Nodule Fresh Weight Research Articles

Glutamine synthetase and glutamate synthase activities in relation to nitrogen fixation in Lotus spp.

Lotus corniculatus, L. tenuis, L. pedunculatus, and L. subbiflorus inoculated with Mesorhizobium loti NZP2037 strain were grown in a growth chamber. The plants dry weight (DW), the nodule fresh weight (FW), the nitrogenase activity, the nodule glutamine synthetase (GS) and glutamate synthase (GOGAT) activities, as well as the leghemoglobin content and the amino acid in the stem were measured 28 days after inoculation. The highest DW of plants was measured in L. tenuis and the highest FW of nodules was measured in L. pedunculatus. Nitrogenase activity in L. tenuis, L. pedunculatus and L. subbiflorus was six fold the activity in L. corniculatus. Nodule GS and GOGAT activities did not follow this same pattern. L. tenuis had the highest values of GS and GOGAT activities in the nodule, and a high nitrogenase activity which is consistent with its high plant DW. The four species of Lotus were compared and no correlation between nitrogen fixation parameters and ammonia assimilation enzymes was found, but the GS/GOGAT ratio has a positive and significant correlation (r²=0.82**) with the amino acid content in stems.

Host Variation in Traits Associated with Crown Nodule Senescence in Soybean

Active N2 fixation in soybean [Glycine max (L.) Merr.] in Minnesota is limited by cool early‐season soil temperatures and by postflowering nodule senescence. This study examined variation in onset of nodule senescence among Maturity Group I soybean cultivars and sought traits associated with this variation. Host genotype markedly affected onset of crown nodule senescence. For most cultivars, crown nodule fresh weight and specific nodule activity (SNA) peaked 31 to 38 d after emergence (DAE) and declined rapidly thereafter. In contrast, maximum crown nodule fresh weight in ‘Hardin’ and ‘Hodgson 78’ did not occur until 52 DAE, and SNA was still high 45 to 52 DAE. Two cultivars, Chippewa and Alpha, that exhibited early change in crown nodule mass and SNA, accumulated glyceollin I 10 to 45 DAE at rates significantly greater than for Hardin and Hodgson 78. The four cultivars also differed in phenylalanine ammonia lyase (PAL) and chalcone synthase (CHS) gene expression, nodule protease activity, and polyamine accumulation. Morphological changes within the nodule paralleled the biochemical differences, with Chippewa nodules 45 DAE showing more conspicuous deterioration than was evident in Hardin. Because crown‐nodule mass and nitrogenase activity in Hardin and Hodgson 78 declined later than in other Maturity Group I cultivars, with less evidence of host–strain incompatibility, these two lines may have value in breeding programs to extend the period of active nodulation and N2 fixation in soybean.

Root Characteristics and Seed Yields of Cowpeas Grown with and without Added Nitrogen Fertilizer

Field experiments were conducted in Oklahoma in 1993 and 1994. Cowpeas [Vigna unguiculata (L.) Walp.] were grown using either noninoculated seed and 23 kg·ha–1 of preplant nitrogen (N) fertilizer (conventional) or Rhizobium-inoculated seed and no preplant N fertilizer (reduced input). Sample plants were excavated at first pod set and analyzed for nodulation and root morphology. Additional plants were harvested at the green-shell stage to determine seed yield and plant N concentration. Conventional and reduced input cowpeas did not differ in dry weight of root mass components, total root dry weight, shoot dry weight, shoot: root ratio, nodule distribution among root morphological components, total nodule fresh weight, plant N concentration, or green-shell seed yield. Most of the nodule fresh weight generally was associated with nodules on the basal and lateral roots. Results indicate that cowpea root characteristics are not necessarily altered by the presence or absence of added N fertilizer at a given location.

Development of N2‐fixing nodules on the wetland legume Lotus uliginosus exposed to conditions of flooding

Seeds of the wetland legume, Lotus uliginosus, were germinated and grown in vermiculite which was either continuously flooded or well‐drained. Plants from both treatments were infected by Mesorhizobium loti strain DUS341 via a ‘classical’ root hair pathway, although some flooded plants appeared to be infected via enlarged epidermal cells. Subsequent to infection by M. loti, nodule meristems, which had developed within the root outer cortex, were penetrated by infection threads that released bacteria into the meristematic cells. The infection threads and infection droplets were immunogold labelled with monoclonal antibodies (MAC265 and MAC236) that recognize epitopes (at approx. 155/170 and 170/210 kDa, respectively) on a glycoprotein component of the matrix that surrounded the bacteria within the threads or droplets. Although labelling of infection threads or infection droplets with MAC236 was stronger than that with MAC265, both antibodies strongly labelled material occluding intercellular spaces in the cortices of developing nodules that had not yet expressed nitrogenase (as determined by a lack of signal after immunogold labelling with an antibody raised against nitrogenase component II). After 60 d, nitrogenase activity, shoot and root dry weights, and nodule fresh weight per plant did not differ between the treatments. After a further 30 d submergence, the flooded stems developed extensive aerenchyma and there was profuse development of (nodulated) adventitious roots. Nodules also formed at the junction of adventitious roots and the subtending stem and these were connected vascularly to a small stalk of tissue which gave rise to both a nodule and an adventitious root. The flooded nodules had prominent lenticels, and possible air pathways from the atmosphere to the nitrogen‐fixing bacteroids are discussed.

Nitrate levels and stages of growth in hypernodulating mutants of Lupinus albus. I. N2 fixation potential

This work aimed to evaluate physiological parameters, nodulation response and N2 fixation rate in mutants of Lupinus albus in comparison with the standard Multolupa cultivar. Two nitrate levels (0 and 5mM) and two evaluation periods (7 and 10 weeks) were used. Significant differences were observed among genotypes, in relation to fresh nodule weight, nitrate levels and growth stages. The overall average for nitrate level differed between them where 5mM severely inhibited the number of nodules, reaching a 49.5% reduction in relation to treatment without nitrate. There were no behaviour differences among genotypes, nor among evaluation periods. Although the level of nitrate did not influence the production of shoot dry matter in relation to the average among levels applied, the L-135 genotype, being an inefficient mutant, reached very low values. There were no significant differences in electron allocation coefficient (EAC) among nitrate levels, nor among genotypes studied. However, the evaluation periods revealed differences, where the EAC for the seventh week had a higher value than that for the tenth week, when a 5mM aplication was evaluated. The N2 fixation rate (N2 FIX) showed the existence of the nitrate interference in fixation, given that the application of 5mM severely reduced. However, there were no differences among the genotypes and it was noted that the fixation rate was much higher in those that received nitrate. The L-88 and L-62 genotypes were the ones that have shown best adaptability in this experiment, thus being able to be recommended for new studies with higher nitrate levels and different evaluation periods. The nitrate (5mM) interferes in the nitrogen fixation rate, given that all the genotypes were affected by the level applied.

Nitrate-Induced Inhibition of Root Nodule Formation and Nitrogenase Activity in the Peanut (Arachis hypogaea L.)

SummaryThe effect of nitrate in the culture solution (0.7, 3.5, 7.0, and 14.0 mM N) on the growth, root nodule formation and nitrogenase activity of the peanut (cv. chico) inoculated with Bradyrhizobium sp. was examined at early developmental stages. The amount of nitrogen derived from fixation and its percentage to total nitrogen in the nitrate-fed plant were evaluated using 15N labeled nitrate. The dry weight and total nitrogen content of the plant at 40 days after transplanting (day 40) varied with the concentration of nitrate in the culture solution. Both of them were higher in the plants grown in the presence of nitrate at the concentrations above 0.7 mM. On the other hand, the number and fresh weight of nodules, and acetylene reduction activity per plant were the lowest in the presence of nitrate at 14.0 mM, relatively high at 0.7 mM, and the highest at 3.5 mM. The proportion of N derived from atmosphere to total nitrogen in the plants examined at days 18 and 33 was 23–31% in the presence of nitrate at 3.5 and 14.0 mM. However, the proportion examined at day 48 was as high as 57–58% at 3.5 mM, and it was only 22–24% at 14.0 mM.

Role of siderophores in chickpea ( Cicer arietinum L.) — Rhizobium symbiosis

Strains of Rhizobium ciceri (wild type as well as mutants) were screened for their ability to produce siderophores under cultural conditions.All strains except a mutant Ca40lRW5 were found to be siderophore producers.A standard strain Cal81 which is being used for large scale production as inoculant for chickpea, found to be a siderophore producer, was used for standardization of conditions for siderophore production.Only hydroxamate type of siderophores were detected under cultural conditions.The level of hydroxamate production increased linearly during log phase of growth up to 8 days.Addition of iron in the medium resulted in decrease in hydroxamate production and at 500 pM iron level, the hydroxamate level decreased by 40%.In other strains tested, decrease in hydroxamate production varied from 30 to 75% whereas in one strain Ca85AZ3 this level of iron completely inhibited hydroxamate production.Strain Cal 81 was most effective in terms of symbiotic nitrogen fixation in absence as well as added iron.Addition of iron stimulated nodule fresh weight in almost all the strains tested.It also resulted in increased symbiotic effectivity in terms of nitrogen gain per plant although the response varied with the bacterial strain.

Effects of Soil Amendment with Crab Shell on the Growth and Nodulation of Soybean Plants (Glycine max Merr.)

Three pot experiments were conducted to investigate the effects of soil amendment with crab shell on (1) soybean (Glycine max Merr., cv. Akishirome) yield after 120 days outdoors, (2) soybean nodulation after 6 weeks in the greenhouse, and (3) soil chemical properties after 10 weeks in the greenhouse. The experimental treatments were addition of crab shell at rates of 0.1, 0.2, 0.3, 0.5, and 1% (w/w), standard fertilizer (NPK) treatment, and an untreated control with neither crab shell nor fertilizer application. The crab shell amendment treatment was applied either two months before seeding (incubated) or immediately before seeding (non-incubated). The soil used was a volcanic ash soil (Andosol) from Kakamigahara. Addition of crab shell just before seeding caused a significant increase in seed yield and 100-seed weight compared to the control, but similar yield to NPK treatment. However, seed yield and 100-seed weight did not differ with the crab shell application rate. Amendment by 0.1–0.3% crab shell without incubation lead to the formation of slightly more nodules than in the NPK and control treatments. Reduced numbers of nodules were observed in soil incubated at 0.5–1%, and in non-incubated soil with 1% crab shell application. Nodule fresh weight and N2-fixation plant−1 were decreased in all incubation treatments and 0.3-1% non-incubation treatments as compared to the NPK treatment and the control. Both nodule fresh weight and N2 fixation were lower in incubated soils than in non-incubated soils, at corresponding rates of crab shell treatment. Dry matter production of soybean plants in soil given the crab shell treatment, especially when incubation was used, was found to be reduced after 6 weeks of plant growth. EC, N, and P availability increased with the increasing rate of crab shell treatment. However, there was a gradual decrease of soil pH following the crab shell treatment. In conclusion, although soybean nodulation and N2 fixation were not improved by crab shell application, seed yields were similar to those obtained with the NPK fertilizer.

Variation in the Pattern of Nitrogen Accumulation and Distribution in Soybean

The contribution of early nodulation and dinitrogen (N2) fixation to overall soybean [Glycine max (L.) Merr.] plant performance the cooler northern states of the U.S. Midwest needs further investigation. In a previous growth chamber study, we identified traits associated with improved early nodulation and N2 fixation in this crop. This study examined the nodulation, N accumulation, and N distribution patterns for the same 19 soybean lines in the field, and relates field performance to previous growth chamber results. The study was conducted in 1992, 1993, and 1994 at Becker, MN, in a soil with 7 to 15 kg NO−3 ha−1. The following data was recorded: nodule fresh weight (NFW), plant dry weight (PDW), and total N accumulation (TNA) at 20 to 60 d after planting (DAP), PDW and TNA at the R3 and R6 growth stages, and grain and seed protein yield at maturity. Differences in NFW, PDW, and TNA were found at most sampling dates, and were indicative of significant genetic differences in N2‐fixing ability. Lines also differed in partitioning of N, with the cultivar Lambert distributing 88% of total N accumulated to reproductive tissue at the R6 growth stage, compared with only 31% for PI 437966. Two canonical variables, one created by analyzing together the NFW data collected at 20, 40, and 60 DAP in the field, the other derived from NFW, PDW, and nodule soluble protein data obtained in the previous growth chamber study each correlated with the seed protein yield of field‐grown plants. This suggests that the measurement of early nodulation and N2 fixation in either the growth chamber or field can serve as an indicator on which to base selection for enhanced N2 fixation.

The Formation of Nitrogen-Fixing Bacteroids Is Delayed but Not Abolished in Soybean Infected by an [alpha]-Ketoglutarate Dehydrogenase-Deficient Mutant of Bradyrhizobium japonicum.

A mutant strain of Bradyrhizobium japonicum USDA 110 devoid of [alpha]-ketoglutarate dehydrogenase activity (LSG184) was used to test whether this tricarboxylic acid cycle enzyme is necessary to support nitrogen fixation during symbiosis with soybean (Glycine max). LSG184 formed nodules about 5 d later than the wild-type strain, and the nodules, although otherwise normal in structure, contained many fewer infected host cells than is typical. At 19 d after inoculation cells infected with the mutant strain were only partially filled with bacteroids and showed large accumulations of starch, but by 32 d after inoculation the host cells infected with the mutant appeared normal. The onset of nitrogen fixation was delayed about 15 d for plants inoculated with LSG184, and the rate, on a per nodule fresh weight basis, reached only about 20% of normal. However, because nodules formed by LSG184 contained only about 20% of the normal number of bacteroids, it could be inferred that the mutant, on an individual bacteroid basis, was fixing nitrogen at near wild-type rates. Therefore, the loss of [alpha]-ketoglutarate dehydrogenase in B. japonicum does not prevent the formation or the functioning of nitrogen-fixing bacteroids in soybean.

Heritability in the early nodulation of F3 and F4 soybean lines

We have previously identified differences among soybean [Glycine max (L.) Merr.] lines in early nodulation and N2 fixation, and related early nodule development and overall plant performance under N-limited field conditions. The objectives of the present study were: 1) to estimate heritabilities for nodule fresh weight (NFW), shoot dry weight (SDW), and root dry weight (RDW) determined 17 d after inoculation (DAI); 2) to examine genetic variation for total nodule soluble protein (TSOLP) at 17 DAI, and SDW and shoot N derived from fixation (SNdfa) at the R5 growth stage; and 3) to assess relationships between early nodulation and subsequent R5 shoot mass and SNdfa. The two populations used were derived from the Minnesota-adapted cultivars Kasota and Parker, and two plant introductions PI 437966 and PI 384469B. Genetic variances within populations were significant for most traits measured at 17 DAI, and at the R5 growth stage. Heritability estimates ranged from 0.79–0.82 for NFW, 0.51–0.56 for SDW, and 0.38–0.40 for RDW, with SDW also correlated with seed size at 17 DAI (r = 0.88**). Additional correlation analyses showed SNdfa correlated with NFW (r = 0.33**) and TSOLP (r = 0.46**). The results of this study suggest that gains from selection are possible for NFW and SDW at early stages of plant development. Key words: Glycine max (L.) Merr., nodulation, nitrogen fixation, heritabilitynot available

Comparative efficieny of Rhizobium/Bradyrhizobium spp. strains in nodulating Cajanus cajan in relation to characteristic metabolic enzyme activities

The comparative symbiotic properties of Rhizobium spp. and Bradyrhizobium spp. strains infecting pigeon pea were evaluated. Bradyrhizobium strains (Cajanus) were found to be superior to Rhizobium strains (Cajanus) and the superiority was ascertained to be due to the higher enzyme activity of the tricarboxylic acid (TCA) cycle in comparison to Rhizobium spp. strains. Moreover, metabolic superiority or rapid growth rate does not necessarily correlate with symbiotic effectiveness. The symbiotic performance of isolates varied with the host cultivar. The dry matter accumulation could be correlated with the total acetylene reduction activities rather than nodule number or nodule fresh weight per plant.

Reduction of Growth and Symbiotic Nitrogen Fixation in Soybean as Affected by the Site of Simulated Acid Rain Application.

Reductions of growth and symbiotic nitrogen fixation were studied in soybean exposed to simulated acid rain (SAR) applied to different sites. Soybean cultivar Okuharawase was grown in 1/5000a pots under a rain shelter. The plants were exposed to SAR from 15 days after sowing (DAS) to the flowering stage at 36 DAS. The sites of SAR application were foliage, soil and both foliage and soil, and the pH of SAR used was 2.7, 3.5, 4.5 and 7.0 (control). At the flowering stage, leaf area per plant, apparent photosynthetic rate and root dry weight per plant decreased in proportion to decrease in the pH of SAR. The degree of decrease was similar between the plants exposed to SAR in foliage and soil and those in foliage, but the application of SAR to soil alone did not cause any change in the parameters. There was a consistent reduction in the number of nodules per plant, the fresh weight of nodules per plant and the rate of symbiotic nitrogen fixation, or acetylene reduction activity (ARA) regardless of the site of SAR application. ARA decreased in proportion to the decrease in the number and fresh weight of nodules. The root dry weight significantly correlated with nodule number or nodule fresh weight. The reduction of growth and symbiotic nitrogen fixation in plants with exposure to SAR in foliage was similar to that with exposure in both foliage and soil, whereas SAR applied to soil alone suppressed modulation only.

Evaluation and improvement of the energy efficiency of nitrogen fixation in Lotus corniculatus nodules induced by Rhizobium loti strains indigenous to Uruguay

In order to evaluate energy efficiency of nitrogen fixation by the Lotus corniculatus/Rhizobium loti symbiosis, Uruguayan R. loti strains were tested for hydrogen-uptake (Hup) status. Nodules induced in L. corniculatus by all eight R. loti strains tested evolved high amounts of hydrogen (2.0–8.7 μ mol H2/h.g nodule fresh weight). This production of hydrogen corresponds to 38–69% of total nitrogenase activity estimated as acetylene reduction, suggesting that hydrogen is not recycled within these nodules. This was confirmed by the lack of hydrogenase activity in bacteroid suspensions. Additionally, no hybridization signals were observed in total DNA restriction digests from these strains when a DNA fragment containing part of hydrogenase structural genes from Rhizobium leguminosarum bv. viciae was used as probe. Cosmid pHU52, containing the complete gene cluster required for hydrogen oxidation in Bradyrhizobium japonicum, was introduced into two R. loti strains. Transconjugants from only one of the strains were able to express hydrogenase activity in vegetative cells incubated under the derepression conditions described for B. japonicum. Bacteroids induced by both transconjugant strains in L. corniculatus and Lotus tenuis expressed hydrogenase activity in nodules. The level of hydrogenase activity induced in L. tenuis nodules was two-fold higher than those induced in L. corniculatus. This implies the existence of a strong host effect on hydrogenase expression in this symbiotic system.

Nodulation, Nitrogen Fixation and Growth of Soybean Plants(Glycine max Merr.) in Soil Supplemented with Chitin or Chitosan.

Pot soil was supplemented with varying concentration of chitin or chitosan to investigate their influence on the nodulation, nitorgen fixation and growth of soybean plants. Significant (P=0.05) reductions in nodule fresh weight and nitrogen fixaton (measured as acetylene-reduction activity) were observed at the early growth stage (28 DAP) for plants grown in soil supplemented with chitin or chitosan as compared to regular pot soil. However, the nodule numbers were not affected at the early growth stage with the exception of 1% of chitin or chitosan. Both nodulation and nitorgen fixation activity at a later growth stage (42 DAP) were markedly enhanced in soils supplemented with 0.10% of either chitin or chitosan. These results suggest that all concentrations of chitin or chitosan have a severe influence on nodule development and N2-fixation activity at the early growth stage in soybean plants. The production of total dry matter was higher in untreated control plants at 28 DAP, while plants treated with 0.10∼0.25% chitin or chitosan produced higher dry matter at 42 DAP. Soybean seed yield increased with the increasing pod number at these levels of treatment. However, the above parameters did not differ greatly between chitin and chitosan at corresponding levels of concentration.

Influence of indoleacetic-acid-producing Bacillus isolates on the nodulation of Phaseolus vulgaris by Rhizobium etli under gnotobiotic conditions

Twenty-two Bacillus spp. isolates from the rhizosphere of Phaseolus vulgaris 'Contender' were identified using Biolog™, gas chromatographic fatty acid methyl ester, and 23S rDNA analyses. Some of the Bacillus isolates produced significant amounts of the phytohormone indoleacetic acid (IAA) when grown in a liquid culture medium supplemented with 100 μg L-tryptophan/L; less IAA was produced in culture medium not supplemented with L-tryptophan. Thin-layer chromatography, high-performance liquid chromatography, gas chromatography – mass spectrometry, and the avena coleoptile bioassay were used to identify and quantify IAA produced by Bacillus isolates. Significant differences were observed in the amounts of IAA produced by different strains of Bacillus, with amounts varying from 0.40 to 4.88 μg/mL. α-Methyltryptophan-resistant mutants of Bacillus exhibited altered IAA production and excreted tryptophan into the growing medium. The IAA-producing Bacillus isolates promoted root growth and (or) nodulation when coinoculated with Rhizobium etli (TAL 182) on Phaseolus vulgaris 'Contender' under gnotobiotic conditions in growth chambers. Coinoculation resulted in increased nodule number, nodule fresh weight, nitrogenase activity, leghemoglobin content, and total soluble protein content in the root nodules of Phaseolus vulgaris. In contrast, coinoculation with α-methyltryptophan mutants resulted in decreased nodulation, indicating that Bacillus isolates have a direct effect on either the Rhizobium or the plant and the effect may not be singularly attributed to their ability to produce IAA in vitro.Key words: Bacillus, indoleacetic acid production, nodulation enhancement.

Host Genetic Variation in the Early Nodulation and Dinitrogen Fixation of Soybean

Soybeans [Glycine max (L.) Merr.] grown on soils low in N can exhibit N deficiency during early plant development. Improved early N2 fixation may alleviate this problem. Our objectives were to (i) examine genetic variation in early nodulation and N2 fixation of soybean, and (ii) identify traits useful in breeding for early nodulation and N2 fixation in this crop. Eighty‐six soybean lines showed considerable genetic variation in efficiency and speed of nodulation with Bradyrhizobium japonicum strain USDA 110. Eighteen lines were then reevaluated for nodulation efficiency with the B. japonicum strains USDA 123, UMR 161, and NA 6407 for differences in nodule number 5 to 12 d after inoculation (DAI) and for differences in plant dry weight, nodule fresh weight, acetylene reduction activity (ARA), nodule enzyme activity, leghemoglobin concentration, and nodule soluble protein concentration (SOLP) 10 to 17 DAI. The 86 lines differed in percentage roots nodulated (PRN; 26–93%) and number of nodules above the root tip mark (NARTM; 0.3–3.0), and in uppermost nodule position (− 17.5 to 19.5 mm). For the 18 lines that were reevaluated, mean PRN and NARTM values were greater for USDA 123 than for USDA 110 (65.1 vs. 57.9%; 1.4 vs. 1.0, respectively). For these lines, correlation analyses showed that PRN correlated with nodule number at 5 (r = 0.69**) but not 12 DAI, while nodule number 12 DAI correlated positively with nodule fresh weight, ARA, enzyme activity, and SOLP 17 DAI. Plant dry weight, nodule fresh weight, and nodule SOLP assayed at 17 DAI are suggested as traits for use in a breeding program to improve early nodulation and N2 fixation in soybean.

Comparison of Root Characteristics of Conventionally Grown Cowpeas and Cowpeas Grown without Added Nitrogen Fertilizer

Field experiments were conducted in Oklahoma in 1993 and 1994. Cowpeas [Vigna unguiculata (L.) Walp.] were grown using either non-inoculated seed and 23 kg·ha–1 of preplant nitrogen fertilizer (conventional) or inoculated seed and no preplant nitrogen fertilizer (reduced input). Sample plants were excavated at first pod set and analyzed for nodulation and root morphology. Additional plants were excavated at the green-shell stage and were analyzed for nitrogen concentration. Conventional and reduced input cowpeas did not differ in nodule distribution among root morphological components, total nodule fresh weight, total root dry weight, or nitrogen concentration. Most nodules generally were located on the basal and lateral roots. Results indicate that cowpea root characteristics are not necessarily altered by differing cultural systems at a given location.

EVALUATION OF HUP+, HUP– AND A TRANSCONJUGANT RHIZOBIUM ON YIELD, NITROGEN FIXATION, AND UPTAKE HYDROGENASE ACTIVITY IN SELECTED CHICKPEA CULTIVARS

Evolution of hydrogen gas (H2) during N2 reduction in root nodules results in inefficient use of energy needed for N2 fixation. Cultivars of chickpea (Cicer arietinum L.) were inoculated with Rhizobium strains with and without genes for uptake hydrogenase (Hup) activity. H2 evolution, acetylene reduction activity, and uptake hydrogenase (Hup) activity were assayed on the resulting nodules. The Hup– strains produced higher plant yields than the Hup+ strains. The +N controls produced significantly higher yields than the –N controls and plants inoculated with Rhizobium strains. Hydrogen uptake activity by Rhizobium strains was influenced by the cultivar characteristics. Expression of the plasmid-borne hup genes (pHU52) of Bradyrhizobium japonicum was modified by the host cultivar. The average nodule fresh weight and shoot and root dry weights of the cultivars significantly increased following inoculation with the transconjugant Hup+ Rhizobium strain. Thus, biological N2 fixation may be enhanced by selecting Rhizobium strains that are appropriately matched to the particular cultivar. Incorporation of transconjugant Hup+ genes can increase rhizobial activity.

Effect of Shading on Nodule Growth and Sugar Distrihution in Faha Bean (Vicia Faba L.)

AbstractThe faba bean (Vicia faba L.) is an important grain legume world wide, yet the relationship between light intensity and nodule development and N2 fixation has received minimal attention. An experiment was conducted to study the effects of shading on faba bean growth, seed yield, nodule development, and the distribution of total N and total sugar among plant parts. Seeds of the V. faba cultivar Xichang Dabai were sown in pots at Xichang. China, in 1986–90. The plants subjected to 50 % and 20 % shading for 145 d had thinner and taller main stem than the control, and fewer flowers, pods, and seeds. Poor dry weight of organs and less developed underground parts were recognized. Shading caused the deficiency of photosynthate (supply), subsequently less nodules formed poorer fresh weight of nodules was produced and nodule senescing advanced. Total N and total sugar contents per plant and in roots, stems and leaves were consistently lower under shading regimes, but the contents in nodules did not change significantly. It seems that the insufficient photosynthate supply limited both N and C metabolism in shading plants by first limiting growth of the whole plant and nodule, and that the lower total N content in faba bean plant under shading regime was attributed to an decreased effective nodule mass and advanced nodule senescing.