Introduction
Groundnut (Arachis hypogaea L.) is a leguminous crop that produces grains inside shells that develop underground. This crop is highly valued for its oil (40–50%) and protein (20–30%) contents and is used for human food, livestock feed, and other industrial uses, including biodiesel production (Taurian et al., Reference Taurian, Anzuay, Ludueña, Angelini, Muñoz, Valetti and Fabra2013; Moda-Cirino et al., Reference Moda-Cirino, Ribeiro, Buratto, Sou and Jr2015; Asante et al., Reference Asante, Ahiabor and Atakora2020). Groundnut is currently cultivated worldwide in tropical, subtropical, and warm temperate regions, with the top five producers in tons of grain with shells being mainland China (18,307,800), India (10,244,000), Nigeria (4 607 669), the USA (2 898 140), and Sudan (2 355 000) (FAO, 2021). In the 2022/2023 crop season, Brazil produced 892 200 tons of grain, with an average productivity of 4,041 kg ha-1 (CONAB, 2023).
Groundnut plants have a high demand for N, which may result from biological N2 fixation (BNF) (Kermah et al., Reference Kermah, Franke, Adjei-Nsiah, Ahiabor, Abaidoo and Giller2018; Peoples et al., Reference Peoples, Giller, Jensen and Herridge2021). This process contributes to economic gains and plays an important role in maintaining soil fertility and the sustainability of agricultural production systems. Under field conditions, symbioses are established both with indigenous and cointroduced rhizobia strains (Muñoz et al., Reference Muñoz, Ibañez, Tonelli, Valetti, Anzuay and Fabra2011; Bouznif et al., Reference Bouznif, Guefrachi, Rodríguez de la Vega, Hungria, Mars, Alunni and Shykoff2019) and may result in abundant nodulation (Castro et al., Reference Castro, Permigiani, Vinocur and Fabra1999; Lanier et al., Reference Lanier, Jordan, Spears, Wells and Johnson2005; Torres-Júnior et al., Reference Torres-Júnior, Leite, Santos, Fernandes Junior, Zilli, Rumjanek and Xavier2014; Asante et al., Reference Asante, Ahiabor and Atakora2020; Jovino et al., Reference Jovino, da Silva, Rodrigues, de Sá Carvalho, Cunh, de Lima, dos Santos, Santos, Ribeiro, de Freitas, Martins and Fernandes-Júnior2022).
Bradyrhizobium spp. have been recognized as the most representative symbiotic rhizobial species for groundnuts (Bouznif et al., Reference Bouznif, Guefrachi, Rodríguez de la Vega, Hungria, Mars, Alunni and Shykoff2019); however, variable results have been reported. In the USA, inoculation was successful at increasing yields in only 7 out of 20 experiments in which groundnuts had not been previously grown (Lanier et al., Reference Lanier, Jordan, Spears, Wells and Johnson2005). In Argentina, inoculation of selected strains did not improve yields in comparison to those in noninoculated fields, and BNF by indigenous rhizobia allowed maximal yields to be reached (Bogino et al., Reference Bogino, Banchio, Rinaudi, Cerioni, Bonfiglio and Giordano2006). For these examples and others elsewhere, the variable responses of the inoculation of groundnut with Bradyrhizobium could be attributed to the variable environmental conditions in which the crop was grown (Castro et al., Reference Castro, Permigiani, Vinocur and Fabra1999; Lanier et al., Reference Lanier, Jordan, Spears, Wells and Johnson2005; Bogino et al., Reference Bogino, Banchio, Rinaudi, Cerioni, Bonfiglio and Giordano2006; Torres-Júnior et al., Reference Torres-Júnior, Leite, Santos, Fernandes Junior, Zilli, Rumjanek and Xavier2014; Asante et al., Reference Asante, Ahiabor and Atakora2020; Jovino et al., Reference Jovino, da Silva, Rodrigues, de Sá Carvalho, Cunh, de Lima, dos Santos, Santos, Ribeiro, de Freitas, Martins and Fernandes-Júnior2022).
Moreover, eubacteria of the phylum Cyanobacteria have called the attention of producers seeking sustainable development (Sutherland et al., Reference Sutherland, McCauley, Labeeuw, Ray, Kuzhiumparambil, Hall, Doblin, Nguyen and Ralph2021; Taira et al., Reference Taira, Baba, Togashi, Berdiyar, Yashima and Inubushi2021) because they can be applied both for wastewater bioremediation (Araujo et al., Reference Araujo, Santiago, Moreira, Dantas Neto and Fernandes2021; Melo et al., Reference Melo, Telles, Ribeiro, de Carvalho Junior and Andrade2022) and as agricultural inoculants, biofertilizers (Gavilanes et al., Reference Gavilanes, Andrade, Zucareli, Horácio, Yunes, Barbosa, Ribeiro Alves, Cruzatti, Maddela and de Fátima Guimarães2020; Horácio et al., Reference Horácio, Zucareli, Gavilanes, Yunes, Sanzov and Andrade2020; Supraja et al., Reference Supraja, Behera and Balasubramanian2020), or foliar fertilizers (Amatussi et al., Reference Amatussi, Mógor, Cordeiroi, Mógor, Marques and de Larai2023). Cyanobacteria were the first organisms to evolve photosynthesis, and some species, such as Anabaena sp. and Nostoc sp., fix N2. Additionally, they have been used as coinoculants with Azospirillum brasilense in maize (Gavilanes et al., Reference Gavilanes, Andrade, Zucareli, Horácio, Yunes, Barbosa, Ribeiro Alves, Cruzatti, Maddela and de Fátima Guimarães2020) and with a mix of Rhizobium tropici and R. freirei plus A. brasilense in common bean (Horácio et al., Reference Horácio, Zucareli, Gavilanes, Yunes, Sanzov and Andrade2020). However, other cyanobacteria do not fix N but may contribute to crop growth via similar mechanisms to those of the most commonly known plant growth-promoting bacteria (Singh et al., Reference Singh, Parihar, Singh, Bajguz, Kumar, Singh, Singh and Prasad2017; Gavilanes et al., Reference Gavilanes, Amaral, García, Araujo-Junior, Zanão Júnior, Nomura, Andrade, Maddela, García Cruzatty and Chakraborty2021).
In our study, two species of cyanobacteria were chosen based on previous work by our group that used groundnut in a field experiment (Andrade et al., Reference Andrade, Colozzi Filho and Gatti2014). The first, Arthrospira (Spirulina) platensis, is a filamentous cyanobacterium that grows under photoautotrophic conditions and does not fix N. When inoculated in Phaseolus aureus or P. mungo seeds (Bhowmik et al., Reference Bhowmik, Dubey and Mehra2010) or applied as a biofertilizer in Begonia semperflorens (Jowkar et al., Reference Jowkar, Bashiri and Golmakani2017), rice (Dineshkumar et al., Reference Dineshkumar, Kumaravel, Gopalsamy, Sikder and Sampathkumar2018), tomato (Supraja et al., Reference Supraja, Behera and Balasubramanian2020), or groundnut (Sivalingam, Reference Sivalingam2020), it increases seed germination and plant growth. The second species, Synechocystis sp., is also a unicellular nonnitrogen-fixing cyanobacterium that primarily inhabits water environments and soil and has been characterized by elevated production of phytohormones when inoculated in the rhizosphere of wheat (Khurshid et al., Reference Khurshid, Zahid and Husnain2017).
To confirm whether microbial inoculants may improve groundnut performance, growth, and grain yield due to their beneficial effects, we carried out field experiments to compare N fertilizer with single inoculation and with double inoculation of B. arachidis with cyanobacteria.
Materials and Methods
Microbial strains
The B. arachidis strain (SEMIA6144, =SM400, =USDA3187, =MAR11), which is used in commercial inoculants in Brazil, was provided by the Microbiological Resource Center, Porto Alegre, Brazil (FEPAGRO). The cyanobacteria were obtained from the Microalgae IPR Collection of the Instituto de Desenvolvimento Rural do Paraná - IAPAR-EMATER (IDR-Paraná). Arthrospira (Spirulina) platensis (IPR7059) was kindly provided by Dr. Iracema de Oliveira Moraes (Fundação Andre Tosello, Brazil), and Synechocystis sp. (IPR7061) was isolated from freshwater in Paraná state, Brazil (Andrade et al., Reference Andrade, Colozzi Filho and Gatti2014).
Experimental site description and design
Field experiments were performed during the summer season of 2011–2012 at experimental stations in the IDR-Paraná under four different edaphic–climate conditions (Supplementary Material Figure S1 available online at https://doi.org/10.1017/S0014479723000285 and Table 1) using two sandy soils (Paranavaí and Umuarama), a sandy-clayey–loamy soil (Ponta Grossa), and a clayey soil (Londrina), which are classified as Paleudult (udic Ultisol), Rhodic Haploperox (perudic Oxisol), and Rhodudult (udic Ultisol), respectively (Soil Survey Staff, Reference Soil Survey Staff2014). The rainfall and temperature during the crop growing season at each experimental site are presented in Supplementary Material Figure S2a–d available online at https://doi.org/10.1017/S0014479723000285.
Table 1. Soil physicochemical properties and most probable number (MPN) groundnut-nodulating of rhizobia (cells g of dry soil-1) of soils and locations of the experimental fields. Soil sampling is done before sowing at top layer of 0 to 0.10 m. Values are on an oven-dried soil mass basis
Soil chemical analyses were performed according to Pavan et al. (Reference Pavan, Bloch, Zempulski, Miyazawa and Zocoler1992), and the soil total N content was calculated (Andrade et al., Reference Andrade, Leal, Ramos and de Goes2015) by using Equation 1:
$$N = C\left( {1.724/20} \right)$$
where N is the soil’s total nitrogen and C is the soil’s total carbon.
The most probable number (MPN) counts of fresh soil samples were performed using groundnut surface disinfected seeds pregerminated before planting in sterilized Leonard jars containing sterilized sand and vermiculite (3:1) and nutrient solution without N.
Before groundnut sowing, the soil was fertilized with 300 kg ha-1 0–20–20 fertilizer, which was applied to the sowing furrows. The N fertilizer treatment involved 100 kg ha-1 N (urea), which was split into 20 kg N ha-1 applied in the furrow at sowing and 80 kg N ha-1 top-dressed on the soil surface at 25–30 d after emergence.
The treatments used were as follows: (i) control (without inoculation and N); (ii) N fertilizer with 100 kg ha-1 N; (iii) B. arachidis (peat); (iv) B. arachidis (fine shale); (v) A. platensis (fine shale); (vi) coinoculation with B. arachidis plus A. platensis (fine shale); (vii) Synechocystis sp. (fine shale); and (viii) coinoculation with B. arachidis plus Synechocystis sp. (fine shale). Inoculation of B. arachidis (SEMIA6144, =SM400, =USDA3187, =MAR11) was achieved by adding inoculant either to peat or fine shale (plus 1% (w/v) of carboxymethylcellulose and polyvinylpyrrolidone) as a solid carrier at a concentration of 109 viable cells g-1. For inoculation, the seeds were moistened with sterile water supplemented with 10% sugar, after which the inoculant was applied to 50 kg of seeds.
A. platensis (IPR7059) and Synechocystis sp. (IPR7061) were grown in BG11 culture medium (Rippka et al., Reference Rippka, Deruelles, Waterbury, Herdman and Stanier1979) for 30 d using a 12-h light phase (temperature of 28.0 ± 2.0 °C) and a 12-h dark phase (22.0 ± 2.0 °C). Inoculation of the seeds with their respective treatments was performed using a suspension of cells of each of the cyanobacteria with approximately 106 cells mL-1 at a rate of 12 mL kg-1 of seeds. Sowing was performed manually in furrows made by disking, using an average of 60 kg seeds ha-1. For weed control, trifluralin was applied one week before sowing according to the crop recommendations. The experiments were arranged in randomized complete blocks with five replications. Each plot had an area of 4.0 m × 6.0 m (24 m2) and consisted of eight furrows with 6 m in length spaced 0.5 m apart.
Plant samplings
At late flowering (R2 stage), 8–12 plants from each plot were uprooted in the second and seventh rows to assess nodulation (nodule number and dry matter weight), shoot dry matter, and shoot N concentration. Nodulation was evaluated according to procedures described by Cardoso et al. (Reference Cardoso, Gomes, Goes, Fonseca-Junior, Dorigo, Hungria and Andrade2009). Nodules and shoot dry weight were measured after drying at 60 °C for 72 h. For grain yield, the plants were harvested from the four central lines (8 m2), and the results are expressed in kg ha-1 with moisture corrected to 8%.
The shoot N concentration and grain N concentration were analyzed by the Kjeldahl digestion method (Bremner and Keeney, Reference Bremner and Keeney1966), and the shoot N uptake was calculated from the N concentration according to Horácio et al. (Reference Horácio, Zucareli, Gavilanes, Yunes, Sanzov and Andrade2020). Grain N accumulated (kg ha-1) was calculated using grain N concentration multiplied by grain yield from each plot, and the crude protein concentration was calculated by multiplying the N content (%) by a factor of 6.25 (Jones, Reference Jones1931). The shelling percentage (%) was calculated from the seed weight per plant relative to the pod weight multiplied by 100.
Statistical analysis
The data were analyzed for a normal distribution and homogeneity of variance and log-transformed when necessary. The data were analyzed by one-way analysis of variance according to the procedures outlined by Snedecor and Cochran (Reference Snedecor and Cochran1980), and the means were compared by the Scott–Knott test (p = 0.05) using the statistical package SISVAR (Ferreira, Reference Ferreira2011).
Results
Data from the soil chemical and rhizobia MPN analyses of the experimental areas are shown in Table 1. Tables 2–5 present data from the four experiments regarding plant development (dry weight of roots and shoots), shoot N concentration (SNC), nodulation (number and weight of nodules), shells and shelling, and N and grain protein concentration (%) of groundnut.
Table 2. Roots dry weight (RDW), shoot dry weight (SDW), shoot N concentration (SNC) (g kg-1), N uptake (mg N plant-1), nodulation (nodule number and dry weight per plant), shell, shelling, grain N concentration (GNC) (g kg-1), grain N accumulated (GNA) (kg ha-1) and grain protein concentration (GPC) (%) of groundnut (cv. IAC Tatu-ST) inoculated with Bradyrhizobium sp. (SEMIA 6144), Arthrospira platensis (IPR7059) and Synechocystis sp. (IPR7061). Sandy–clayey–Loamy soil (Ponta Grossa, PR). Values are means of five repetitions
ns = not significant (p > 0.05); *Significant at (p < 0.05). Means followed by the same letter, within each column, are not different by Scott-Knott test, p = 0.05. †Urea (100 kg N ha-1); ‡Peat or §Fine shale is used as carrier of microbial inoculants. ¶Cell suspension.
Table 3. Root dry weight (RDW), shoot dry weight (SDW), shoot N concentration (SNC) (g kg-1), N uptake (mg N plant-1), nodulation (nodule number and dry weight per plant), shell, shelling, grain N concentration (GNC) (g kg-1), grain N accumulated (GNA) (kg ha-1), and grain protein concentration (GPC) (%) of groundnut (cv. IAC Tatu-ST) inoculated with Bradyrhizobium sp. (SEMIA6144), Arthrospira platensis (IPR7059) and Synechocystis sp. (IPR7061). Sandy soil (Paranavaí, PR). Values are means of five repetitions
ns = not significant (p > 0.05); *Significant at (p < 0.05). Means followed by the same letter, within each column, are not different by Scott-Knott test, p = 0.05. †Urea (100 kg N ha-1); ‡Peat or §Fine shale is used as carrier of microbial inoculants. ¶Cell suspension.
Table 4. Roots dry weight (RDW), shoot dry weight (SDW), shoot N concentration (SNC) (g kg-1), N uptake (mg N plant-1), nodulation (nodule number and dry weight per plant), shell, shelling, grain N concentration (GNC) (g kg-1), grain N accumulated (GNA) (kg ha-1), and grain protein concentration (GPC) (%) of groundnut (cv. IAC Tatu-ST) inoculated with Bradyrhizobium sp. (SEMIA6144), Arthrospira platensis (IPR7059) and Synechocystis sp. (IPR7061). Sandy soil (Umuarama PR). Values are means of five repetitions
ns = not significant (p > 0.05); *Significant at (p < 0.05), **Significant at (p < 0.01). Means followed by the same letter, within each column, are not different by Scott-Knott test, p = 0.05. †Urea (100 kg N ha-1); ‡Peat or §Fine shale is used as carrier of microbial inoculants. ¶Cell suspension.
Table 5. Roots dry mass (RDW), shoot dry mass (SDW), shoot N concentration (SNC) (g kg-1), N uptake (mg N plant-1), nodulation (nodule number and dry weight per plant), shell, shelling, grain N concentration (GNC) (g kg-1), grain N accumulated (GNA) (kg ha-1), and grain protein concentration (GPC) (%) of groundnut (cv. IAC Tatu-ST) inoculated with Bradyrhizobium sp. (SEMIA6144), Arthrospira platensis (IPR7059) and Synechocystis sp. (IPR7061). Clayey soil (Londrina, PR). Values are means of five repetitions
ns = not significant (p > 0.05); *Significant at (p < 0.05). Means followed by the same letter, within each column, are not different by Scott-Knott test, p = 0.05. †Urea (100 kg N ha-1); ‡Peat or §Fine shale is used as carrier of microbial inoculants. ¶Cell suspension.
Discussion
There were no effects of inoculation treatment or N fertilizer on nodule weight or number in relation to the control, except in the sandy soils (Paranavaí and Umuarama) (Tables 2–5), indicating that naturalized bradyrhizobia was highly competitive in inducing nodule formation in groundnut.
Our nodulation data are in line with previous results obtained elsewhere in Brazil (Santos et al., Reference Santo, da Silva, Silva, Freitas, da Silva, Bezerra, de Lyra and Ferreira2017) and in Argentina (Castro et al., Reference Castro, Permigiani, Vinocur and Fabra1999; Bogino et al., Reference Bogino, Banchio, Rinaudi, Cerioni, Bonfiglio and Giordano2006), the USA (Lanier et al., Reference Lanier, Jordan, Spears, Wells and Johnson2005), and Ghana (Asante et al., Reference Asante, Ahiabor and Atakora2020). Most likely due to the promiscuous nature of groundnut plants and the high competitiveness of native rhizobial strains (Bouznif et al., Reference Bouznif, Guefrachi, Rodríguez de la Vega, Hungria, Mars, Alunni and Shykoff2019), only a few very effective strains stand out for their BNF rates and ability to promote plant growth (Bogino et al., Reference Bogino, Banchio, Rinaudi, Cerioni, Bonfiglio and Giordano2006; Torres-Júnior et al., Reference Torres-Júnior, Leite, Santos, Fernandes Junior, Zilli, Rumjanek and Xavier2014; Jovino et al., Reference Jovino, da Silva, Rodrigues, de Sá Carvalho, Cunh, de Lima, dos Santos, Santos, Ribeiro, de Freitas, Martins and Fernandes-Júnior2022). The literature has noted that groundnut is a very promiscuous microsymbiont, as this crop establishes symbioses with both native and introduced rhizobia at the same rates (Torres-Júnior et al., Reference Torres-Júnior, Leite, Santos, Fernandes Junior, Zilli, Rumjanek and Xavier2014; Li et al., Reference Li, Wang, Zhang, Young, Wang, Sui and Chen2015; Muñoz et al., Reference Muñoz, Ibáñez, Tordable, Megías and Fabra2015; Bouznif et al., Reference Bouznif, Guefrachi, Rodríguez de la Vega, Hungria, Mars, Alunni and Shykoff2019).
Despite initial evidence of plant growth stimulation in other crops, in our study, there was a greater (but not significant) grain yield in the sandy soil (Paranavaí) due to coinoculation with Synechocystis sp. (IPR7061) and only significant effects on nodule weight in sandy soil (Umuarama) due to coinoculation of the B. arachidis strain (SEMIA6144) with A. platensis (IPR7059). In rhizobial inoculation, the stimulus from the cyanobacteria A. platensis (IPR7059) or Synechocystis sp. (IPR7061) or any other plant growth-promoting microorganism may not be the factor limiting the plant growth and grain yield of groundnut.
In the sandy-clayey–loamy soil (Ponta Grossa), there was a response to single inoculation with B. arachidis (SEMIA6144), in which the yield increased by 14%–24%, and to N fertilizer (urea), with a 20% increase, showing that yield was limited by rhizobial inoculation or N fertilizer (Figure 1a). In the clayey soil (Londrina), single inoculation with B. arachidis (SEMIA6144) or Synechocystis sp. (IPR7061) or coinoculation with B. arachidis and A. platensis (IPR7059) increased the amount of N accumulated in the grains, which was greater than that in the noninoculated and N-fertilized soils (Table 5), indicating that BNF is essential for grain yield in these soils (Figure 1d).
Figure 1. Grain yield (kg ha-1) of groundnut (cv. IAC Tatu-ST) seeds inoculated with Bradyrhizobium sp. (SEMIA6144), Arthrospira platensis (IPR7059) and Synechocystis sp. (IPR7061). Cropping seasons 2011/2012. (a) Sandy-clayey–loamy soil (Ponta Grossa), (b) sandy soil (Paranavaí), (c) sandy soil (Umuarama), and (d) clayey soil (Londrina). The bars are the means of five repetitions, and the same letter indicates that the data were not significantly different according to the Scott–Knott test (p = 0.05). The N fertilizer (100 kg N ha-1) was urea.
Coinoculation of rhizobia with other microorganisms has often been suggested as an additive strategy for improving rhizobial symbiosis and BNF in grain legumes due to complementary mechanisms of plant growth promotion (Ahemad and Kibret, Reference Ahemad and Kibret2014; Gavilanes et al., Reference Gavilanes, Andrade, Zucareli, Horácio, Yunes, Barbosa, Ribeiro Alves, Cruzatti, Maddela and de Fátima Guimarães2020). There have been several successful examples of coinoculation of groundnut with Bradyrhizobium and other collaborative microorganisms, e.g., Azospirillum brasilense (Gericó et al., Reference Gericó, Tavanti, de Oliveira, Lourenzani, de Lima, Ribeiro, dos Santos and dos Reis2020), Bacillus spp. (Figueredo et al., Reference Figueredo, Tonelli, Taurian, Angelini, Ibanez, Valetti, Munoz, Anzuay, Ludueña and Fabra2014; Preyanga et al., Reference Preyanga, Anandham, Krishnamoorthy, Senthilkumar, Gopal, Vellaikumar and Meena2021; Kaschuk et al., Reference Kaschuk, Auler, Vieira, Dakora, Jaiswal and da Cruz2022), Serratia marcescens, and Trichoderma harzianum (Badawi et al., Reference Badawi, Biomy and Desoky2011). The benefits of coinoculation may be direct on nodule formation or indirect; for example, a meta-analysis revealed that coinoculation of bradyrhizobia and Bacillus spp. significantly increases the sizes of roots and shoots of groundnut (Kaschuk et al., Reference Kaschuk, Auler, Vieira, Dakora, Jaiswal and da Cruz2022).
Among grain legumes, groundnut, and common bean are the crops with the highest gap in BNF to meet their N demands (Palmero et al., Reference Palmero, Fernandez, Garcia, Haro, Prasad, Salvagiotti and Ciampitti2022), whereas crops such as soybean promptly respond to inoculation with elite strains and obtain sufficient N2 fixation to satisfy their N demands (Kaschuk et al., Reference Kaschuk, Nogueira, de Luca and Hungria2016).
Studies on the introduction of efficient Bradyrhizobium strains have shown that it is a limiting factor for nodule efficiency and plant growth in groundnut. Inoculation with B. arachidis (SEMIA6144) resulted a greater grain yield compared to the control, similar to the effects of both N fertilizer (urea) and selected efficient Rhizobium strains isolated from Brazilian soils, for instance, the 23M isolate (Santos et al., Reference Santo, da Silva, Silva, Freitas, da Silva, Bezerra, de Lyra and Ferreira2017) and the Bradyrhizobium sp. strain ESA123 (Jovino et al., Reference Jovino, da Silva, Rodrigues, de Sá Carvalho, Cunh, de Lima, dos Santos, Santos, Ribeiro, de Freitas, Martins and Fernandes-Júnior2022).
In our study, the cyanobacteria IPR7059 and IPR7061 were chosen to be coinoculated with B. arachidis (SEMIA6144) because these genera are related to plant growth in other crops (Bhowmik et al., Reference Bhowmik, Dubey and Mehra2010; Khurshid et al., Reference Khurshid, Zahid and Husnain2017; Sivalingam, Reference Sivalingam2020; Supraja et al., Reference Supraja, Behera and Balasubramanian2020). According to Liebig’s law of the minimum, crop yield is limited by the lack of one or more factors that are present in lesser quantities. Usually, N is one of the most limiting factors for grain legume crops. However, under the conditions of our experiments, nodulation and BNF were not the limiting factors for groundnut yield. Moreover, interestingly, indicators of plant growth (root and shoot dry weights, shoot N concentration (SNC), shells, and shelling) hardly changed because of the inoculation treatments, but their averages varied among the different locations. In fact, studies have indicated that the performance of rhizobial symbioses in groundnut is influenced by cultivar, rhizobial strain, soil physical and chemical attributes, land use history, and weather conditions (e.g., Lanier et al. Reference Lanier, Jordan, Spears, Wells and Johnson2005, Bogino et al. Reference Bogino, Banchio, Rinaudi, Cerioni, Bonfiglio and Giordano2006, Santos et al. 2008, Torres-Junior et al. 2014, Santos et al. Reference Santo, da Silva, Silva, Freitas, da Silva, Bezerra, de Lyra and Ferreira2017, Asante et al. Reference Asante, Ahiabor and Atakora2020).
Considering the different locations (soil and climate), the average yield of groundnut was much lower in the sandy-clayey–loamy soil (Ponta Grossa) (1852 kg ha-1) and in the sandy soil (Paranavaí) (1328 kg ha-1) (Figure 1a and b) and were comparable to the values in lesser productive areas in the world, which includes the national average of Brazil (FAO, 2021). However, the grain yield in the other sandy soil (Umuarama) was 3819 kg ha-1, and that in the clayey soil (Londrina) was 3657 kg ha-1 (Figure 1c and d); these values are comparable to the average yields of the most productive countries, for example, Argentina, with 3455 kg ha-1 in 2019 (FAO, 2021), and Brazil (Moda-Cirino et al., Reference Moda-Cirino, Ribeiro, Buratto, Sou and Jr2015; CONAB, 2023).
The lower grain yields in Paranavaí and Ponta Grossa than in Londrina and Umuarama may be due to the lower amount of rainfall during the stages in which the plants needed water for development (Supplementary Material Figure S2a–d available online at https://doi.org/10.1017/S0014479723000285). Additionally, when analyzing the soil chemical characteristics of the field experiments, it was found that the two locations with higher grain yields (Umuarama and Londrina) had greater soil phosphorus (P) availability than did Paranavaí and Ponta Grossa (Table 1). In these two field experiments, lower soil P content should be highlighted as a possible cause of lower groundnut yields since it is an important factor for symbiosis. In our experiments, these findings provide evidence that soil P availability may be limiting groundnut.
Overall, the naturalized rhizobial population capable of nodulating groundnut, which ranged from 1.12 to 9.8 × 103 per g of soil, did not appear to influence the response to inoculation with bradyrhizobia.
In Londrina, where the soil P supply was greater (Table 1), the inoculation of B. arachidis (SEMIA6144) increased yields in comparison to those in the noninoculated and N-fertilized treatments (Figure 3d). This result suggested that P limitation is a possible explanation for the slight effects of rhizobial inoculation, although the experiment was not designed to test this hypothesis. Indeed, in Ghana, the influence of P fertilization on groundnut yields was demonstrated by Asante et al. (Reference Asante, Ahiabor and Atakora2020) using the USDA3456 and BR3267 strains and by Yaro et al. (Reference Yaro, Mahama, Kugbe and Berdjour2021) using the BR3267 strain.
Hence, we conclude that coinoculation of B. arachidis with the cyanobacteria A. platensis (IPR7059) or Synechocystis sp. (IPR7061) overcame the limitations for increasing nodulation, BNF, and grain yield in groundnut in only one out of four field experiments, where the weather and soil P content conditions were better for the crop. The success of groundnut inoculation depends on the interaction of soil factors (e.g., P) and weather conditions. Subsequent studies of rhizobial inoculation should include P fertilization and consortia with microorganisms that increase the availability of P (Taurian et al., Reference Taurian, Anzuay, Ludueña, Angelini, Muñoz, Valetti and Fabra2013). In our study, the lack of response of groundnut to N fertilization in increasing grain yield should be highlighted, which indicates the importance of inoculation with selected strains of Bradyrhizobium, as this crop has also been included in rotation with sugarcane to improve soil fertility and is also used in the food industry.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0014479723000285
Availability of data and materials
The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.
Acknowledgements
The authors are grateful to Maria Aparecida de Matos, Paulo S. Aguilar, Alisson W. Sanzovo, Aretusa D. R. Mendes, and Cezar F. Araujo Junior. M.H. and D.S. Andrade (315060/2020-4) acknowledge the research fellowships received from CNPq.
Authors’ contributions
All the authors contributed to the study’s conception and design. Material preparation, data collection, and analysis were performed by DSA and GML. The first draft of the manuscript was written by DSA, GK, and MH. All the authors commented on previous versions of the manuscript. All the authors read and approved the final manuscript.
Financial support
This study was partially supported by Petrobras SIX, São Mateus do Sul, PR – Agricultural Shale Project at Instituto Agronômico do Paraná (IAPAR) and by the CNPq (National Council for Scientific and Technological Development, Brazil), Project INCT-CNPq (MPCPAgro 465133/2014-2) and Fundação Araucária (SETI-043/2019).
Competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
