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Article

Effects of Inoculating the Diazotrophic Endophyte Bradyrhizobium sp. AT1 on Different Cultivars of Sweet Potato (Ipomoea batatas [L.] Lam.)

by
Junko Terakado-Tonooka
1,2,*,
Fukuyo Tanaka
3,
Toshihiko Karasawa
1,
Akihiro Suzuki
4 and
Yoshinari Ohwaki
1
1
Central Region Agricultural Research Center, National Agriculture and Food Research Organization (NARO), 2-1-18 Kannondai, Tsukuba 305-8666, Ibaraki, Japan
2
Japan Society for the Promotion of Science (JSPS), Kojimachi Business Center Building, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan
3
Research Center for Advanced Analysis, National Agriculture and Food Research Organization (NARO), 3-1-3 Kannondai, Tsukuba 305-8604, Ibaraki, Japan
4
Faculty of Agriculture, Saga University, Honjo-Machi, Saga 840-8502, Japan
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(4), 963; https://doi.org/10.3390/agronomy13040963
Submission received: 31 December 2022 / Revised: 21 March 2023 / Accepted: 22 March 2023 / Published: 24 March 2023
(This article belongs to the Special Issue Microbial Root Symbionts in Plant Production – Getting to the Bottom)
Browse Figure
Versions Notes

Abstract

:
Owing to the worldwide shortage of nitrogen (N) fertilizers, diazotrophic endophytes have received increasing attention as biofertilizers. In this study, we investigated the inoculation effects of a diazotrophic endophyte (Bradyrhizobium sp. AT1) on three different cultivars of sweet potato (cvs. Beniazuma, Ayamurasaki, and Kokei No. 14) under pot, container, and different field conditions. Following inoculation, the root length was increased in cvs. Beniazuma and Ayamurasaki but suppressed in cv. Kokei No. 14 in pots, filled with a mixture of vermiculite, potting soil, and pearlite. AT1 inoculation also increased shoot growth in cv. Beniazuma and tuber formation in cv. Ayamurasaki in containers filled with vermiculite, potting soil, and light-colored Andosol. In field experiments, carried out at two field sites with the three cultivars, AT1 inoculation increased the growth of cvs. Beniazuma and Ayamurasaki, but it had almost no effect on cv. Kokei No. 14. In addition to growth promotion, inoculation of micropropagated sweet potato cv. Beniazuma with AT1 led to N derived from air (Ndfa) and acetylene reduction activity (ARA) five months after inoculation. Our studies indicate that AT1 inoculation can enhance the growth of sweet potato and promote N2 fixation.

1. Introduction

Sweet potato (Ipomoea batatas [L.] Lam.) is grown globally with stable yields owing to the crop’s great adaptability. This plant is known as an emergency crop that can grow in infertile N-deficient soils and with little chemical fertilizer [1,2]. It is also reported that the N balance of the soil after sweet potato cultivation cannot account for the total N uptake of the crop [1,2], suggesting that some N may be derived from the air via biological N2 fixation.
Many diazotrophic endophytes have been isolated from field-grown sweet potato for use in sustainable agriculture [3,4,5,6,7,8]. Using information regarding the expression of bacterial nitrogenase reductase (nifH) genes [9], we previously isolated Bradyrhizobium sp. AT1 from sweet potato cv. Ayamurasaki as a N2-fixing endophyte [8]. AT1 fixed N2 inside plants and increased the fresh weight of micropropagated sweet potato plants (cv. Beniazuma). Although N has been suggested to play a crucial role in tuber development in sweet potato [10], effects of N2-fixing endophytes on the yield of sweet potato cannot be studied in small pots. To utilize this endophytic bacterium as a biofertilizer in practice, investigations of its effects on different plant cultivars under field conditions are needed.
In this study, we investigated the effects of AT1 inoculation on three different cultivars of sweet potato (cvs. Beniazuma, Ayamurasaki, and Kokei No. 14) under field, pot, and container cultivation conditions. The contribution of AT1 to the growth and yield of sweet potato is discussed. The present information might help establish techniques for using N2-fixing endophytes to enhance the production of sweet potato while reducing the input of chemical N fertilizer.

2. Materials and Methods

2.1. Bacterial Strain and Culture Conditions

For inoculation, Bradyrhizobium sp. AT1 [8,11] was cultured in YM medium at 28 °C with continuous shaking for 5 days. The cells were harvested by centrifugation at 5000 rpm for 10 min, when the turbidity (A660) reached approximately 0.6, washed twice with sterile distilled water, and suspended in sterile Hoagland N-free solution or distilled water (1 × 108 cells mL−1).

2.2. Colonization by Bradyrhizobium AT1-SR in Three Cultivars of Sweet Potato

To ascertain the infection rate of AT1 in three sweet potato cultivars (cvs. Beniazuma, Ayamurasaki, and Kokei No. 14), AT1-SR, a strain resistant to streptomycin and rifampicin, was selected. It was isolated on yeast mannitol agar (YMA) plates [12] containing 100 ppm streptomycin at 28 °C for 7 days. Then, the bacterial colonies were inoculated on YMA plates containing 100 ppm streptomycin and rifampicin at 28 °C for 7 days. After the selection of AT1-SR, its growth rate in YM medium was determined (Figure S1).
Apical stem cuttings were soaked in bacterial suspensions of AT1-SR (1 × 108 cells mL−1) or distilled water overnight. The inoculated and control plants were planted in a glass bottle (58 mm in diameter, 450 mL) filled with sterilized vermiculite and 80 mL of Hoagland N-free solution supplemented with 1 mmol KNO3 per plant. The plants were fertilized with Hoagland N-free solution and grown in a growth chamber programmed with a 16 h/8 h light/dark photoperiod and 28 °C/25 °C day/night temperature cycle.
Fourteen days after inoculation, roots and shoots (3 cm above the cut end) were harvested (four replicates per plant), and bacterial colonies were counted. The tissue samples of each plant were surface-sterilized with 0.2% NaOCl for 3 min, rinsed four times with sterile distilled water, and then ground in a sterilized mortar with a pestle in 20 mM phosphate buffer (pH 6.8). The homogenate was serially diluted and plated on YMA containing 100 ppm of streptomycin and rifampicin, each. Colonies were counted after 10 days of inoculation at 28 °C.

2.3. Growth Experiment in Pots and Containers

Apical stem cuttings of three sweet potato cultivars (cvs. Beniazuma, Ayamurasaki, and Kokei No. 14) were soaked in bacterial suspensions of AT1 or distilled water overnight and planted in pots (90 mm in diameter, 360 mL; one plant per pot) filled with vermiculite: Napura potting soil (YANMAR Co., Ltd., Osaka, Japan): pearlite, (2:1:3, v/v/v) and containers (55 × 30 × 32 cm3, 25 L; six plants per container) filled with vermiculite: Napura potting soil: light-colored Andosol (2:1:3, v/v/v). Andosol was collected from the experimental field of NARO (36°02′ N, 140°10′ E, Tsukuba, Japan). The applied fertilizer in Napura potting soil contained N (15 mg plant−1), P (150 mg plant−1), and K (30 mg plant−1). The plants were grown outdoors (NARO) and watered periodically with tap water from June to October in 2021. After 4 months, the plants were harvested (six replicates/group) and separated into shoots, roots, and storage roots. The root morphologies (total root length, root tips, and root forks) of pot-grown sweet potato plants were measured (WinRHIZO, Regent Instruments Inc., Quebec, Canada). Samples were dried at 80 °C for 48 h in a forced-air oven and weighed.

2.4. Growth Experiments in the Field

Field experiments were performed at NARO (36°02′ N, 140°11′ E, Tsukuba, Japan; average annual temperature 14 °C, average annual precipitation 1396 mm) in 2012 and Saga University (33°14′ N, 130°17′ E, Saga, Japan; average annual temperature 17.5 °C, average annual precipitation 2253 mm) in 2014 from June to October. The applied fertilizers were ammonium sulphate, superphosphate, and potassium sulphate at the rate of N (40 kg ha−1), P (40 kg ha−1), and K (40 kg ha−1) in three separate plots (3 × 5 m2 concrete frames filled with gray lowland soil; total N content = 0.13%) at NARO and N (0 kg ha−1), P (40 kg ha−1), and K (40 kg ha−1) in two separate plots (3 × 14.5 m2 concrete frames filled with fine gray lowland soil; total N content = 0.17%) at Saga University.
Apical stem cuttings of the cultivars Beniazuma, Ayamurasaki, and Kokei No. 14 were soaked in suspensions of AT1 or distilled water overnight and planted on ridges (ridge width, 0.6 m; height, 0.2 m; five plants per ridge at 0.3 m intervals).
After 4 months, the plants were harvested (three plants were taken from each ridge) and dissected into vines and storage roots. Samples were dried at 80 °C for 48 h in a forced-air oven and weighed. The N concentration was measured by ANCA-SL in the single N mode.

2.5. Inoculation of Micropropagated Sweet Potato with AT1 for Evaluation of N2 Fixation

Micropropagated sweet potato (cv. Beniazuma) plants [8] with 3–4 leaves were transplanted into glass bottles (58 mm in diameter, 450 mL) containing sterilized vermiculite supplemented with 0.25 mmol KNO3 (2.00 atom % 15N) per plant. Seedlings were inoculated with 1 mL of a cell suspension of AT1 or distilled water (five replicates/treatment). These plants received periodically N-free Hoagland solution in a growth chamber programmed with a 16 h/8 h light/dark photoperiod and 28 °C/25 °C day/night temperature cycle.
Five months after transplantation, the nitrogenase activity of the plants was evaluated using the acetylene reduction assay [13] and 15N dilution techniques [14]. The shoots and roots were harvested and placed in 20 mL vials sealed with silicon septa, and pure acetylene was injected at a final concentration of 10% (v/v). The ethylene concentrations in the vials were measured after 3 days of incubation at 28 °C using a GC-14B gas chromatograph (Shimazu Co., Ltd., Kyoto, Japan) equipped with Porapak N 80–100 mesh and a flame ionization detector connected to a chromatography data computer system. Ethylene evolution from non-inoculated plants treated with acetylene was also assayed.
The shoots and roots were dried at 80 °C for 48 h in a forced-air oven and weighed. The samples were ground to a fine powder, and the N content and 15N atom % were measured by ANCA-SL (SerCon, Ltd., Cheshire, UK) in the single N mode. The contribution of N derived from air (Ndfa) was calculated as follows: %Ndfa = 1 − (15N atom % excess of inoculated plant/15N atom % excess of the control plant) × 100.

2.6. Statistical Analysis

We conducted a Student’s t-test using the Excel statistical software package (Ekuseru-Toukei 2015. Social Survey Research Information Co., Ltd., Tokyo, Japan).

3. Results

3.1. Colonization of the Apical Stem Cuttings of Three Cultivars of Sweet Potato by AT1-SR

The population density of inoculated AT1-SR in the newly formed roots of sweet potato (cvs. Beniazuma, Ayamurasaki, and Kokei No. 14) was in the range of 1 × 103/g fresh weight (Table 1), and no differences were observed among the cultivars. AT1-SR colonies were not detected in the shoots (3 cm above the cut end) of inoculated plants after 14 days of inoculation.
We confirmed acetylene reduction activity (ARA) and population density of AT1 in tissue extracts (Table S1). Production of phytohormones (cytokinin, abscisic acid, auxin and gibberellin) by strain AT1 was low or undetectable under the culture conditions (Table S2).

3.2. Growth Experiments in Pots and Containers

In pots filled with a mixture of vermiculite, Napura potting soil, and pearlite, the total root length of the inoculated plants was higher in cv. Beniazuma but lower in cv. Kokei No. 14 4 months after transplanting (Table 2). Tuberous roots were not produced under this condition. The number of root tips was not altered by the inoculation, whereas the number of root forks (number of root bifurcations) was decreased in cv. Kokei No. 14. The dry weight of shoots and roots was decreased in cv. Kokei No. 14 by AT1 inoculation (Table 2). The root length was also affected by AT1 infection irrespectively of root diameter (Figure 1).
In containers filled with a mixture of vermiculite, Napura potting soil, and light-colored Andosol, the shoot dry weight of cv. Beniazuma was significantly increased after 4 months of AT1 inoculation. The dry weights of roots and tuberous roots in cv. Ayamurasaki were also increased by AT1 inoculation. However, these positive effects were not observed in Kokei No. 14 even though its root and tuber yield were similar to those of cv. Ayamurasaki in both control and AT1-inoculated plants (Table 3).

3.3. Growth Experiments in the Field

Under field conditions, AT1 inoculation increased the dry weight of vines and tubers and the N content of vines in cv. Ayamurasaki under 40 kg ha−1 N fertilization in Tsukuba (Table 4). AT1 inoculation also increased the dry weight and N content of tubers in cvs. Beniazuma and Ayamurasaki in Saga without N fertilization (Table 4). However, the dry weight and N content of cv. Kokei No. 14 were not increased by AT1 inoculation compared to the non-inoculated plants at either location even though its tuber yield was higher than those of cvs. Beniazuma and Ayamurasaki (Table 4).

3.4. Growth, ARA, and 15N Dilution Analysis of Micropropagated Sweet Potato

After 5 months of AT1 inoculation, shoot and root dry weights were higher than for non-inoculated plants (Table 5). The N content of inoculated plants was also higher than that of non-inoculated plants (Table 5).
The 15N isotope abundance in the shoots and roots of inoculated plants was significantly lower than that of non-inoculated plants, and the shoots and roots may have received 14.2% and 13.8% Ndfa, respectively (Table 5).
Five months after inoculation, roots reduced more acetylene than when not inoculated (Table 6). We also confirmed the O2 concentration in the roots of sweet potato, which was less than 5% in some locations (Figure S2). Acetylene reduction activity was not observed in the shoots.

4. Discussion

4.1. Growth Stimulatory Effects of AT1 on Different Sweet Potato Cultivars

The utilization of endophytic bacteria as biofertilizers is promising for sustainable agriculture. In the present study, we used Bradyrhizobium sp. AT1, which was isolated from the tuberous roots of sweet potato, as a diazotrophic endophyte and evaluated its effects on host plant growth. Plant growth promoting bacteria (PGPB) stimulate plants by various means such as N2 fixation, nutrient solubilization, siderophore production, and the synthesis of plant growth regulators [15].
AT1 inoculation shows promising effects on the growth of sweet potato plants, but effects of growth stimulation vary among the cultivars. In the present study, growth stimulation by AT1 inoculation was observed in cvs. Beniazuma and Ayamurasaki in pots, containers, or fields located in geographically distinct regions. Growth stimulation induced by AT1 inoculation might partly be attributable to N2 fixation, as we observed ARA in the roots of cv. Beniazuma and an increased N accumulation in cvs. Ayamurasaki and Beniazuma. In sweet potato, N deficiency can occur in the late stage of cultivation because the amount of applied basal fertilizer often fails to meet the demand. N2 fixed by endophytic bacteria can be utilized by the host plant and potentially used for growth under growth-limiting N conditions [16,17,18]. Contrarily, we did not observe any growth stimulation in cv. Kokei No. 14 following AT1 inoculation under the present conditions.
Under the employed pot conditions, cv. Kokei No. 14 produced longer roots and a higher number of forks than cvs. Beniazuma and Ayamurasaki when not inoculated. Long, well-branched roots are favorable for nutrient absorption, translating into vigorous early growth. It was also reported that the tuberous growth of cv. Kokei No. 14 is faster than that of cv. Ayamurasaki under field conditions [19]. These traits of initial hypertrophy may allow cv. Kokei No. 14 to grow well under N-limited conditions without relying on N2 fixation by endophytes.
Successful colonization of plants by inoculated bacteria is a crucial step, and it has a significant impact on the effectiveness of the endophyte. Inoculated AT1 colonized the roots of three cultivars of sweet potato to similar degrees, suggesting stable associations between sweet potato and AT1. Although we found that the extracts of cv. Kokei No. 14 promoted the ARA of AT1 in vitro, ARA inside cv. Kokei No. 14 has not been confirmed yet. The beneficial function of the inoculated endophyte was probably mediated by the native microbial community, plant genotype, age, and environmental conditions such as climate and soil type [6,7,20,21,22,23,24]. There is a need to investigate the distribution of endophytes and study their interactions with the host plant and their abundance [7,25,26]. In some diazotrophic bacteria, N2 fixation occurs in association with a non-diazotrophic endophyte [27]. We must also consider co-inoculating AT1 with other bacteria when developing and assessing biofertilizers. Application of materials, such as biochar, and adjusting the amounts and forms of applied fertilizer may also improve biological N2 fixation [17,28].

4.2. Stimulation of Tuber Growth by AT1 Inoculation

In the present study, both shoot growth and tuber development were stimulated by AT1 inoculation in cvs. Beniazuma and Ayamurasaki in pots, containers, and under different field conditions. The sweet potato plant has a complex root system composed of fibrous roots, pencil roots, and storage roots. Under ideal growth conditions, adventitious roots become storage roots. It is suggested that stimulation of adventitious root development and elongation at the initial stage is important for storage root production in sweet potato [29,30]. Storage root growth has been demonstrated to be affected by several environmental factors, including soil moisture, temperature, light, photoperiod, carbon dioxide, and N application [31,32,33,34,35,36,37]. However, unlike cassava (Manihot esculenta), the effects of endophytes on tuber development have not been examined in sweet potato [38].
Stimulation of tuber growth may occur via increased photosynthesis, although this effect does not always correspond with shoot growth induced by AT1 inoculation. Many studies demonstrated that the development of storage roots is controlled by endogenous phytohormones, including cytokinin [39,40,41,42,43,44,45,46], abscisic acid [41,44,45,47], auxin [44,45,48,49], and gibberellin [44,45,47,50], depending on the concentration.
The production of phytohormones is also reported as a mechanism for PGPB-induced growth promotion in plants [51]. The phytohormone levels (cytokinin, abscisic acid, auxin, and gibberellin) in strain AT1 were low or undetectable under our culture conditions. Although some rhizobia synthesized phytohormones [52], our results do not imply that AT1 produces phytohormones, at least not actively compared to phytohormone-producing bacteria [53,54,55,56]. Nevertheless, other possible mechanisms by which AT1 stimulates the expression of genes related to phytohormones and/or other factors inducing tuberous root formation are worth investigating.

4.3. Biological N2 Fixation Inside Sweet Potato Roots upon AT1 Inoculation

In the present study, growth promotion and the contribution of N2 fixation to plant N nutrition were quantified for micropropagated sweet potato 5 months after AT1 inoculation, demonstrating the long-lasting beneficial effects of the diazotrophic endophyte. The limits to endophytic N2 fixation in vivo are not well understood, although some experiments have indicated the availability of energy [57,58,59,60] and/or oxygen as limiting factors [8,60]. We previously reported ARA of strain AT1 cultured in sweet potato extracts under microaerobic conditions (<5% O2) [8]. In this experiment, we also measured the O2 concentration in the roots of sweet potato, which was less than 5% in some locations. The O2 concentration in the roots of sweet potato is thus favorable for biological N2 fixation. In the next step, we must elucidate the interaction between the plant and diazotrophic endophyte in the root tissue because local N availability affects the initiation of storage roots through lateral root proliferation [10,29].
Although additional studies are needed, this study suggests that AT1 has the potential to stimulate the growth and increase the yield of tuberous roots in a cultivar-dependent manner that may be supported by N2 fixation inside the roots. Tuber crops are a globally important source of carbohydrates; however, less attention has been paid to improving the agronomic traits of these crops compared with cereals. Our results showed stimulation of tuber yield of sweet potato by inoculation of an N2-fixing endophyte. The use of N2-fixing endophytes as biofertilizer would help establish the techniques to increase the yield of tuber crops while reducing chemical N fertilizer application and contributing to sustainable agricultural production. Furthermore, the clarification of the unexplored mechanisms of stimulation of tuberous root development by N2-fixing endophytes would shed light on complex interactions between plants and microbes belowground. Further studies are warranted to clarify the mechanism of interaction between endophytic bacteria and sweet potato, especially focused on tuberous root initiation and development.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy13040963/s1, Figure S1: Growth curves of Bradyrhizobium sp. AT1 and Bradyrhizobium AT1-SR. Table S1: Acetylene reduction activity and population density of Bradyrhizobium sp. AT1 in extracts of sweet potato (leaves, stems, and storage roots). Table S2: Quantification of phytohormones in bacterial cells or culture medium by LC/MS/MS. Figure S2: O2 levels across roots of sweet potato plants.

Author Contributions

J.T.-T. and Y.O. conceptualized the study and designed the experiments. J.T.-T., F.T. and Y.O. performed the experiments; J.T.-T. wrote the original manuscript; F.T., T.K., A.S. and Y.O. edited the manuscript and provided intellectual advice for the research. J.T.-T. and Y.O. acquired the funding. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by a Grant-in-Aid from the Japan Society for the Promotion of Science (JSPS) Fellows (20J40207 to J.T.) and Grant-in-Aid for Scientific Research (C) from JSPS (26450487 to Y.O.).

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful to Tadakatsu Yoneyama (University of Tokyo, Tokyo, Japan) for fruitful discussion and constructive comments. We also appreciate Toshikazu Kuranouchi, Yasuhiro Takahata, Kazunori Taguchi, and Mio Nishinaka (NARO) for supplying sweet potato lines and Junko Ishikawa and Satoshi Ohno (NARO) for supporting the analysis. We would like to thank Genebank, NARO for providing Bradyrhizobium sp. Th46A (MAFF210138), and B. diazoefficiens USDA110 (MAFF 210161) and the laboratory of Crop Science, Faculty of Agriculture, at Saga University for helping us with the field experiment. We also thank Shinichi Tokuda (NARO) for technical advice and Yumiko Sasaki and Setsuko Tanaka (NARO) for technical assistance.

Conflicts of Interest

The authors declare no conflict of interest.

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Agronomy 13 00963 g001 550
Figure 1. Effect of Bradyrhizobium sp. AT1 inoculation on root length of three cultivars of sweet potato. The length is shown separately for different diameter classes. * Significant differences between the inoculation treatments at the 95% confidence level as determined by Student’s t-test, n = 6.
Figure 1. Effect of Bradyrhizobium sp. AT1 inoculation on root length of three cultivars of sweet potato. The length is shown separately for different diameter classes. * Significant differences between the inoculation treatments at the 95% confidence level as determined by Student’s t-test, n = 6.
Agronomy 13 00963 g001
Table
Table 1. Population density of Bradyrhizobium AT1-SR in the three cultivars of sweet potato 2 weeks after inoculation.
Table 1. Population density of Bradyrhizobium AT1-SR in the three cultivars of sweet potato 2 weeks after inoculation.
SiteNumber of Cells (1 × 103 CFU g fw−1)
BeniazumaAyamurasakiKokei No. 14
ShootsN.D.N.D. N.D.
Roots1.6 ± 0.23.3 ± 0.74.1 ± 0.7
Statistical analysis was performed by Student’s t-test, n = 4; fw, fresh weight; N.D., Not detected.
Table
Table 2. Effects of Bradyrhizobium sp. AT1 inoculation on root development and shoot growth of three cultivars of sweet potato grown in pots *.
Table 2. Effects of Bradyrhizobium sp. AT1 inoculation on root development and shoot growth of three cultivars of sweet potato grown in pots *.
TreatmentSweet Potato Cultivar
BeniazumaAyamurasakiKokei No. 14
Root length (cm)Control1466 ± 1171376 ± 832039 ± 199
AT11781 ± 156 **1413 ± 1811379 ± 263 **
Root tips (number) Control4388 ± 3482545 ± 1753319 ± 714
AT14825 ± 3812600 ± 2893129 ± 369
Root forks (number)Control9322 ± 11959169 ± 103813,776 ± 2149
AT111,470 ± 14579808 ± 20108169 ± 1702 **
Shoots (g dw)Control4.69 ± 0.424.62 ± 0.284.45 ± 0.76
AT14.20 ± 0.843.80 ± 0.442.12 ± 0.24 **
Roots (g dw)Control0.40 ± 0.060.26 ± 0050.37 ± 0.18
AT10.43 ± 0.080.24 ± 0.030.24 ± 0.05 **
* Pots were filled with vermiculite, Napura potting soil, and pearlite. ** Significant differences between the inoculation treatments at the 95% confidence level as determined by Student’s t-test, n = 6; dw, dry weight.
Table
Table 3. Effect of Bradyrhizobium sp. AT1 inoculation on total plant dry weight of three cultivars of sweet potato grown in containers *.
Table 3. Effect of Bradyrhizobium sp. AT1 inoculation on total plant dry weight of three cultivars of sweet potato grown in containers *.
TreatmentDry Weight (g Plant−1)
BeniazumaAyamurasakiKokei No. 14
ShootsControl4.26 ± 0.858.46 ± 0.877.78 ± 1.44
AT1 12.11 ± 1.41 **9.23 ± 1.306.35 ± 0.72
Roots + Tubers Control2.08 ± 0.682.66 ± 1.185.64 ± 1.19
AT12.52 ± 1.15 5.37 ± 1.23 **5.29 ± 1.29
* containers were filled with vermiculite, Napura potting soil, and Andosol. ** Significant differences between the inoculation treatments at the 95% confidence level as determined by Student’s t-test, n = 6.
Table
Table 4. Effects of Bradyrhizobium sp. AT1 inoculation on the growth and N content of field-grown sweet potato.
Table 4. Effects of Bradyrhizobium sp. AT1 inoculation on the growth and N content of field-grown sweet potato.
RegionSiteTreatmentDry Weight (g Plant−1)N (g Plant−1)
BeniazumaAyamurasakiKokei No. 14BeniazumaAyamurasakiKokei No. 14
TsukubaVinesControl623 ± 130321 ± 54474 ± 917.14 ± 1.222.87 ± 0.477.87 ± 2.06
AT1688 ± 97526 ± 58 *664 ± 1818.77 ± 2.557.41 ± 1.31 *10.76 ± 4.52
TubersControl255 ± 47361 ± 29454 ± 731.45 ± 0.292.24 ± 0.292.46 ± 0.23
AT1298 ± 40472 ± 47 *415 ± 441.60 ± 0.572.61 ± 0.422.07 ± 0.61
SagaVinesControl371 ± 42155 ± 23430 ± 687.12 ± 0.73.10 ± 0.615.74 ± 1.07
AT1558 ± 109187 ± 30386 ± 757.68 ± 1.593.21 ± 0.526.26 ± 0.99
TubersControl201 ± 19312 ± 44664 ± 680.76 ± 0.071.94 ± 0.392.79 ± 0.45
AT1511 ± 44 *414 ± 43 *553 ± 841.97 ± 0.24 *2.65 ± 0.33 *2.05 ± 0.21
* Significant differences between the inoculation treatments at the 95% confidence level as determined by Student’s t-test, n = 3.
Table
Table 5. Effects of Bradyrhizobium sp. AT1 inoculation on dry weight and N nutritional parameters of sweet potato (cv. Beniazuma) 5 months after transplantation.
Table 5. Effects of Bradyrhizobium sp. AT1 inoculation on dry weight and N nutritional parameters of sweet potato (cv. Beniazuma) 5 months after transplantation.
SiteTreatmentDry Weight
(g Plant−1)		N (mg Plant−1)15N Atom % Excess% Ndfa **
ShootsControl0.31 ± 0.023.26 ± 0.310.78 ± 0.02-
AT10.38 ± 0.01 *3.42 ± 0.17 *0.67 ± 0.01 *14.2 ± 0.4
RootsControl0.43 ± 0.014.05 ± 0.100.80 ± 0.03-
AT10.56 ± 0.06 *5.23 ± 0.38 *0.69 ± 0.03 *13.8 ± 0.5
* Significant differences between the inoculation treatments at the 95% confidence level as determined by Student’s t-test, n = 5; ** Nfda, nitrogen derived from air.
Table
Table 6. Acetylene reduction activity of sweet potato cv. Beniazuma shoots and roots 5 months after Bradyrhizobium sp. AT1 inoculation.
Table 6. Acetylene reduction activity of sweet potato cv. Beniazuma shoots and roots 5 months after Bradyrhizobium sp. AT1 inoculation.
InoculationAcetyleneARA (nmol C2H4 g fw−1 h−1)
ShootsRoots
−+0.02 ± 0.0070.02 ± 0.007
++0.03 ± 0.0080.56 ± 0.12 *
+−N.D.N.D.
* Significant differences between the inoculation treatments at the 95% confidence level as determined by Student’s t-test, n = 5; fw, fresh weight; ARA, acetylene reduction activity; N.D., Not detected.
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Terakado-Tonooka, J.; Tanaka, F.; Karasawa, T.; Suzuki, A.; Ohwaki, Y. Effects of Inoculating the Diazotrophic Endophyte Bradyrhizobium sp. AT1 on Different Cultivars of Sweet Potato (Ipomoea batatas [L.] Lam.). Agronomy 2023, 13, 963. https://doi.org/10.3390/agronomy13040963

AMA Style

Terakado-Tonooka J, Tanaka F, Karasawa T, Suzuki A, Ohwaki Y. Effects of Inoculating the Diazotrophic Endophyte Bradyrhizobium sp. AT1 on Different Cultivars of Sweet Potato (Ipomoea batatas [L.] Lam.). Agronomy. 2023; 13(4):963. https://doi.org/10.3390/agronomy13040963

Chicago/Turabian Style

Terakado-Tonooka, Junko, Fukuyo Tanaka, Toshihiko Karasawa, Akihiro Suzuki, and Yoshinari Ohwaki. 2023. "Effects of Inoculating the Diazotrophic Endophyte Bradyrhizobium sp. AT1 on Different Cultivars of Sweet Potato (Ipomoea batatas [L.] Lam.)" Agronomy 13, no. 4: 963. https://doi.org/10.3390/agronomy13040963

APA Style

Terakado-Tonooka, J., Tanaka, F., Karasawa, T., Suzuki, A., & Ohwaki, Y. (2023). Effects of Inoculating the Diazotrophic Endophyte Bradyrhizobium sp. AT1 on Different Cultivars of Sweet Potato (Ipomoea batatas [L.] Lam.). Agronomy, 13(4), 963. https://doi.org/10.3390/agronomy13040963

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