also differed from parental in their ability to increase numbers of root hairs (Hai phenotype) either when inoculated onto plants, or when supernatants of bacteria exposed to seed exudates were used as inoculants. This results lead to the hypothesis that a modification in some of the “common” nodulation genes had occurred. However, the increase in Hai phenotype with CPAC 7 was dependent on the soybean cultivar, indicating a possible alteration in some genotypic specific nodulation gene. Apparently, there were no differences in Nod metabolites produced by strains CPAC 15 and SEMIA 566, but a more detailed chemical analysis would be required to rule out subtle differences. On the contrary, significant differences were found between CPAC 7 and the parental strain CP 1809, in the profile of Nod metabolites. Consequently, it may be possible that diffusable molecules, responsible for Hai phenotype, would be related to nodulation ability, competiviveness, and N2 fixation, resulting in the higher yields that have been associated with CPAC 7 and CPAC 15. For the CPAC 7 strain, the increase in Hai phenotype could be atributed to the differences found in the Nod molecules. Consequently, a high degree of physiological and genetic variability can result from the adaptation of rhizobial strains to the soil. Also, this variability can be found under laboratory conditions, when searching single colonies with specific properties. ei]Section editor: R O D Dixon

Differential Metabolism of Dimethylsulfoniopropionate and Acrylate in Saline and Brackish Intertidal Sediments

In anoxic Spartina alterniflora-dominated sediments along a naturally occuring salinity gradient (the Cooper River estuary, South Carolina, U.S.A.), dimethylsulfoniopropionate (DMSP) was metabolized to dimethyl sulfide (DMS) and acrylate by sediment microbes. The rate of DMSP degradation and acrylate mineralization by sediment microbes was similar at all sites along this 25-km transect. However, sediments amended with acrylate (or DMSP) showed significantly higher rates of N2 fixation (measured as acetylene reduction activity) (ARA) in the saline sediments downstream than brackish sediments. These results are consistent with the fact that acrylate stimulated the rates of both denitrification and CO2 production in the saline sediments at the mouth of the river more than tenfold over rates in brackish sediments. Enrichment experiments indicate that microbes capable of using DMSP or acrylate were not present in upstream sediments despite the fact that microbial biomass, percent organic matter, and both glucose-stimulated ARA and denitrification were highest upstream. It appears that acrylate utilizing, N2 fixing, and denitrifying populations are insignificant in the lower salinity sediments of the estuary. These results may reflect the availability of DMSP, which averaged 10.3 nmol g wet wt-1 of saline sediments and levels less than our detection limit (1 &mgr;M) in brackish sediments.

Recent Changes of N2-Fixation in Lake 227 in Response to Reduction of the N:P Loading Ratio

Continuous artificial additions of nitrogen (N) and phosphorus (P) to Lake 227 at a loading ratio of 11:1 (atom: atom; 5:1 by mass) from 1975 to 1989, combined with natural inputs, resulted in variable mean annual total N and total P concentrations and changes in phytoplankton species composition. Summer phytoplankton occasionally shifted from N2-fixing cyanobacteria to non-N2-fixing forms. In 1990, additions of N fertilizer were stopped in order to test the hypothesis that low N:P loading rates cause selection for heterocystous cyanobacteria and stimulate N2-fixation. Five years of phytoplankton nutrient-status measurements, together with estimates of N2-fixation before and after lowering the N:P loading ratio, verified the importance of this ratio in producing blooms of N2-fixing cyanobacteria. Nitrogen input via biological N2fixation increased, and summer dominance by heterocystous cyanobacteria returned as a consequence of lowering the N:P ratio of external inputs. Seasonally, N return from the epilimnetic sediments increased after a period of high planktonic N2-fixation rates. This regenerated N reduced the abundance of heterocystous cyanobacteria and lowered late summer N2-fixation rates. Over several years, preferential regeneration of N relative to P maintained the N:P ratio of the internally cycling nutrient pool, despite the reduction in the external N:P loading ratio.

N2 fixation in a young Alnus incana stand, based on seasonal and diurnal variation in whole plant nitrogenase activity

N2 fixation by grey alder, Alnus incana (L.) Moench, was studied in the field during two growing seasons in northern Sweden. Alders were planted in a nitrogen-poor soil. Each alder had its root system enclosed in an open-ended cylinder that was closed with a gas-tight lid around the stem base to serve as cuvette during nitrogenase activity (acetylene reducing activity) measurements. To follow the seasonal variation, nitrogenase activity was measured at noon on 15 occasions for each alder in 1987 and on 15 occasions in 1988. Diurnal variation in nitrogenase activity was studied at six occasions, but no obvious pattern in the diurnal variation was found. Nitrogenase activity began shortly after leaf emergence at the very end of May, increased in June, stayed high although with some variation through July and August, declined during September, and was zero in early October. Cumulative nitrogenase activity over the season was converted to cumulative N2 fixation after determination of molar ratio nitrogenase activity to N2 fixation. This conversion was facilitated as the Frankia chosen as symbiont was lacking hydrogenase activity. Control experiments showed that the introduced symbiont was the only infective Frankia in the soil. N2 fixation was estimated to be 0.23 and 2.83 g N/(alder∙year) in the 1st and 2nd year, respectively. Despite its young age, A. incana was apparently capable of high N2 fixation rates at the high latitude studied. Key words: Alnus incana, hydrogenase, intact plants, N2 fixation, seasonal variation, spreading of Frankia.

Nitrogen accumulation in sole and mixed stands of sweet-blue lupin (Lupinus angustifolius L.), ryegrass and oats

Nitrogen fixation was measured in monocropped sweet-blue lupin (Lupinus angustifolius), lupin intercropped with two ryegrass (Lolium multiflorum) cultivars or with oats (Avena sativa) on an Andosol soil, using the 15N isotope dilution method. At 117 days after planting and at a mean temperature below 10°C, monocropped lupin derived an average of 92% or 195 kg N ha−1 of its N from N2 fixation. Intercropping lupin with cereals increased (p<0.05) the percentage of N derived from atmospheric N2 (% Ndfa) to a mean of 96%. Compared to the monocropped, total N fixed per hectare in intercropped lupin declined approximately 50%, in line with the decrease in seeding rate and dry matter yield. With these high values of N2 fixation, selection of the reference crop was not a problem; all the cereals, intercropped or grown singly produced similar estimates of N2 fixed in lupin. It was deduced from the 15N data that significant N transfer occurred from lupin to intercropped Italian ryegrass but not to intercropped Westerwoldian ryegrass or to oats. Doubling the 15N fertilizer rate from 30 to 60 kg N ha−1 decreased % Ndfa to 86% (p<0.05), but total N fixed was unaltered. These results indicate that lupin has a high potential for N2 fixation at low temperatures, and can maintain higher rates of N2 fixation in soils of high N than many other forage and pasture legumes.

Contemporaneous N 2 Fixation and Oxygenic Photosynthesis in the Nonheterocystous Mat-Forming Cyanobacterium Lyngbya aestuarii

The nonheterocystous filamentous cyanobacterial genus Lyngbya is a widespread and frequently dominant component of marine microbial mats. It is suspected of contributing to relatively high rates of N(2) fixation associated with mats. The ability to contemporaneously conduct O(2)-sensitive N(2) fixation and oxygenic photosynthesis was investigated in Lyngbya aestuarii isolates from a North Carolina intertidal mat. Short-term (<4-h) additions of the photosystem II (O(2) evolution) inhibitor 3(3,4-dichlorophenyl)-1,1-dimethylurea stimulated light-mediated N(2) fixation (nitrogenase activity), indicating potential inhibition of N(2) fixation by O(2) production. However, some degree of light-mediated N(2) fixation in the absence of 3(3,4-dichlorophenyl)-1,1-dimethylurea was observed. Electron microscopic immunocytochemical localization of nitrogenase, coupled to microautoradiographic studies of CO(2) fixation and cellular deposition of the tetrazolium salt 2,4,5-triphenyltetrazolium chloride, revealed that (i) nitrogenase was widely distributed throughout individual filaments during illuminated and dark periods, (ii) CO(2) fixation was most active in intercalary regions, and (iii) daylight 2,4,5-triphenyltetrazolium chloride reduction (formazan deposition) was most intense in terminal regions. Results suggest lateral partitioning of photosynthesis and N(2) fixation during illumination, with N(2) fixation being confined to terminal regions. During darkness, a larger share of the filament appears capable of N(2) fixation.

Proteolysis and nitrogen fixation in faba‐bean (Vicia faba) nodules under water stress

Nodulated faba‐beans (Vicia faba L. var. minor) exhibiting high rates of N2 fixation (133 μmol C2H4 g−1 dry weight h−1), were subjected to water restriction. A loss of C2H2 reduction due to water stress was always associated with a decline of the leghemoglobin content for each of the 4 decreasing values of Ψmod. Electron micrographs showed ultrastructural alterations of the fixing tissue, which affected both partners and increased with the severity of water stress. In the nodule cytosol, the alkaline proteolysis approximately doubled when Ψmod decreased from −0.55 MPa to −1.55 MPa. Concomitantly, an increase of the nodule intracellular pH from 6.3 to 7.0 was observed. Proteolysis was due to serine proteases, exhibiting a pH‐optimum of 8 and which actively degraded purified leghemoglobin in vitro (Km=100 μM). The degradation of leghemoglobin during water stress may contribute to the loss of C2H2 reduction and may affect the pattern of recovery upon rewatering.

Increasing Soybean Productivity through Selection for Nitrogen Fixation

AbstractNitrogen fixation and accumulation are major factors of soybean [Glycine max (L.) Merr.] production. Investigations were carried out to determine whether higher N2 fixation could be considered as a criterion for the improvement of productivity. A selection for N2 fixation, which was based on acetylene reduction estimates, was carried out at Growth Stage R5. A set of 210 F2 plants issued from three crosses and developed in nutrient solutions was studied in phytotronic chambers, under temperature and mineral N nutrition conditions that were favorable for the maximum expression of the symbiosis. The seed yield of the F4‐derived lines was evaluated under normal population density on a Typic Udifluvent loamy‐sand soil (pH 6.0, organic matter 1.5%, and total N 1 g kg−1). Seed yield and N2 fixation were expressed with respect to the mean value of the genotypes with the same date of maturity. Seed yields of the F4‐ derived lines were significantly correlated with the N2 fixation of the F2 parent plants. This correlation was observed for all of the plants investigated including all crosses (r = 0.294**, significant at P = 0.01) and for each of the three crosses: Weber × Maple Arrow (r = 0.433**), Weber × Jiling 14 (r = 0.394*, significant at P = 0.05), Weber × Kingsoy (r = 0.597**). The seed yield of the F4 lines was also significantly correlated with nodule volume and nodule dry weight, particularly at each cross level. High nodulation ability and high N2 fixation rates appear to increase soybean seed yield. Early breeding for these traits may therefore be expected to result in a genetic gain in seed yield in the progeny.

Isolation of Soybean Lines Capable of Nodulation and Nitrogen Fixation under High Levels of Nitrate Supply1

Results from numerous studies indicate that the proportion of soybean [Glycine max (L.) Men.] crop N derived from N2 fixation ranges between 25 and 50% and rarely exceeds 75%. The remainder is taken up as NO−3 from the soil and the net effect of soybean cropping is depletion of the soil N pool. Therefore, a program was commenced to identify genotypes of soybean which were capable of high levels of nodulation and high rates of N2 fixation in the presence of NO−3. In the first year of screening, 489 genotypes of diverse origin were grown in sand, free of Bradyrhizobium japonicum prior to inoculation with effective strain CB 1809 (USDA136), in unreplicated, 14‐L free‐draining pots in a glasshouse and supplied with nutrients containing either zero or 2.5 mMNO−3. Plants were sampled at R2 for plant growth (shoot mass), for nodulation, and for the proportion of plant N derived from N2 fixation (relative abundance of ureides in xylem exudate and extracts of shoot axes and nodulated roots). From the original 489 genotypes, 66 “NO−3‐tolerant”, 9 “NO−3‐sensitive” and 12 others were selected for a second year of screening on the basis of an overall ranking index which related nodulation and N2 fixation (relative ureides) activity at 2.5 mMNO−3 to activity at zero NO−3. In year two, treatments were replicated twice; plant culture and sampling procedures were otherwise identical to those used in the first year. Variation in symbiotic activity at high NO−3 supply was again evident with between 5‐ and 13‐fold differences in the nodulation and ureide indices recorded. In both screenings, genotypes of Korean origin ranked first overall and provided three of the top 10 lines. Of the original 19 Korean genotypes, 15 (80%) were screened in year 2 and nine (47%) were identified for future evaluation. Only 5% of the other 470 genotypes were selected as “NO−3‐tolerant” after two years of screening. Isolation of genotypes, superior in nodulation and N2 fixation in the presence of NO−3 under both glasshouse and field conditions, should be achieved with further screening in a high‐NO−3 field soil.

Fast‐Growing Rhizobium fredii are Poor Nitrogen‐Fixing Symbionts of Soybean1

Soybean‐Rhizobium genotype combinations with high rates of N2 fixation are useful in soybean (Glycine max L. Merr.) breeding programs. Nitrogen fixation was examined in 13 soybean cultivars and wild soybean (G. soja Sieb. &amp; Zucc.) plant introductions inoculated with Bradyrhizobium japonicum strain 61A76 and four fast‐growing strains of Rhizobium fredii. Five‐week‐old plants were assessed for nodule morphology, acetylene reduction rate, nodule number, nodule fresh weight, and plant‐top dry weight. All fast‐growing strains formed effective nodules on cultivars ‘Peking’, ‘Virginia’, ‘Hardee’, and the G. soja plant introductions. In contrast, three of the fast‐growing strains formed ineffective nodules on the other cultivars. One strain, USDA191, formed effective nodules on all cultivars. No nodules were formed by the fast‐growing R. fredii on ‘Harosoy rj1 rj1’. Rhizobium fredii strain USDA191 fixed N, as well as the slow‐growing control with cultivars Peking, Virginia, ‘Harosoy 63’, and ‘Rampage’, and G. soja plant introductions PI 342622A and PI 101404B, but fixed less NJ than the slow‐growing control with cultivars ‘Evans’, ‘Williams’, Hardee, ‘Hill’, and G. soja PI 407217 and PI 81762.

Nitrogen fixation in an oligotrophic, saline desert lake: Pyramid Lake, Nevada

High rates of nitrogen fixation by a short‐lived but dense unialgal bloom of the planktonic blue‐green Nodularia spumigena provided 99.5% of the alga’s needs and 81% of Pyramid Lake’s annual total combined nitrogen input in 1979. The bloom was spatially very heterogeneous. Bloom size, duration, and presumably N2 fixation vary from year to year, but in 1979 about 900 t of nitrogen were fixed in 2 months in this large deep lake. The annual rate of N2 fixation was about 2 g m−2. In this year of low inflow the Truckee River provided 54 t of inorganic nitrogen and 83 t of organic nitrogen. Planktonic N2 fixation has not been measured during high inflow years and may have been small relative to river input. Lakewide average heterocyst to vegetative cell (h : c) ratios followed seasonal trends in N2 fixation, but synoptic samples showed only a weak relation between h : c and N2 fixation. N2 fixation was induced by low epilimnetic levels of inorganic nitrogen and ended before lake overturn in the fall. High rates of N2 fixation were confined to the upper 5% of the epilimnetic volume and thus occurred only in calm weather when Nodularia colonies floated to the lake surface. Access to freshly dissolved atmospheric CO2 may account for the near‐surface dependence, since the lake pH is normally about 9.2. We predict that Nodularia will not show the same degree of near‐surface dependence in near‐neutral lakes or in the ocean.

Nodulation and nitrogen fixation of Phaseolus vulgaris L. grown in minesoil as affected by soil compaction and N fertilization

Abstract The objectives of this growth chamber study were to assess the influence of three soil compaction levels, as determined by bulk density, and four N fertilizer treatments on growth, nodulation and N2 fixation of P. vulgaris in minesoils. Shoot and root growth was severely retarded and nodulation essentially absent in N unfertilized, uninoculated (control) plants. With inoculated plants, root and shoot mass increased and nodule mass decreased as bulk density was decreased from 1.55 g/cm3 (unamended) to 1.40 g/cm3 (amended with 50% by volume sand). Root and shoot mass increased as N fertilizer rates increased from 0 to 100 kg N/ha. Nodule mass increased at 25 kg N/ha, but higher N rates decreased nodule mass compared to unfertilized plants. There was no significant correlation between bulk density and N2 fixation, as measured by the acetylene reduction assay. In general, N2(C2H2) fixation was the greatest at 35 days from seeding; however, at 21 days, unfertilized plants showed high N2(C2H2) fixation rates. Fertilizer N at 25 kg N/ha increased acetylene reduction as compared to unfertilized plants, while at 50 and 100 kg N/ha the rate of N2(C2H2) fixation decreased.

The Glycine‐Glomus‐Rhizobium symbiosis

Soybean [Glycine max, (L.) Merr, cv. Lancer] plants were grown in a sterile rooting medium watered daily with a nutrient solution containing 4, 20, 100, or 500 μM, P. Plants were inoculated with Rhizobium japonicum, strain 61A118 and grown in the presence or absence of the endomycorrhizal fungus Glomus fasciculatus, Gerdemann et Trappe. Plants grown at the highest P regime had six times higher shoot dry weight than those grown in the lowest P regime. Nodulation did not occur at 4 μM P. Nodule dry weight increased 200‐fold from the 20 to the 500 μM P treatment. Percentage P in shoots and nodules differed significantly among all treatment levels. Acetylene reduction by nitrogenase increased logarithmically with increasing amounts of P. Hydrogen evolution was not detectable at the 20 μM P level. The relative efficiency of nitrogen fixation increased with increasing P stress. Infection by Glomus fasciculatus, at the 500 μM P level was negligible and did not affect the parameters measured. At all other treatment levels the mycorrhizal plants had significantly higher rates of N2 fixation, plant and nodule mass and P content.

Nitrogen sources for neotropical nasute termites: Fixation and selective foraging.

Studies with Nasutitermes and Rhynchotermes (Isoptera, Nasutitermitinae) in a Costa Rican rainforest suggest that nitrogen fixation by gut symbionts may play a significant role in termite nutrition. Leaf-litter feeders ingest more and fix less nitrogen than do wood-litter feeders; both species feed preferentially on N-rich foods in their foraging repertoire. Unusually high N2 fixation (acetylene reduction) rates were found in Nasutitermes soldiers, thereby reducing their nutritional dependence on workers. Finally, N2 fixation rates of termites diminished rapidly within 24 h after removal of a colony from the field, underlining the importance of conducting future N2 fixation studies under field conditions.

Interdependence of Nitrogen Nutrition and Photosynthesis in Pisum sativum L

Physiological responses to infection by strains of Rhizobium leguminosarum which differed in their capacity to reduce N(2) were determined in 26-day-old pea plants (Pisum sativum L. cv. Alaska) grown under uniform environmental conditions in the absence of combined N. The highest N(2) reduction rates, calculated from H(2) evolution and C(2)H(2)-dependent C(2)H(4) production measurements, were approximately 6-fold greater than the lowest. Higher N(2) fixation rates were associated with greater CO(2) exchange rates (R(2) = 0.92) and carboxylation efficiency (R(2) = 0.99). Increases in the apparent relative efficiency of N(2) fixation [1-(H(2) evolved in air/C(2)H(2) reduced)] (acteroid efficiency) were associated with increases in whole-plant N(2) fixation efficiency (N(2)/CO(2) reduction ratio) (R(2) = 0.95). Whole-plant dry weight and total N content were related by regression analysis (R(2) = 0.98); both parameters were enhanced by increased N(2) fixation in a manner analogous to previously reported increases caused by greater external applications of NH(4) (+). These data reveal that photosynthetic parameters in genetically uniform host plants grown under identical environmental conditions are affected by N(2) fixation characteristics of the rhizobial symbiont. The measured efficiencies of micro- and macrosymbiont are directly related under uniform environments.