Strain CIAT Research Articles

Nodulation and biological nitrogen fixation of common bean rhizobia from Santa Catarina Plateau soils

Curitibanos town is the third regular bean producer (Phaseolus vulgaris L.) in Santa Catarina. However, the regional production decreased due to problems such as the nitrogen fertilizers cost. Farmers do not use seed inoculation because of inoculant’s rhizobia low performance and adaptation to edaphoclimatic conditions. Thus, 36 rhizobia were isolated, characterized, and evaluated by in vitro indole acetic acid (IAA) production and phosphate solubilization, as auxiliary tools to the rhizobia selection. Ninety-two percent of the isolates produced IAA and 64% solubilized calcium phosphate. The isolates selected for field trials during two consecutive years were RBZ14 and RBZ15. RBZ14 produced an IAA higher amount than strain CIAT899 (recommended for inoculants) and RBZ15 formed a lower amount. For phosphate solubilization, all showed similar performance. In the first year, the isolates increased leaf N contents in the TAA Dama cultivar. RBZ15 increased the productivity. In the second year, in the IPR Tuiuiú cultivar, the isolates also increased leaf N contents, and there was no difference at productivity but increments of 120 and 205 kg.ha-1 with RBZ14 and RBZ15 inoculations, respectively. Therefore, it suggested their potential for BNF in different genotypes and that evaluated mechanisms may help to the selection of a more efficient rhizobia.

Synergistic Interaction of Rhizobium tropici, Rhizophagus irregularis and Serendipita indica in Promoting Snap Bean Growth

The overuse of chemical pesticides and fertilizers in crop farming has led to a decrease in crop quality and negative impacts on soil and the environment. It is crucial to adopt alternative strategies to maintain soil and environmental quality while enhancing crop growth and yield. To explore this, a study was conducted under greenhouse conditions to investigate the effect of Rhizobium tropici CIAT 899 alone, as well as in association with mycorrhizae (Rhizophagus irregularis) and endophytic fungus (Serendipita indica), on the growth, yield, and nutrient status of snap bean plants. At harvest, the rhizobial strain CIAT 899 demonstrated the highest effectiveness. It significantly increased the number of nodules in both Contender and Garrafal Enana varieties by 6.97% and 14.81%, respectively, compared with the control without inoculation. Furthermore, the results indicated that co-inoculation of Rhizobium and symbiotic fungi had positive effects on nitrogen content, phosphorus availability, and overall plant growth. Regardless of the variety, plants inoculated with R. tropici CIAT 899 and Serendipita indica exhibited the highest values for plant growth parameters. This combination resulted in 168% and 135% increases in root dry biomass, as well as 140% and 225% increases in the number of pods for Contender and Garrafal Enana, respectively, compared with the control at harvest. Additionally, this study highlights the potential benefits of combining R. tropici with either Serendipita indica or Rhizophagus irregularis in terms of nitrogen and phosphorus uptake. These symbiotic microorganisms demonstrated synergistic interactions with snap bean plants, leading to improved mineral nutrition and enhanced growth. Overall, these findings suggest that utilizing these symbiotic microorganisms can effectively enhance the mineral nutrition and growth of snap bean plants.

Impact of Fe + 2 ions on structural integrity of A0A6P1CI42_RHITR NifA protein from Rhizobium tropici strain CIAT 899

A0A6P1CI42_RHITR, a protein originating from Rhizobium tropici strain CIAT 899, has emerged as a key player in leguminous plant symbiosis and nitrogen fixation processes. Understanding the intricate details of its structure and function holds immense significance for unraveling the molecular mechanisms underlying its biological activities. In this study, we employed molecular modeling and molecular dynamics (MD) simulations to investigate the A0A6P1CI42_RHITR protein, with a specific emphasis on the influence of Fe-atoms, linker structural integrity, and conformational changes within the GAF domain. Our findings unveiled noteworthy conformational changes in the A0A6P1CI42_RHITR protein, particularly within the GAF domain, when Fe-atoms were present compared to its apo form. Significant conformational rearrangements after an initial 20 ns, accompanied by the opening of the ligand substrate accommodating loop in the GAF domain influenced by Fe-atoms was observed. At the residue level, the investigation revealed substantial activity variations in individual residues, particularly in those contributing to the GAF domain from positions 51 to 223. Intriguingly, the presence of Fe-atoms led to controlled movement of conserved cysteine residues at positions 467 and 472, indicating a correlation between interlinker domain motion and the activity of the GAF domain loop responsible for substrate accommodation. Moreover, in the presence of Fe-atoms, the distance between Cys467 and Cys472 residues was maintained, ensuring the overall structural integrity of the interdomain loop necessary for protein activation. Conversely, in the apo form, a sudden flip motion of cysteine residues’ thiol groups was observed, leading to a loss of structural integration. Overall, our study utilizing molecular modeling and MD simulations offers valuable insights into the structural dynamics and functional implications of the A0A6P1CI42_RHITR protein. Communicated by Ramaswamy H. Sarma

Scopping review: use of biofertilizers and humic substances in Phaseolus vulgaris (Fabaceae) for dry zones

The common bean crop in dry areas is affected by the climate change and the low availability of nutrients that limits its yield. The objective of this review was to analyze the scope of the scientific literature on the use of inoculants based on Plant growth promoting rhizobacteria (PGPR) and humic substances (HS) in sustainable agronomic management of common bean crop and drought tolerance, for the generation of recommendations applicable to the production cultivation in dry areas from low tropic. An exploratory review on the use of PGPR and SH in common bean was carried out, since the first publications until December 2022 in the databases: Science direct, SciElo, SpringerLink, Scopus, Pubmed and Proquest. The co-inoculation of rhizobia with other PGPR was the most frequent technique in the reviewed articles. However, the combined application with SH allows greater tolerance to the water stress caused by drought. Rhizobia species most reported as efficient were Rhizobium tropici, Rhizobium etli and the strain CIAT 899 (R. tropici). In addition, the strain CIAT 899 was found to be the most useful in inoculant formulations for common beans under drought conditions in Brazil. In Colombia, only one registered product based on Rhizobium phaseoli was found for common bean, although there are no reports of evaluation of this strain under drought stress conditions.

Rhizobacteria control damping-off and promote growth of lima bean with and without co-inoculation with Rhizobium tropici CIAT899.

Rhizoctonia solani compromises the production of lima bean, an alternative and low-input food source in many tropical regions. Inoculation of bacterial strains has been used, but research on their biocontrol and growth promotion potential on lima bean is scarce. The objective of this study was to evaluate the effects of inoculation with rhizobacterial strains of the genera Bacillus, Brevibacillus, Paenibacillus, Burkholderia, Pseudomonas, and Rhizobium in combination or not with N2-fixing Rhizobium tropici on the control of damping-off disease and growth promotion in lima bean plants. Greenhouse experiments were conducted to evaluate the inoculation with bacterial strains with biocontrol potential in combination or not with R. tropici in substrate infected with R. solani CML 1846. Growth promotion of these strains was also assessed. Strains of Brevibacillus (UFLA 02-286), Pseudomonas (UFLA 02-281 and UFLA 04-885), Rhizobium (UFLA 04-195), and Burkholderia (UFLA 04-227) co-inoculated with the strain CIAT 899 (Rhizobium tropici) were the most effective in controlling R. solani, reducing the disease incidence in 47-60% on lima bean. The promising strains used in the biocontrol assays were also responsive in promoting growth of lima bean under disease and sterile conditions. A positive synergistic effect of co-inoculation of different genera contributed to plant growth, and these outcomes are important first steps to improve lima bean production.

Influence of common bean genotypes and rhizobia interaction for nodulation and nitrogen fixation

Bacteria from the rhizobia group are able to associate symbiotically with bean crop, forming nodules in the root, in which the biological nitrogen-fixing process occurs. However, the efficiency of this process has been low and it can be attributed to genetic and environmental factors. Thus, the objective of this study was to evaluate the capacity of nodulation of local varieties and commercial common bean cultivars inoculated with a Rhizobium tropici strain used in commercial inoculants and rhizobia isolates from common bean root nodules. The experiment was carried out in a factorial scheme (2x4), in a randomized block design with four replicates. It was tested two local varieties and two commercial cultivars, inoculated with the reference Rhizobium tropici strain CIAT899 and the RBZ14 strain isolated from common bean nodules grown in soils of Southern Brazil, in adapted Leonard-type pots. The CIAT899 strain promoted either higher mass of viable nodules and higher nitrogen accumulation in the aerial part. The black group local variety showed better response than the cultivar TAA Dama for nodule viability, suggesting more efficiency for nodulation. The interaction between genotypes (local varieties and commercial cultivars) and bacteria showed the specificity of the complex symbiotic relationship of biological nitrogen fixation in common bean, requiring further studies of these interactions.

The Cell Membrane of a Novel Rhizobium phaseoli Strain Is the Crucial Target for Aluminium Toxicity and Tolerance.

Soils with low pH and high aluminium (Al) contamination restrict common bean production, mainly due to adverse effects on rhizobia. We isolated a novel rhizobium strain, B3, from Kenyan soil which is more tolerant to Al stress than the widely used commercial strain CIAT899. B3 was resistant to 50 µM Al and recovered from 100 µM Al stress, while CIAT899 did not. Calcein labeling showed that less Al binds to the B3 membranes and less ATP and mScarlet-1 protein, a cytoplasmic marker, leaked out of B3 than CIAT899 cells in Al-containing media. Expression profiles showed that the primary targets of Al are genes involved in membrane biogenesis, metal ions binding and transport, carbohydrate, and amino acid metabolism and transport. The identified differentially expressed genes suggested that the intracellular γ-aminobutyric acid (GABA), glutathione (GSH), and amino acid levels, as well as the amount of the extracellular exopolysaccharide (EPS), might change during Al stress. Altered EPS levels could also influence biofilm formation. Therefore, these parameters were investigated in more detail. The GABA levels, extracellular EPS production, and biofilm formation increased, while GSH and amino acid level decreased. In conclusion, our comparative analysis identified genes that respond to Al stress in R. phaseoli. It appears that a large portion of the identified genes code for proteins stabilizing the plasma membrane. These genes might be helpful for future studies investigating the molecular basis of Al tolerance and the characterization of candidate rhizobial isolates that perform better in Al-contaminated soils than commercial strains.

Inoculation with Rhizobium tropici can totally replace N-fertilization in the recently released BRSMG Uai bean cultivar

The BRSMG Uai common bean cultivar (Phaseolus vulgaris L.) unites traits required by producers, such as high yield and upright stands, the latter of which is considerably improved in relation to previously released cultivars such as BRSMG Madrepérola and Pérola. However, the potential of the cultivar to form symbiotic relationships with rhizobia to fix N2 has not yet been assessed. Here, field experiments were conducted to assess the efficiency of biological nitrogen fixation (BNF) in the cultivar, and to compare this to the efficiencies of other genetic materials widely grown throughout Brazil (BRSMG Madrepérola and Pérola). Experiments were conducted on two Oxisols in Minas Gerais State, Brazil (one during the dry season in the south, and the other during the winter season in the Alto-Paranaíba region), under a no-tillage system over maize stover. A randomized block experimental design was implemented in a 3 × 3 factorial arrangement with four replicates. Individual and combined Analysis of Variance were performed and the data were normalized. Homogeneity of the residual mean squares was observed. Significant effects of factors were identified by Scott-Knott tests and F tests. The BRSMG Uai cultivar responded to inoculation with the rhizobial strain CIAT 899, with yields similar to those achieved upon fertilization with 80 kg ha-1 of N-urea and to the yields of the BRSMG Madrepérola and Pérola cultivars inoculated with the same rhizobial strain. In terms of commercial use, this cultivar may be successful under either of the management practices adopted for N supply, and from the perspective of plant breeding, it has potential for inclusion in breeding programs directed toward improving BNF.

Co-inoculation of Rhizobium and Bradyrhizobium promotes growth and yield of common beans

Some strains of Bradyrhizobium benefit the biological N2 fixation (BNF) and growth of common beans (Phaseolus vulgaris L.) when co-inoculated with Rhizobium tropici under greenhouse conditions. Here, we evaluated the benefit of Bradyrhizobium diazoefficiens strain CPAC 7 co-inoculated with Rhizobium tropici strain CIAT 899 on common bean in the field. Three Bradyrhizobium strains were co-inoculated with R. tropici CIAT 899 in one experiment in the greenhouse and two in the field. The greenhouse experiment combined each of three Bradyrhizobium strains (29w, USDA 110, and CPAC 7) in co-inoculation with CIAT 899 in Leonard jars. Then, two of these strains, 29w and CPAC 7, were co-inoculated with Rhizobium on-seeds (1.2 × 106 CFU seed−1) or both on-seed and in-furrow (1.08 × 108 CFU m−1) in a field experiment. Finally, a second field experiment evaluated strain CPAC 7 at doses ranging from 1.5 × 105 to 1.2 × 106 CFU seed−1. Co-inoculation with strain CPAC 7 improved shoot biomass accumulation in Leonard jars. In the field, co-inoculation of CPAC 7 with CIAT 899 on-seed enhanced grain yield by 20% compared with inoculation of CIAT 899 only (p < 0.10). The percentage of N derived from the air (%Ndfa) was between 50 and 60%, with only a tendency towards co-inoculation benefit. Given that the %Ndfa of the inoculated plants was not statistically different between treatments, the increased grain yield was not entirely due to BNF but also due to an increase of soil N uptake. In the second field trial, larger doses of inoculum increased nodulation and root biomass, but significant differences in grain yield were not observed. Our results show that the Rhizobium-Bradyrhizobium co-inoculation can enhance growth and grain yield of common beans and indicate that a plant growth-promoting mechanism besides BNF may be involved in the beneficial co-inoculation effect. However, further adjustments are needed to ensure a better response under field conditions.

Seed size influences the promoting activity of rhizobia on plant growth, nodulation and N fixation in lima bean

ABSTRACT: This study evaluated the activity of rhizobia isolates inoculated in large (18 mm) and small (11 mm) seeds on lima bean growth, nodulation and N fixation. Selected rhizobia isolates were compared with a reference strain CIAT899 and two controls without inoculation. Large seeds contributed for highest plant growth, nodulation and N fixation than small seeds. The isolates UFPI-59, UFPI-18 and UFPI-38 promoted the highest values of shoot and root dry weight, respectively. The isolates UFPI-32 promoted the highest values of nodule number, while UFPI-59 promoted the highest values of nodule dry weight. The isolates UFPI-38 and UFPI-59 promoted the highest accumulation of N. This study showed that seed size really influences lima bean growth, nodulation and BNF. Considering rhizobia isolates, UFPI-59, UFPI-38, and UFPI-18 contributed for plant growth, promoted better nodulation and effectiveness on biological N fixation.

Bacterial Consortium and Microbial Metabolites Increase Grain Quality and Soybean Yield

The effects of Bradyrhizobium inoculation on soybean growth and productivity are well known, but plant responses to consortia of other beneficial microbes and microbial molecules have not yet been well explored. Therefore, the main aim of this study was to evaluate the effect of different combinations of beneficial bacteria with and without microbial secondary metabolites (MSM) on two soybean cultivars in three cropping seasons under tropical field conditions. The bacterial consortia consisted of Bradyrhizobium japonicum (strain SEMIA 5079) plus Bradyrhizobium diazoefficiens (strain SEMIA 5080) inoculated with different combinations of Bacillus subtilis (strain QST 713), Azospirillum brasilense (strains Ab-V5 and Ab-V6), and MSM (metabolites enriched in lipo-chitooligosaccharides (LCOs) extracted from B. diazoefficiens (strain USDA 110) and from Rhizobium tropici (strain CIAT 889)). Standard inoculation of Bradyrhizobium combined with Azospirillum brasilense and microbial secondary metabolites increased leaf total N (7.1%), total P (11.1%), and N-ureide (16.5%); nodule number (NN, 26%) and dry weight (NDW, 22%); root (RDW, 15.4%) and shoot dry weight (SDW, 6%); 100-seed weight (3.7%); grain yield (up to 516 kg ha−1); grain crude protein concentration (2.4%); and the agronomic efficiency index (AEI) (11%). Inoculation with bacterial consortia and metabolites increased grain yield and quality, representing a promising technology for sustainable soybean cropping in tropical regions.

Nitrogen in Shoots, Number of Tillers, Biomass Yield and Nutritive Value of Zuri Guinea Grass Inoculated with Plant-Growth Promoting Bacteria

The objective of this study was to evaluate the effects of plant growth-promoting bacteria (PGPB) strains of Azospirillum brasilense, Pseudomonas fluorescens and Rhizobium tropici on biomass yield, number of tillers, nitrogen accumulation and nutritive value of shoots of Megathyrsus (syn. Panicum) maximus cultivar BRS Zuri (Zuri Guinea grass). For that, one experiment was performed for 14 months to evaluate inoculation and re-inoculation with Azospirillum brasilense strains Ab-V5 and Ab-V6, Pseudomonas fluorescens strain CCTB 03 and of co-inoculation with Rhizobium tropici strain CIAT 899 + A. brasilense strain Ab-V6 combined with N-fertilizer (100 kg of N ha-1). Shoot dry weight yield (SDWY), number of tillers (NT), total N concentration (TNC), total N uptake (TNU) and nutritive value of Zuri Guinea grass was evaluated for eight cuts, and inoculation increased all parameters. In the NT, the treatments inoculated with PGPB were superior to the positive non-inoculated control receiving N-fertilizer, by up to 36%. For the accumulated of SDWY the treatment re-inoculated with P. fluorescens CCTB 03 after each cut was statistically superior tin 7% the positive control. The PGPB when combined N-fertilizer also increased SDWY, NT, the relative chlorophyll index, TNC, total N uptake, neutral detergent fiber, acid detergent fiber, crude protein and in vitro digestibility dry matter of Zuri Guinea grass. The results indicate that PGPB can represent a sustainable alternative for reducing the use of N-fertilizers. The lower effects of re-inoculation with PGPB on the nutrition or yield of Zuri Guinea grass, demonstrating that the determination of the method of application and periodicity of inoculation still require investigation.

Yield, yield components and nutrients uptake in Zuri Guinea grass inoculated with plant growth-promoting bacteria

The objective of this study was to evaluate the effects of strains of Azospirillum brasilense, Pseudomonas fluorescens and Rhizobium tropici on biomass yield and nutrients uptake of shoots and roots of Megathyrsus (syn. Panicum) maximus cultivar BRS Zuri (Zuri Guinea grass) inoculated with plant growth-promoting bacteria (PGPB). Treatments consisted of inoculation and re-inoculation with A. brasilense strains Ab-V5 and Ab-V6, P. fluorescens strain CCTB 03 and of co-inoculation with R. tropici strain CIAT 899 + A. brasilense Ab-V6, with or without N-fertilizer (100 mg dm-3). Evaluations were performed on three cuts for the determination of root and shoot dry weight yield, morphological compositions, tiller mass, number of tillers, and nutrient uptake. Inoculation with bacteria in association with N-fertilizer increased N, NH4+, Ca, Fe, Mn and Zn accumulation in shoots and P and K uptake in roots. P. fluorescens and co-inoculation with R. tropici CIAT 899 + A. brasilense Ab-V6 increased the relative chlorophyll index in relation to the non-inoculated control. As expected, PGPB were not able to fully replace N-fertilization. However, when combined with N-fertilizer, the PGPB increased yield, the relative chlorophyll index, and the uptake of N, NH4+, Ca, Zn, Mn and Fe of Zuri Guinea grass. The results indicate that PGPB can represent a sustainable alternative for reducing the use of N-fertilizers. There were no effects of re-inoculation with PGPB on the nutrition or yield of Zuri Guinea grass, demonstrating that the determination of the method of application and periodicity of inoculation still require investigation.

Effects of growth‐promoting bacteria on soybean root activity, plant development, and yield

AbstractRhizobia and other plant growth‐promoting rhizobacteria (PGPR) have been broadly used as inoculants in agriculture, resulting in morphofunctional improvements in roots and grain yield. This study was carried out during two cropping seasons under field and greenhouse conditions in Brazil to verify the effects of inoculation of two soybean cultivars with PGPR and secondary microbial metabolites (SMMs) on root activity and nodulation, plant development, and grain yield. Inoculation and co‐inoculation treatments consisted of Bradyrhizobium japonicum strain SEMIA 5079 and B. diazoefficiens strain SEMIA 5080 inoculated together, in combination with Bacillus subtilis strain QST 713, Azospirillum brasilense strains Ab‐V5 and Ab‐V6, and SMMs extracted from B. diazoefficiens strain USDA 110 and Rhizobium tropici strain CIAT 889. Root systems were evaluated by direct (optical reading) and indirect (rubidium nitrate application, 85RbNO3) methods. Increases of up to 1.6% in root diameter (0.01‐ to 0.5‐mm class), 28.5% in length, 19.7% in root volume, 17.8% in root surface area, 29% in the number of nodules, 27.2% in nodule dry weight, 13.5% in root dry weight, and 3.8% in shoot dry weight. Greater exploration and activity within and between rows following inoculation at up to 40 and 10 cm in depth, respectively, were observed in plants co‐inoculated with the standard inoculation (only Bradyrhizobium spp.) + SMMs + A. brasilense, resulting in a yield increase of 485 kg ha−1. The results emphasize the biotechnological potential of using secondary metabolites of rhizobia with inoculants containing rhizobia and PGPR to improve the growth and soybean yield in tropical conditions.

Evaluation of Rhizobia strains UFLA 02 100 and CIAT 899 in line LEP 02 11 of Phaseolus vulgaris L.

The growing quest for sustainability in agricultural production systems has been pushing researchers to develop new technologies under different soil and climatic conditions. Based on this, and knowing the socioeconomic importance of the bean, the use of biological nitrogen fixation (BNF) is explored. Thus, the quest for quantitative knowledge of BNF at the phenological stages in a new bean line using promising and commercial strains becomes necessary. The objective of this study was to evaluate the strains UFLA 02 100 and CIAT 899 in the different phenological stages of the LEP 02 11 common bean strain using the biological nitrogen fixation. The experiment was conducted in a greenhouse on the Maringá-PR campus of the State University of Maringá. The design was a randomized complete block design in a 6x4 factorial scheme with four replications. The factor A comprised six nitrogen sources: Control, strain UFLA 02 100, strain CIAT 899, Nitrogen 60 kg ha-1, strain UFLA 02 100 + Nitrogen 30 kg ha-1, strain CIAT 899 + Nitrogen 30 kg ha-1 and the B factor is the phenological stages V4, R5, R6 and R8. The analyzed variables were: dry mass of nodules (DMN, grams/plant), dry mass of shoot (DMS, grams/plant), nitrogen content in shoot (NCS, %) and accumulation of nitrogen in shoot (ANS, grams/plant). For the statistical analysis of the data, the means test was used for the variables in the stages. The CIAT 899 strain statistically outperformed UFLA 02 100 for most variables and it was found that nitrogen fertilization negatively affected nodulation.&#x0D;

Regulation of hsnT, nodF and nodE genes in Rhizobium tropici CIAT 899 and their roles in the synthesis of Nod factors and in the symbiosis

Rhizobium tropici strain CIAT 899 possesses outstanding agronomic properties as it displays tolerance to environmental stresses, a broad host range and high effectiveness in fixing nitrogen with the common bean (Phaseolus vulgaris L.); in addition, it carries intriguing features such as five copies of the regulatory nodD gene, and the capacity to synthesize a variety of nodulation factors (NFs), even in a flavonoid-independent manner, when submitted to abiotic stresses. However, the roles of several nod genes of the repertoire of CIAT 899 remain to be determined. In this study, we obtained mutants for the hsnT, nodF and nodE genes of CIAT 899 and investigated their expression, NF structures and symbiotic properties. Either in the presence of the flavonoid apigenin, or of salt the expression of hsnT, nodF and nodE in wild-type CIAT 899 was highly up-regulated in comparison to the mutants of all five copies of nodD, indicating the roles that regulatory nodD genes play in the activation of hsnT, nodF and nodE; however, NodD1 was recognized as the main inducer. In total, 29 different NF structures were synthesized by wild-type CIAT 899 induced by apigenin, and 36 when induced by salt, being drastically reduced by mutations in hsnT, nodF and nodE, especially under osmotic stress, with specific changes related to each gene, indicating that the three genes participate in the synthesis of NFs. Mutations in hsnT, nodF and nodE affected differently symbiotic performance (nodule number and shoot dry weight), according to the host plant. Our results indicate that the expression of hsnT, nodF and nodE genes of CIAT 899 is mediated by nodD genes, and although these three genes do not belong to the main set of genes controlling nodulation, they contribute to the synthesis of NFs that will impact symbiotic performance and host specificity.

Response of dry bean (Phaseolus vulgaris L.) to inoculation with indigenous and commercial Rhizobium strains under organic farming systems in Minnesota

Organic farmers recognize the importance of using Rhizobium to meet crop N fertility needs and to reduce use of chemical fertilizers. Field experiments were conducted during the 2015 and 2016 seasons at different organic fields to assess the effect of indigenous Rhizobium leguminosarum bv. phaseoli (OrgK9) and commercial R. tropici (CIAT899) strains on symbiotic performance and grain yield of bean varieties, Eclipse and Redhawk. Averaged for both varieties, nodule numbers on plants inoculated with CIAT899 (35.2 ± 3.7 and 26.3 ± 2.6 /plant) and OrgK9 (29.6 ± 2.5 and 26.8 ± 2.1 /plant) were significantly greater (P < 0.0001) than the control (18.9 ± 2.2 and 10.5 ± 3.1) in 2015 and 2016, respectively. Bean genotype Eclipse cultivated at Lamberton and inoculated with OrgK9 had (P < 0.0001) greater number of nodules compared with CIAT899-inoculated or non-inoculated plants. The N-content (3.3 ± 0.04%) of plants inoculated with CIAT899 was greater (P < 0.0001) compared with plants inoculated with OrgK9. Strain CIAT899 increased nodule occupancy by 29 and 62% and 4 and 26% on Red Hawk cultivated in the Lamberton and Farmington fields compared with the control in season 2015 and 2016, respectively. The majority of nodules formed on Red Hawk (76%) and Eclipse (64%) at Farmington were occupied by CIAT899 and OrgK9 in 2015 and 2016, respectively. Eclipse inoculated with OrgK9 had a significantly greater seed yields (2683 ± 402 and 2546 ± 247 kg/ha) compared with control (2265 ± 996 and 2058 ± 51 kg/ha) in both years, respectively. These results clearly indicate that symbiotic performance and grain yield of dry bean can be significantly increased by using Rhizobium inoculation under organic farming systems.

Inoculação e aplicação de diferentes doses de nitrogénio na cultura do feijoeiro

The objective of this research was to evaluate the development and productivity of bean crop (Phaseolus vulgarisL.), when submitted to the inoculation and application of nitrogen fertilizer (N) at sowing and/or top dressing (cover). The experimental design was in randomized blocks (RBD) in 4 (N sowing, N top dressing, N sowing and top dressing and without N application) x 2 (with and without inoculation of strain CIAT 899T) factorial combination with three replications. At 28 days after sowing, all treatments received the foliar application of 80.0 g ha-1of molybdenum. In the same period, the treatments containing N top dressing received the application of 40 kg ha-1. At flowering time, the parameters, dry matter of the aerial part (DMAP), shoot nitrogen content (TNPA), number (NN) and dry matter of nodules (MSN) were evaluated. After 120 days (harvest time) the following parameters were evaluated: number of pods (NVP) and grains per plant (NGP) and grain yield (PG - kg ha-1). Inoculation proved to be a viable practice when it was not used mineral nitrogen. The use of inoculated seeds and also top dressing of nitrogen fertilization in cover promoted good development and productivity, however, not comparable to the full dose of mineral N.

Response of common bean (Phaseolus vulgaris L.) to nitrogen, phosphorus and rhizobia inoculation across variable soils in Zimbabwe

Common bean is an important crop with potential to curb malnutrition in poor Sub-Saharan African populations. Yields of common bean (Phaseolus vulgaris L.) are, however poor, limited by low soil phosphorus (P), nitrogen (N) and poor biological N2-fixation. On-farm experiments were carried out to study the effect of N, P and rhizobia inoculation on common bean yield and yield components during the 2014/2015 and 2015/2016 cropping seasons in Eastern Zimbabwe. Experiments were conducted on five farmers’ fields located in two agroecologies; three fields were considered to be degraded with soil organic carbon (SOC) < 4 g kg−1 and available P < 6 mg kg−1, while the two non–degraded sites had SOC > 7 g kg−1 and available P > 15 mg kg−1. Two common bean varieties (Gloria and NUA45) were tested in a split-plot arranged in randomized complete block design. The main plot factor was the combination of N (0 and 40 kg ha−1) and P (0 and 20 kg ha−1), and the sub-plot factors were variety (Gloria and NUA 45) and inoculation with Rhizobium tropici strain CIAT899 (+/− inoculum). At planting, both N and P were applied at 20 kg ha−1, with an additional 20 kg ha−1 N top dressing applied at flowering. Analysis of variance indicated common bean did not respond to rhizobia inoculation (P > 0.05) whilst P significantly increased the number of nodules and active nodules per plant (P < 0.001), and grain yield. Application of 40 kg ha−1 N significantly increased the number of pods per plant, number of seeds per pod, and grain yields. A significant NP interaction was only observed on grain yield for non-degraded soils. Co-application of N and P in non-degraded sites increased grain yields from 0.27 to 1.48 Mg ha−1during the first season and from 0.37 to 2.09 Mg ha−1during the second season. On degraded sites, NP application resulted in uninspiring grain yield gains of 0.09 to 0.19 Mg ha−1 during the first season, and from 0.16 to 0.28 Mg ha−1 in the second season. In general, effects of N or P were not significantly different, suggesting that farmers could invest in either of these nutrients for increased common bean grain yields. Strategically, P investments would be more logical as residual P effects to rotational cereals improve overall cropping system performance. The response of common bean to inoculation in Zimbabwe still needs to be widely investigated for these and other varieties.

English

Common bean (Phaseolus vulgaris L.) is the most important legume for human consumption in many countries of South and Central America, Asia and Africa. The crop can benefit from the biological nitrogen fixation process, especially when inoculated with elite rhizobial strains. Strains belonging to the “Rhizobium tropici group” are preferred because they show high tolerance to abiotic stresses, but their survival in liquid formulations is poor, limiting their use by farmers. In this study, response surface methodology (RSM) was used to develop liquid formulations for the commercial strains CIAT 899 (R. tropici) and PRF 81 (Rhizobium freirei). A significant interaction between dibasic potassium phosphate (K2HPO4) and yeast extract was observed, and to reach higher cell concentration, one should employ low concentrations of yeast extract and high concentrations of K2HPO4. A basic formulation which may represent the basis for the development of liquid inoculants for the common bean crop was developed. Key words: Biological nitrogen fixation, inoculation, common bean, Phaseolus vulgaris, Rhizobium freirei.