Potassium-mobilizing bacterial isolates: A sustainable approach to enhance growth parameters and yield of Bt cotton

Introduction

Cotton (Gossypium hirsutum L.) is one of the most important commercial cash crop and important fiber crop of global significance cultivated in more than seventy countries. It is an important raw material of economy in terms of both employment generation, foreign exchange and hence it is popularly known as "White gold or friendly fiber". The cotton plant belongs to the genus Gossypium of the family Malvaceae. It is grown for fiber and seed. Cotton is a multipurpose crop that supplies five basic products such as lint, oil, meal, seed and hulls and is popularly known as "King of Fiber". Cotton is one of the principal crops of India and plays a vital role in the country's economic growth by providing substantial employment and making significant contributions to export earnings. Globally, cotton is cultivated on approximately 33.5 million hectares, with an estimated production of 24.5 million metric tons in 2024. Leading producers include India, China, the United States, Brazil, and Pakistan, which together account for over 75% of global cotton production. India leads in cultivation area, while the United States and Brazil have higher productivity per hectare. In India Maharashtra is the leading cottonproducing state, accounting for over 25% of the national production, followed by Gujarat and Telangana (USDA Foreign Agricultural Service 2024) [22] . Sustainable agricultural practices are essential to meet the dual challenges of increasing crop productivity and maintaining environmental health. Bt cotton (Gossypium hirsutum), a genetically modified crop with inherent pest resistance, has become a cornerstone of cotton farming globally due to its ability to minimize losses caused by bollworms and other pests. However, the realization of optimal growth and yield in Bt cotton depends significantly on efficient nutrient management, particularly potassium (K), a key macronutrient that regulates physiological processes, stress tolerance, and reproductive success in plants (Zorb et al. 2014) [26] . Potassium exists in several forms in the soil, including mineral K, non-exchangeable K, exchangeable K, and dissolved or solution K (K + ions). Plants can only directly take up solution K. Among the three major essential nutrients required by plants, one of them is potassium. It is involved in numerous biochemical and physiological processes in plants like stomatal regulation for plants depend upon K to regulate the opening and closing of stomata. The activation of enzymes by potassium and its involvement in adenosine tri phosphate (ATP) production is important in regulating the rate of photosynthesis; sugars produced in photosynthesis must be transported through the phloem to other parts of the plants for utilization and storage. The plants transport system uses energy in the form of ATP if K is inadequate, less ATP is available, transport system breaks down. The enzyme responsible for synthesis of starch is activated by K, hence it plays crucial role in water and nutrient transport (Doman and Geiger, 1979; Pettigrew, 2008) [5,15] . The potassium content of Indian soils varies from less than 0.5% to 3.00% (Mengel and Kirkby, 1987) [12] . In Indian soil the soluble K form are present in approximately 2% and insoluble are present in range of 98% in the form of minerals like biotite, feldspar, mica, muscovite, vermiculite (Goldstein,1994). Despite its abundance in most agricultural soils, potassium is predominantly found in insoluble forms, limiting its bioavailability to plants (Basak and Biswas, 2009) [2] . The conventional use of potassium fertilizers has been effective in addressing this issue but raises concerns about environmental sustainability and soil health. Over-reliance on chemical inputs can degrade soil structure, reduce microbial diversity, and lead to nutrient imbalances. These challenges have prompted the exploration of biological alternatives such as potassiummobilizing bacterial isolates (KMBIs). Many bacteria such as Acidothiobacillus ferrooxidans, Paenibacillus spp., Bacillus mucilaginosus, B. edaphicus, and B. circulans have capacity to solubilize K minerals (e.g., biotite, feldspar, illite, muscovite, orthoclase, and mica). KSB can dissolve silicate minerals and release K through the production of organic and inorganic acids, acidolysis, polysaccharides, complexolysis, chelation, and exchange reactions. Etesami et al. (2017) [6] In addition to enhancing potassium availability, potassium solubilizing microbes improve soil microbial activity, nutrient cycling, and overall soil fertility, thus benefiting the growth and health of plants. Various andA. terreus, along with yeast species such as Torulaspora globosa (Yadhav and Sindhu 2016) [24] Since, potassium is a limiting factor in a crop production and cotton is seen as a potential cash crop for many farmers. Keeping this in the view present study was undertaken for understanding the role of potassium-mobilizing bacterial isolates on growth parameters and yield of Bt cotton" in field condition.

Materials and Methods

In order to evaluate the potassium solubilization potential of different microbial strains in vitro condition and in liquid medium, laboratory experiments were carried out using Aleksandrov media. The experiments were carried out during 2022 in Soil Biodiversity Research Laboratory, Department of Soil Science and Agricultural Chemistry, VNMKV, Parbhani. The observations on colony diameter, halozone diameter, clearing zone on the solid media were recorded systematically. Further, the data on soluble potassium in liquid medium was noted and change in pH of broth was recorded. On the basis of halo zone formation promising microbial strains were selected for further experiment. Thereafter, experiments were conducted to evaluate potassium solubilizing potential of different microbial isolates under field condition. The field trials were conducted during Kharif 2022 and 2023 on Vertisol (Typic Haplustert) at same place at research farm, All India Network Project on Soil Biodiversity and Biofertilizer, Vasantrao Naik Marathwada Krishi Vidyapeeth, Parbhani. The experiment was laid out in randomized block design with 10 treatments replicated three times. The recommended dose of phosphorus and potassium along with half dose of N were applied at the time of sowing and half dose of N after one month of sowing. The inorganic sources of fertilizers urea, single super phosphate (SSP) and murate of potash (MOP) were used for experiment. The laboratory stock cultures (KSB-W1 (Bacillus sp,) KSB-PD-3-A, (Bacillus sp), KSB-NP-3 (Bacillus sp), KSB-M-1 (Pseudomonas sp), KSB-M-2 (Pseudomonas sp,) KSB-M-3 (Sinorhizobium metallidans)), Fraturia auruntia, Psudomonas strita, were selected on the basis of their potash solubilizing ability in laboratory condition. After germinating the cotton seedlings (15 DAS) liquid broth of isolates (10 -8 CFU ml -1 ) diluted in water was applied @ 5L ha -1 through drenching. Five plants from each plot were randomly selected were used to collect data for growth and yield attributes. From the net plot cotton yield per hectare was reported. Data were statistically analyzed by the standard procedure of Panse and Sukhatme (1985) [14] .

Results and Discussion

Growth parameters 1. Plant height

The effect of inoculation of potassium solubilizing microbial inoculants with recommended dose of fertilizers on plant height was recorded at critical growth stages of Bt cotton and data is presented in Table 1. The data shows that plant height was significantly influenced by inoculation of different bacterial isolates. Increase in plant height from flowering till to harvest due to inoculation of different potassium solubilizing microbial inoculants the significantly highest plant height of cotton in pooled analysis, was recorded in treatment T 9 RDF + Frateuria aurantia (84.28,108.35,113.47 and 118.63 cm) at respective critical growth stages which was found at par with T 3 , T 4 , T 5 , T 6 , T 7 and T 10 at flowering and boll development stage, at boll brusting; T 3 , T 7 and T 10 , at harvest T 3 , T 6 , T 7 and T 10 respectively. The lowest values were obtained (44.79, 61.14, 63.26 and 67.71 cm) with treatment T 1 which is absolute control (71.92,95.76,100.64 and 105.54 cm) followed by treatment T 2 (only RDF) at respective critical growth stages of cotton. The treatment results reveal that inoculation with specific potassiumsolubilizing bacteria (KSB) significantly enhanced plant height compared to both the absolute control (T 1 ) and the sole RDF treatment (T 2 ). Overall, the data suggest that potassiummobilizing bacteria, particularly Frateuria aurantia, Pseudomonas striata, and Bacillus spp., significantly enhance the plant height of Bt cotton by improving potassium solubilization, nutrient uptake, and the provision of growthpromoting compounds. Our findings supported by Hamed et al. (2019) [8] .

Number of leaves

Data on mean number of leaves as influenced by the application of different potassium solubilizing microbial inoculants are presented in Table 2. Inoculation of potassium solubilizing bacteria i.e. Frateuria aurantia along with RDF (T 9 ) and Pseudomonas striata along with RDF (T 10 ) recorded significantly maximum number of leaves of cotton. In the pooled analysis, the highest number of leaves in Bt cotton was recorded in treatment T 9 ( [8] . Results showed that Bacillus circulance (BC) and Bacillus megatherium (BM) strains in combination with chemical fertilizers had an effective synergistic impact on plant growth traits and yield parameters of cotton. Similarly, (Prajapati and Modi 2012) [18] reported that use of KSB having multi-faceted beneficial traits would be highly effective for improving growth.

Number of branches

It is evidenced from the data presented in Table 3 that the potassium solubilizing microbial inoculants have significant impact on the number of branches at different growth stages of cotton during both years. Number of branches were increased from flowering stage till to the harvest stage of crop. In the pooled analysis, significantly the highest number of branches in Bt cotton was observed in treatment T 9 (15. Potassium is essential for cellular functions, including cell division and elongation, which contribute to branch formation and development. The increased branch numbers in treatments with Frateuria aurantia may also be due to this bacterium's ability to produce plant growth-promoting substances, such as indole-3-acetic acid (IAA) and gibberellins, which further stimulate growth and branching. The efficient K-solubilizing microbes (KSM) should be applied for solubilization of a fixed form of K to an available form of K in the soils. This available K can be easily taken up by the plant for growth and development (Verma et al. 2017) [23] .

Chlorophyll content

The chlorophyll a and b are virtually essential pigments for the conversion of light energy to stored chemical energy. The amount of solar radiation observed by a leaf is a function of the photosynthetic pigment content, thus chlorophyll content can directly determine photosynthetic potential and primary production. In addition, chlorophyll gives an indirect estimation of the nutrient status because much of leaf nitrogen is incorporated in chlorophyll. Thus, according to necessity, chlorophyll content in fresh leaves of cotton were estimated at flowering stage of crop and data is presented in Table 4. The table illustrates the effect of potassium-mobilizing bacterial (KSB) isolates on chlorophyll content (Chl-a, Chl-b, and total chlorophyll) of Bt cotton. Among the treatments, T 9: RDF + Frateuria aurantia showed the significantly highest values for Chl-a (1.54 mg g⁻¹), Chl-b (0.83 mg g⁻¹), and total chlorophyll (2.37 mg g⁻¹), indicating its superior ability to enhance photosynthetic pigment content. This was closely followed by T 10: RDF + Pseudomonas striata, which was at par with T 9 for Chl-b (0.80 mg g⁻¹) and total chlorophyll (2.19 mg g⁻¹). Other notable treatments include T 6: RDF + KSB -M -1 (Pseudomonas spp.), which recorded 1.37 mg g⁻¹ Chl-a, 0.78 mg g⁻¹ Chl-b, and 2.15 mg g⁻¹ total chlorophyll, as well as T 7: RDF + KSB -M -2 (Pseudomonas spp.) and T 8: RDF + KSB -M -3 (Sinorhizobium metallidans), both with similar total chlorophyll values (2.13 mg g⁻¹). In contrast, the lowest chlorophyll content was observed in T 1: absolute control, with Chl-a (1.12 mg g⁻¹), Chl-b (0.55 mg g⁻¹), and total chlorophyll (1.67 mg g⁻¹), reflecting the lack of nutrient enhancement in the absence of fertilizers or bioinoculants. Our results corroborate with the conclusions of Sheng and He (2006) [20] revealed that bacterial inoculation significantly increased chlorophyll content and potassium uptake. Imran et al. (2022) [11] concluded that K enriched compost with efficient KSB inoculant can be used to enhance maize growth and productivity in semi-arid agroecosystems. Increased chlorophyll content in levels and photosynthetic efficiency, which ultimately contributed to higher crop productivity.

Root parameters

The table no.5 presents the effect of potassium-mobilizing bacterial (KSB) isolates on root parameters of Bt cotton, including oven dry weight of shoot and root, root volume, rootshoot ratio, and root density. Among the treatments, T 9: RDF + Frateuria aurantia exhibited the best performance, with the highest values for oven dry weight of shoot (45.83 g), root (2.43 g), root volume (16.25 cc), and root-shoot ratio (18.88). The treatment T 10: RDF + Pseudomonas striata was statistically at par with T 9 for oven dry weight of root (2.38 g), root volume (15.46 cc), root-shoot ratio (17.54). Conversely, the lowest values across all parameters were recorded in T 1: absolute control, with oven dry weight of shoot (14.17 g), root (1.43 g), root volume (5.91 cc), root-shoot ratio (9.90). Highest root density of cotton was noticed in treatment T 1 (0.229, 0.253, and 0.240. g/c) treatment strictly control no RDF no KSB inoculant. Followed by treatment T 2 (0.156,0.155 and 0.160 g/cc) with only RDF in pooled data. Whereas, lowest root density among all treatment recorded in treatment T 9 ((RDF + Frateuria aurantia) followed by T 10 (RDF + Pseudomonas striata) and T 3 (RDF + KSB W1-Bacillus spp.) as pooled data. The application of KSB isolates, particularly Frateuria aurantia and Pseudomonas striata, significantly improved root parameters compared to the control and RDF-only treatments. These enhancements likely contribute to better nutrient acquisition and crop performance. Findings supported by (Prajapati and Modi 2016) [16] The inoculated cucumber plants vine length and root length were more than controls, where root elongation is associated with the production of IAA in early stages. The IAA content was increased in inoculated plants as compared to control and so increased root length, shoot length due to bacterial phytohormones, and the lack of essential nutrient cause the elongation of roots to obtain nutrient (Hamid, 2008 and Hassan et al. 2009) [9,10] . Work done by P.P. Nihala Jabin and Syed Ismail (2017) [13] inoculation of KSB strains which can synergistically solubilized rock K that were added into soil and make them much more available for uptake by plant root and leading to plant growth. Further, Prajapati (2015) [17] indicated the largest increase in plant height in soil inoculated with KSB strain compared to the uninoculated soil. Our results are also concurring with the findings of Zhang and Kong (2014) [25] who reported that seedlings inoculated with KSB showed greater height than uninoculated seedlings. Our results are also corroborating with the findings of earlier workers. Chishi, (2010) [4] found that inoculation of KSB showed higher number of leaves as compared with control. Basavesha (2013) [3] also found that the inoculation of potassium solubilizing bacteria increased the number of leaves in plant significantly superior over control. It may be due to inoculation of KSB strains which can synergistically solubilized Rock K that were added into soil and make them much more available for uptake by plant root and leading to plant growth (Sheng, 2005) [21] . The decrease in number of leaves at later stage might be a result of senescence. [16] . Our findings also supported by (Priyavardhini et al.2022) revealed that application of K2O 40 gm + KSB 20 ml/ kg Soil (SA) + KSB 30 ml/ kg Seed (SI) was found to be more productive (5.78 t/ha) for receiving of higher yield of Maize.

Yield and yield attributes

Conclusion

The application of potassium-mobilizing bacterial (KSB) isolates demonstrates a sustainable and effective strategy for enhancing the growth parameters and yield of Bt cotton. The study revealed significant improvements in growth and yield attributes with potassium mobilizing bacterial strains showcasing their potential to optimize nutrient uptake and plant productivity. Treatment RDF + Frateuria aurantia consistently outperformed all others, achieving the highest values for yield including seed cotton yield (1930 kg/ha) and dry matter yield (2892 kg/ha), with an impressive 29.10% and 21.02% increase over RDF, respectively. Additionally, RDF + Pseudomonas striata also exhibited strong performance across parameters, proving it to be at par with Frateuria aurantia in several aspects and minimum growth and yield improvement observed in uninoculated treatment. These potassium solubilizing (mobilizing) microorganisms can be explored as bio input for improving plant growth and also to improve the potassium availability and reduces the losses of nutrient by making them more available with potassium solubilizing mobilizing bacterial isolates in cotton.

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