screening for phosphate solubilizing bacteria inhabiting the rhizoplane of rice grown in acidic soil...

SCREENING FOR PHOSPHATE SOLUBILIZING

BACTERIA INHABITING THE RHIZOPLANE OF RICE

GROWN IN ACIDIC SOIL IN BANGLADESH

ANIMESH SARKAR1, TOFAZZAL ISLAM

2*, GOKUL CHANDRA BISWAS3,

SHOHIDUL ALAM4, MIKAIL HOSSAIN

4 and NUR MOHAMMAD TALUKDER4

1Department of Food Engineering and Tea Technology, Shahjalal University of Science

and Technology, Sylhet-3114, Bangladesh2Department of Biotechnology, Bangabandhu Sheikh Mujibur Rahman Agricultural

University, Gazipur-1706, Bangladesh3Department of Genetic Engineering and Biotechnology, Shahjalal University of Science

and Technology, Sylhet-3114, Bangladesh4Department of Agricultural Chemistry, Bangladesh Agricultural University,

Mymensigh-2202, Bangladesh

(Received: 2 November 2011; accepted: 20 January 2012)

The objectives of the research were to isolate phosphate solubilizing bacteria

(PSB) from the rhizoplane of rice (Oryza sativa L.) cv. BRRIdhan 29 cultivated in

acidic soils of Tangail in Bangladesh and evaluate their performances in phosphate

solubilization in both in vitro and in vivo conditions. A total of 10 bacterial strains

were isolated and purified by repeated streak culture on nutrient agar medium. Upon

screening, five isolates (OS01, OS03, OS07, OS08 and OS10) showed varying levels

of phosphate solubilizing activity in agar plate and broth assays. Among them, the

strain OS07 (B1) and two previously isolated PSB strains B2 and B3 were selected for

evaluation for their performances in rice alone or in combination of TSP (triple super

phosphate: P1) and rock phosphate (P2). Plant height and the number of tillers per plant

were significantly increased by all PSB isolates when used in combination with TSP

but PSB alone did not influence much on plant height and the number of tillers except

B1. The levels of mineral nutrients content in rice plant tissues were generally in-

creased by the application of the PSB in combination with TSP, while the perfor-

mances of B1 isolate was superior in all aspects to B2 and B3 isolates.

Keywords: phosphate solubilizing bacteria, triple super phosphate, rock

phosphate, levels of nutrient elements content, rice plant

1217-8950/$20.00 © 2012 Akadémiai Kiadó, Budapest

Acta Microbiologica et Immunologica Hungarica, 59 (2), pp. 199–213 (2012)DOI: 10.1556/AMicr.59.2012.2.5

* Corresponding author; E-mail: [email protected]

Introduction

Phosphorus (P) is an essential nutrient limiting for plant growth which

mostly remains in insoluble forms in the acidic soils [1, 2]. Soils may have large

reserve of total P, but the amounts of P available to plants is usually a tiny propor-

tion [3]. A large portion of inorganic P fertilizer applied to soil is rapidly immobi-

lized and become unavailable to plants due to fixation with calcium (Ca), alu-

minium (Al) and iron (Fe). In most tropical soils, phosphate (PO43–) predomi-

nantly present as inorganic compounds with Ca, Fe or Al salts. Iron and alu-

minium phosphates predominate under acidic condition while calcium phosphates

are predominate in neutral to alkaline soils. The low availability of P to plants is

due to the vast majority in insoluble forms and plant takes up P only in the form of

soluble orthophosphate ions such as H2PO4– and HPO4

2– [2, 4]. Thus, the release

of insoluble and fixed forms of P is an important aspect of increasing soil P avail-

ability. Plant root-associated PSB have been considered as one of the possible al-

ternatives to solubilize the inorganic phosphate fertilizers for promoting plant

growth and yield [5–9]. Seed or soil inoculation with PSB is known to improve the

solubilization of fixed soil P and increase crop yields [1, 2, 10, 11].

Greater portion of the Bangladeshi soil contain low to medium amount of

available P. Availability of P is low at both low and high pH values under upland

conditions and high under wetland rice cultivation [12]. Again P availability in

Bangladeshi soils is low in rabi season (dry season, typically from November to

February) due to low temperature and increases in kharif (warm and wet season,

typically from March to October) season with the rise of temperature [13]. Phos-

phorus recovery is usually very low (8–20%) in rice in Bangladesh [12]. There-

fore, applications of phosphate fertilizers are essential for better crop yield. Due to

high price, the resource-poor farmers of Bangladesh often fail to apply phosphate

fertilizers at recommended doses into the soil [12]. On the other hand, availability

of P is a serious problem posed due to fixation which lowers the utilization effi-

ciency of added P fertilizer by crop plants [2, 12]. It is also imperative that man-

agement of P fertilizers in agricultural environments is improved (particularly in

more highly P fertilized environments) so that any adverse environmental effects

due to P losses are minimized [14]. A significant reduction in the use of phosphate

fertilizer could be achieved if solubilization of soil-insoluble P is made available

to crop plants [6, 8, 9].

However, very few attempts have been made to isolate the potential PSB

from the rhizosphere of rice cultivated in acidic soils in Bangladesh [15–17].

Hence, scant information is available concerning phosphate solubilizing bacteria

Acta Microbiologica et Immunologica Hungarica 59, 2012

200 SARKAR et al.

and their ability to colonize rice roots especially in acidic soils. The cultivar

BRRIdhan 29 is a rice variety produced from the Bangladesh Rice Research Insti-

tute (BRRI), which is well adapted throughout the country even in acidic soils.

The objectives of this study were (i) to isolate PSB from the rhizoplane of rice cv.

BRRIdhan 29 cultivated in acidic soils in Bangladesh, and (ii) to evaluate their ef-

ficiency in phosphate solubilization in both in vitro and in vivo conditions.

Materials and Methods

Collection and preparation of rice roots for isolation of bacteria

Root sample of rice seedlings (Oryza sativa L.) cv. BRRIdhan 29 were col-

lected randomly from four locations of a farmer field of the village Rasulpur of

Madhupur upazilla in Tangail district in Bangladesh for screening of PSB in root

rhizoplane. Root samples were collected and washed with sterilized distilled water

to remove the adhesive soil particles. The soil free root samples (ca. 10 gram) were

homogenized by a vortex mixer for 15 sec in 20 ml of sterile distilled water in a

sterile test tube. Homogenate was diluted 100-fold by serial dilution down to 10–4.

Exactly 100 µl aliquot of each sample was spread over surface of nutrient (Nissui

under Code no. 05511) agar plate with sterilized glass spreader. Each root homog-

enate is replicated 3 times. All plates were incubated at 25°C for 2 days and contin-

ued observation for 4 days. The culture was repeated 3 times to purify the bacterial

isolates.

Screening phosphate solubilizing bacteria

The Pikovskaya’s medium [18] was used to screen the bacteria having

phosphate solubilizing activity. Each bacterial isolate was stabbed in four places

on an agar plate using a sterile toothpick or with an inoculating loop. The halo

zone of solubilized P and colony diameters were measured after 14 days of incuba-

tion of the plates at 25°C. Phosphate solubilizing capacity was calculated in terms

of Phosphorus Solubilization Index (PSI = A/B, where A was total diameter of the

halo zone, and B was colony diameter). The isolates showing PSI = 2 are consid-

ered as PSB [16]. Evaluation of each isolate was carried out for three times and di-

ameters of halo zones were averaged to calculate PSI.


PHOSPHATE SOLUBILIZING BACTERIA 201

Quantitative estimation of phosphate solubilization in brothby rice rhizoplane bacteria

The quantitative bioassay was carried out using Erlenmeyer flasks contain-

ing 100 mL nutrient broth containing tricalcium phosphate (500 mg/L) as P source

which was inoculated with PSB approximately 108–109 CFU/mL. The flasks were

incubated for 2 days at 30ºC in a shaker at 180 rpm. Then the culture broth was

centrifuged for 10 minutes at 3500 rpm to separate the bacterial cells. The

supernatant was decanted and filtered through Whatman No. 41 filter paper. The

available P content in the supernatant was estimated using stannous chloride

colorimetric method [19] by measuring absorbance at a wavelength of 660 nm and

the bacterial cells were used for root inoculation.

Field trial of isolated phosphate solubilizing bacteria

Pot (of 6 kg soil capacity) culture experiment was conducted in the net

house of the Department of Agricultural Chemistry of Bangladesh Agricultural

University of Bangladesh. The cultivated clay loamy soils had the average pH 5.3,

organic carbon 1.5%, organic matter 2.6%, available P 4.0 mg/L, exchangeable K

64 mg/L, and water holding capacity 27%. Chemical analyses of plant samples

(for the determination of N, P, K, Ca, Mg, S, Mn and Zn) were conducted in the

same department using standard protocols as described by Jackson [19]. A part of

the analytical work was conducted at the central laboratory of BAU and Soil

Chemistry Laboratory of BARI, Gazipur. The roots of rice seedling cv. BRRIdhan

29 were inoculated with the PSB isolates, B1, and B2 (collected from the previous

work in Agricultural Chemistry Department, Bangladesh Agricultural University)

and B3 (collected from the Bangladesh Institute of Nuclear Agriculture) before

transplanting in pots containing acidic red soil of Madhupur in boro season (dry

season rice, grown from December to April) [13].

The experiment was laid out in complete randomized design (CRD) with

three replications and two factors viz. PSB inoculants, i.e. inoculation of without

PSB strain (B0), with PSB strain B1, B2 and B3 and different sources of P fertilizer,

i.e. without P fertilizer (P0), TSP (P1) and rock phosphate (P2) at the recommended

dose of P (12 kg P/ha). The mineral elements N, P, K, S and Zn were applied as

urea, TSP (triple super phosphate) and rock phosphate, MOP (muriate of potash),

gypsum and zinc sulphate (ZnSO4), respectively, at recommended doses (95 kg N,

12 kg P, 45 kg K, 8 kg S and 1 kg Zn ha–1) [12, 20]. Before pot preparation for rice


202 SARKAR et al.

seedlings transplantation, all levels of nutrient elements were applied to soil sam-

ples except N. Nitrogen was applied in two splits: the first 1/3 was applied at final

pot preparation and the 2/3 rest was added after 30 days of transplantation. Weed-

ing, intercultural operation, and application of irrigation water were done in the

pot for ensuring and maintaining proper growth and development of the crop.

Temperature, humidity, and day length of the growing season were 11–30°C,

40–67%, 12–15 h, respectively.

Plant height and number of tillers hill–1 as growth parameter were recorded.

From each pot, plant samples were randomly collected at 15th, 30th and 45th day

of transplantation (DOT) and properly tagged for recording necessary data and

representative samples were kept for chemical analysis. Plant samples were oven

dried at 70ºC for about 72 h until constant weight of the sample was achieved, and

then ground by an electrical mill and analyzed for total N, P, K, Ca, Mg, S, Fe, Mn,

and Zn using the standard methods of Jackson [19].

Results

Isolation and screening of bacteria

Ten bacterial strains were isolated from the rhizoplane of rice collected

from a farmer’s field and purified by repeated streak culture on nutrient agar me-

dium. Initially, all the strains were tested for their phosphate solubilizing activity

by agar assay using Pikovskaya’s medium supplemented with 1.5% bactoagar

[18]. Among ten isolates, five (OS01, OS03, OS07, OS08 and OS10) superior iso-

lates were further assessed for estimation of phosphate solubilizing index (PSI)

(Table I). These five selected strains (on the basis of PSI) were inoculated in

Pikovskaya’s liquid medium to investigate their phosphate solubilizing perfor-

mances in the broth culture and the amounts of phosphate solubilized from

tricalcium phosphate by bacteria isolates were determined by the stannous chlo-

ride method (Table I).

Quantitative evaluation of phosphate solubilizing activities of rice isolates

The results in Table I indicate that the isolates showed varying levels of

halo zones on Pikovskaya’s agar medium in plate assay. The strain OS07 pro-

duced the biggest clear zone (PSI = 4.7 mm) in agar medium. But in broth assay,



the highest amount of phosphate solubilization was observed by the strain OS03

(140.1 ± 0.3 mg L–1), which was very close to that of strain OS07 (139.3 ± 7.0

mg L–1).

Performance of PSB on the growth of rice

To study the effects of selected PSB on growth and nutrient elements up-

take by rice plants, a pot experiment was conducted using three (B1, B2 and B3)

selected PSB isolates. The characters of rice plants growth were recorded at 15th,

30th and 45th DOT. The results showed that all the bacterial strains except B1 de-

creased plant height at different DOT compared to control treatment (Table II). On

the contrary, the bacterial strains increased the number of tillers pot–1 at different

DOT except B3 at 15 DOT (Table II). Among the three strains, the performance of

B1 was the best in increasing the number of tillers pot–1 but the variation in in-

crease of plant height and numbers of tillers were not significantly influenced by

the bacterial strains (Table II). At 45th DOT, the plant height measurements due to

the influence of the treatments (B0, B1, B2 and B3) of PSB were 69.3, 62.7, 65.4

and 67.5 cm, respectively, while the number of tillers pot–1 at 45th DOT were

14.7, 17.4, 13.8 and 15.4, respectively. The single effect of two different sources

of P fertilizer significantly increased both plant height and number of tillers pot–1

at 45th DOT (Table III). Both plant height and number of tillers pot–1 were highest

due to the application of TSP (P1) (Table III).

The interactions of different bacterial isolates and different sources of P did

not have significant influence on the variation in plant height and number of tillers

pot–1. At 45th DOT, the plant height varied from 57.3 cm to 77.8 cm in the treat-

ments B1P0 and B3P1, respectively (Table IV). The P1 treatment alone and in com-

bination with each one of the three PSB displayed the superior performances in


204 SARKAR et al.

Table I

Phosphate solubilizing index and amount of phosphate solubilization by different bacterial isolates

in agar and broth assay using Pikovskaya’s medium

Bacterial isolates Phosphate solubilization index Phosphate solubilization

(PSI)† in agar assay (mm) in broth assay (mgL–1)

OS01 3.5±0.2†† 128.0±3.0††

OS03 4.1±0.3 140.1±5.3

OS07 4.7±0.1 139.3±7.0

OS08 1.1±0.4 87.8±4.0

OS10 2.4±0.3 79.8±6.0

† PSI = (Halo + colony diameter) / colony diameter†† Mean±SE (standard error)



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increasing plant height compared to that of P2 treatments. Similar effects of P1 and

P2 alone and in combination with each one of the PSB were observed in number of

tillers pot–1. The plant height and number of tillers pot–1 were the highest and the

lowest in treatments of B3P1 and B1P0, respectively, at the 45th DOT (Table IV).

Content of mineral elements in rice plant tissues

Table V shows that the application of different bacterial isolates increased

N, P, Mg, S and Zn contents as compared to control treatment. The content of K

also increased in all treatments except in B1. On the other hand, Ca content in-

creased in all treatments except B2 which was similar to control B0. Also, it indi-

cates that Fe and Mn contents increased in all the treatments except in the treat-

ment with B3.

Results in Table VI demonstrate the application of two different sources of

P fertilizer alone increased the content of N, P, K, Ca, Fe and Mn, while it de-

creased in Mg content compared to the control treatment. Also, S content was in-

creased in P1 (TSP) treatment, and it was similar to control treatment (P0) when

treated with P2 (rock phosphate). Zinc content remained almost unchanged in the

treatments of P0 and P1, but it was improved in P2 treatment. Variations in the con-

tent of P, K, Ca, Mg, Mn and Zn were statistically significant at 1% level due to the

single effect of P (Table VI).

Table VII shows that the content of nutrient elements of N, K, Ca, Mn and

Zn in rice plant BRRIdan 29 at tillering stage varied significantly due to interac-

tion effect of different PSB isolates and sources of P (TSP and rock phosphate),

but the concentrations of P, Mg, S and Fe in rice plant at tillering stage were not

significantly influenced by the interaction effect of bacterial strains and sources of

P (Table VII). The percent N content in rice plant at tillering stage varied from

0.95 in B2P0 to 1.43% in B3P1; K from 0.57 in B1P0 to 0.66% in B3P1; Ca from 0.16

in B0P0 to 0.21% in B1P0, B1P1, B1P2, B2P1, B3P1 and B3P2; Mn from 1.30 in B0P1

and B3P2 to 2.92 mg g–1 B0P2 and Zn from 0.04 in B0P0 and B0P1 to 0.12 mg g–1 in

B3P2.

The variation in contents of P, S and Fe in rice plant at tillering stage was

not significantly influenced by the interaction of different sources of PSB and P

fertilizers. However, the content of P ranged from 0.04% in B0P0 to 0.16% in B1P1;

Mg ranged from 0.68% in B0P1, B0P2 and B2P2 to 0.78% in B0P0, B1P0 and B2P1; S

from 0.16% in B0P0 and B1P0 to 0.23% in B2P1, and Fe from 0.13 mg g–1 in B3P2 to

0.36 mg g–1 in B2P1 treatment of plant sample. It was noticed that the rice plant at


206 SARKAR et al.



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7


208 SARKAR et al.T

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60

.04

c±0

.00

2

B1

1.3

9a±

0.0

30

.13

a±0

.00

30

.61

c±0

.01

20

.21

a±0

.00

0.7

5±

0.0

37

0.1

8±

0.0

10

.24

b±

0.0

11

2.4

6a±

0.1

60

.05

b±

0.0

02

B2

1.2

5b

±0

.03

0.1

2b

±0

.00

20

.62

b±

0.0

12

0.1

8c±

0.0

00

.74

±0

.04

90

.20

±0

.01

0.3

1a±

0.0

12

.12

b±

0.1

60

.05

b±

0.0

03

B3

1.3

4a±

0.0

30

.12

b±

0.0

02

0.6

4a±

0.0

12

0.1

9b

±0

.00

0.7

5±

0.0

43

0.1

9±

0.0

10

.16

c±0

.00

61

.46

c±0

.16

0.0

7a±

0.0

05

CV

(%)

7.1

78

.70

2.7

12

.24

7.6

87

.14

11

.60

8.9

71

0.0

1

LS

Dat

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lev

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.09

0.0

08

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95

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87

0.0

70

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†D

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9

atti

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inc

(%)

(%)

(%)

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(mg

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(mg

g–

1)

(mg

g–

1)

P0

1.2

3±

0.0

28

0.0

8c±

0.0

02

0.6

1c±

0.0

12

0.1

7c±

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01

0.7

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48

0.1

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04

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10

1.8

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±0

.13

0.0

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.00

1

P1

1.2

9±

0.0

31

0.1

3a±

0.0

03

0.6

4a±

0.0

12

0.2

1a±

0.0

02

0.7

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±0

.03

60

.20

±0

.00

50

.24

±0

.01

11

.88

b±

0.1

20

.05

b±

0.0

01

P2

1.2

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0.0

32

0.1

2b

±0

.00

30

.62

b±

0.0

12

0.1

9b

±0

.00

10

.71

b±

0.0

32

0.1

8±

0.0

04

0.2

4±

0.0

11

2.2

0a±

0.2

70

.07

a±0

.00

2

CV

(%)

7.1

78

.70

2.7

12

.24

7.6

87

.14

11

.60

8.9

71

0.0

1

LS

Dat

5%

lev

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.07

0.0

08

0.0

95

0.0

10

.04

60

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32

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20

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†D

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(mg

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(mg

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P-s

ou

rces

B0P

01

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ab±

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24

0.0

4f±

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03

0.6

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11

0.1

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20

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40

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B0P

11

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28

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20

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11

0.2

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±0

.00

20

.68

±0

.02

30

.17

±0

.00

40

.22

±0

.01

61

.30

e±0

.11

0.0

4d

±0

.00

1

B0P

21

.38

ab±

0.0

27

0.0

6e±

0.0

01

0.6

3c±

0.0

07

0.2

0b

±0

.00

30

.68

±0

.03

90

.18

±0

.00

50

.24

±0

.01

82

.92

a±0

.24

0.0

5c±

0.0

01

B1P

01

.26

b-d

±0

.04

30

.09

d±

0.0

01

0.5

7g

±0

.01

20

.21

a±0

.00

20

.78

±0

.04

30

.16

±0

.00

40

.26

±0

.01

92

.77

ab±

0.2

70

.05

c±0

.00

1

B1P

11

.12

d±

0.0

32

0.1

6a±

0.0

03

0.6

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0.0

14

0.2

1a±

0.0

02

0.7

3±

0.0

34

0.1

9±

0.0

04

0.2

1±

0.0

17

2.7

0ab

±0

.24

0.0

6b

±0

.00

1

B1P

21

.36

a-c±

0.0

31

0.1

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±0

.00

30

.62

d±

0.0

14

0.2

1a±

0.0

02

0.7

5±

0.0

38

0.1

7±

0.0

05

0.2

5±

0.0

14

1.9

2c±

0.1

70

.06

b±

0.0

02

B2P

00

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e±0

.02

70

.09

d±

0.0

01

0.6

4b

±0

.01

20

.17

c±0

.00

20

.76

±0

.02

70

.18

±0

.00

60

.25

±0

.01

31

.57

d±

0.1

10

.06

b±

0.0

02

B2P

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.21

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0.0

22

0.1

5ab

±0

.00

30

.64

b±

0.0

10

.21

a±0

.00

20

.78

±0

.04

90

.23

±0

.00

60

.36

±0

.01

72

.14

c±0

.18

0.0

5c±

0.0

01

B2P

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.12

d±

0.0

27

0.1

2c±

0.0

02

0.6

0f±

0.0

14

0.1

6d

±0

.00

10

.68

±0

.03

20

.17

±0

.00

50

.32

±0

.01

82

.66

b±

0.1

60

.04

d±

0.0

01

B3P

01

.31

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0.0

21

0.0

8d

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.00

20

.64

b±

0.0

13

0.1

7c±

0.0

01

0.7

6±

0.0

36

0.1

8±

0.0

06

0.1

7±

0.0

08

1.6

1d

±0

.12

0.0

5c±

0.0

01

B3P

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a±0

.03

60

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0.0

02

0.6

6a±

0.0

07

0.2

1a±

0.0

02

0.7

6±

0.0

41

0.2

0±

0.0

05

0.1

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0.0

09

1.4

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±0

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1

B3P

21

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b-d

±0

.03

30

.14

b±

0.0

02

0.6

2d

±0

.00

60

.21

a±0

.00

20

.73

±0

.03

70

.17

±0

.00

50

.13

±0

.00

81

.31

e±0

.11

0.1

2a±

0.0

05

CV

(%)

7.1

78

.70

2.7

12

.24

7.6

87

.14

11

.60

8.9

71

0.0

1

LS

Dat

5%

lev

el0

.17

0.0

16

0.0

05

0.0

08

0.1

20

.09

0.2

60

.23

50

.00

6

tillering stage accumulated higher content of N, P and K is due to the application

of B3P1 while, higher accumulation of Ca was found when the rice plants grew in

the presence of B1 in combination with P1 and P2, B2P1 and B3P2. The application

of B2P1 improved Mg, S and Fe content in plant tissues, while the treatments of

B1P0 and B3P2 increased the concentrations of Mn and Zn in rice plants, respec-

tively.

Discussion

In this study, we isolated five potent PSB strains (OS01, OS03, OS07,

OS08 and OS10) from the rhizoplane of rice grown in acidic soils. The efficien-

cies of these PSB isolates were evaluated and compared on basis of their phos-

phate solubilizing index (PSI) on agar assay (Table I) and the amount of phospho-

rus solubilization by Pikovskaya’s broth assay [16, 21–23]. The PSB exhibited

higher PSI in agar assay also displayed higher P solubilization activities from

tricalcium phosphate in broth assays with few exceptions. Among them, the strain

OS07 was found to be almost the best performer in solubilizing insoluble P in agar

assay and broth assay culture conditions. In an earlier survey, Islam et al. [16]

stated that among 30 bacteria were isolated from the rhizoplane of rice cv.

BRRIdhan 29 cultivated in Mymensingh, Bangladesh and from the seedlings ob-

tained from surface-sterilized seeds of BRRIdhan 29. Upon screening, 6 isolates

showed varying levels of phosphate solubilizing activity in both agar plate and

broth assays using National Botanical Research Institute’s phosphate medium and

among them 2 isolates were identified as Acinetobacter sp. BR-25 exhibited high-

est phosphate solubilizing activity followed by Klebsiella sp. BR-15. They grew

rapidly in the liquid medium at pH 5 and 7 but almost no growth occurred at pH 3.

The pH value of the culture medium was decreased with bacterial growth suggest-

ing that they might secrete organic acids to solubilize insoluble phosphorus. In our

study, we found that some PSB isolates had high performances in agar assay and

on the same time low performance in the broth assay or vice versa (as in Table I).

Almost similar results were obtained by Gupta et al. [24]. They found that some

isolates with little clear zone on solid medium exhibited high efficiency for dis-

solving insoluble phosphate in liquid medium and some showed large clearance

zone in agar but low phosphate solubilization in liquid medium which was ob-

served in current study. However, Islam et al. [16, 17] found almost direct correla-

tion between halo zones in agar medium and amount of phosphate solubilization

from tricalcium phosphate in broth culture conditions. It seemed that PSB isolated

by Islam et al. [16, 17] and some of our isolates are different in respect of their ac-


210 SARKAR et al.

tivities on agar and broth assays. Further study is needed to identify PSB isolates

by 16S rRNA gene sequencing.

To evaluate the effect of PSB application on growth and nutrient uptake by

rice plants, we assessed three PSB isolates in a pot experiment with different

sources of P. We found that not only plant height and number of tillers affected,

but also the PSB inoculation greatly enhanced different nutrients uptake by rice

plants. The study clearly showed that plant height and the number of tillers were

increased by all PSB strains used in combination with TSP. Almost similar effects

of PSB on plant growth and nutrient uptake were observed by Ashrafuzzaman et

al. [23]. In our study, we found that application of different PSB isolates (Table V)

significantly increased N, P, Mg and Zn contents in rice plant tissue compared to

control (without PSB inoculation). The content of N, K, Ca, Mn and Zn in rice

plant BRRIdan 29 at tillering stage varied significantly due to interaction effect of

different PSB isolates and P (TSP and rock phosphate), but the concentrations of

P, Mg, S and Fe in rice plant at tillering stage were not significantly influenced by

the interaction effect of bacterial strains and sources of P.

One of the novel findings of the present study includes microorganisms in-

volved in P solubilization as well as better scavenging of soluble P can enhance

plant growth by increasing the N content, enhancing the availability of other nutri-

ent elements and probably by the production of plant growth promoting sub-

stances [23, 27–29]. Higher accumulation of Ca in rice plant observed in current

study was due to application of TSP with PSB than that of rock phosphate [28–30].

In conclusion, this study was successful to isolate five potential PSB iso-

lates from the rhizoplane of rice plants roots grown in acidic soils in Bangladesh.

The PSB isolates displayed promising effects on solubilizing insoluble P in labo-

ratory assays (agar and broth assay) and also enhance growth and nutrient uptake

by rice plants in pot culture in acidic soils. Further studies are needed to identify

the PSB isolates using the molecular techniques including 16S rRNA gene se-

quencing and elucidate their mechanism of P solubilization from tricalcium phos-

phate. A large-scale field study also needed for recommending PSB strain OS07 as

a biofertilizer for practical use in rice cultivation in acidic soils.

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