The main threat to crop production in many countries is salinization (Munns and Tester, 2008) and 33% of the world’s potential arable land already affected by salinity in different level, whereas 950 million ha of salt-affected lands occur in arid and semi-arid regions including almost 60% cotton fields of Uzbekistan (FAO, 2008. www. fao.org/ag/agl/agll/spush). Although, the cotton is considered one of the most salt tolerant crop in arid regions but the increasing of salination retards it’s growth and yield seriously. The increase of salinity caused by irrigation of cultivated lands with saline water, poor agricultural management and drainage systems, and low precipitation. In Uzbekistan, by 1990 the area of the irrigated lands has increased by 1, 6 times, as much as the agriculture production has increased, although salinity and drought became the main factors of crop losses that causing a concern in coping with increasing food requirements (Shanker and Venkateswarlu, 2011; Davranova et al., 2013). Moreover, the massive use of agrochemicals has damaged the natural resources and environment in Aral Sea basin of Uzbekistan which is almost impossible to recover in the future. The ability of plants to take up water are inhibited by salinity stress, causing ion imbalance and, in turn, resulting in a reduction of root and shoot growth (Munns, 2002). Several reports indicated that salinization is the serious threat to sustainable agricultural crop production system and to soil resources (Egamberdiyeva et al., 2007). The improving soil quality and fertility by novel technologies is pivotal for the increase of cotton production in Uzbekistan. Importance of bacterial fertilizers is increasing in worldwide, as it may substantially reduce the use of chemical fertilizers and pesticides which often contribute to the pollution of crop fields and environmental ecosystems (Egamberdieva, 2012).

Current study revealed the prospects of local produced biofertilizers to increase cotton production and it’s capabilities to improve growing condition of cotton in both favourable and salinated arid conditions of Uzbekistan. Moreover, these potential biological fertilizers protect the environment as eco-friendly and cost effective inputs for the farmers, also reduce the use of chemical fertilizers.

1 Results
The problem of salinity is becoming more alert due to improper agricultural management practices, irrigation of cultivated lands with saline water, poor drainage system and low precipitation (Zhang et al., 2012). It is known that cotton seed germination and seedling periods are very sensitive to any level salinity (Ahmad et al., 2002). Several studies have suggested that inoculation with beneficial bacteria all roots have the remarkable ability to secrete exudates like phytohormones or and/or ammonium production into the rhizosphere in response to biotic and abiotic stresses (Bertin et al., 2003; Egamberdieva, 2009). Behl et al., (2012) observed nitrogen-fixing and phosphate-mobilizing bacteria, as well as mycorrhizal fungi, can influence plant nutrition beneficially and thus be used as biofertilizers in agriculture. Biomass and yield improvement due to symbioses depends on nutrient absorbing efficiency of the bacterial symbiont (Khaitov and Teshaev, 2015). A research was conducted in saline and non saline conditions of Uzbekistan revealed the positive effects of bacterial fertilizers on cotton germination, growth and yield under long-term irrigated cotton monoculture.

The use of the four bacterial fertilizers improved seedling growth in non-stressed conditions and also ameliorated the negative effects caused by salt stress on cotton seedlings. Cotton seed germination declined significantly (18%) in saline soil conditions (56%) of Sirdarya region compare favourable (74%) soil condition of Tashkent region.  

The cotton seed germination increased significantly after application of bacterial fertilizers BIST and Subtin compare to control both on saline and not saline soils (Figure 1). Bacterial fertilizer Subtin showed the highest result around 90% seed germination on not saline condition and on saline condition germination rate was 80%. While in control germination rate was 56% on saline condition and 74% on favourable not saline condition. From the results we revealed that growth stimulation of cotton by bacterial inoculants is affected by salinity thereby bacterial fertilizers had ability to improve growth conditions of cotton seedlings. Some authors have pointed out that most bacterial strains help the cotton plant to uptake K, N and P as well as many microelements from the soil, thereby, serve as biological stimulators (Khaitov and Teshaev, 2015; Egamberdieva, 2009).

 

 
The salinity is unfavourable for cotton plants which positively responding to bacterial inoculation. Several researchers have reported that the application of bacterial fertilizers based on endogenous beneficial strains can stimulate plant cell proliferation, which in turn results in an enlarged root system and enhances the utilization of important nutrients from the soil (Egamberdieva, 2009; Khan et al., 2011).

The shoot and root growth stimulated significantly after application of bacterial fertilizers compare to control. All bacterial fertilizers showed stimulation effect in saline and favourable conditions. Among used bacterial fertilizers Subtin and BIST showed more increase of plant dry shoot and root weights. The application of cotton with bacteria fertilizers significantly increased the shoot dry weights from 6 to 16% as compared to the control.

Bacterial fertilizers affected positively the early plant growth of cotton. The inoculation of cotton seed with bacterial fertilizers Best significantly (p˂0.05) increased the shoot and root dry weights from 13% and 6% as compared to the control in not saline and 16 and 7% in saline soil condition. Bacterial fertilizer Fitobisol also positively influenced in saline soil condition and significantly increased shoot and root dry weight up to 16 and 7% respectively in non saline condition and 14 and 7% in saline condition. Yer malhami has a little effect in cotton shoot and root dry weight while increasing 9 to 2% over the control in favourable non saline soil condition but in saline soil condition shoot and root dry weight increased 10 and 3% respectively. The most effective plant growth promoting bacterial fertilizer was Subtin which generated shoot and root dry weight 19 and 7 % increase in non saline condition while in saline condition these increases was 17 and 10% in over the control (Figure2).

 

 
In our study we revealed that increased salinity causes to decreased root weight. A similar result was observed by Demir and Arif (2003), who reported that the root growth of safflower was more inhibited by salinity than shoot growth.

Most of bacterial fertilizers reacted antagonistically against plant pathogens and significantly decreased root rot deceases (data not shown). Thereby, served to providing healthy and uniform growth of cotton plants in field condition. According to Kamilova et al, (2008) reported that beneficial bacterial strains produce indole acetic acid and are able to reduce the infection rate of soil borne diseases. These beneficial bacteria, called plant growth promoting rhizobacteria (PGPR), inhabit plant roots and affect plant growth promotion by mechanisms such as increased solubilization and uptake of nutrients and/or production of plant growth regulators (Farajzadeh et al, 2012). Several authors reported that the inoculation with azotobacteria also positively affects the plant growth and yields due to an increase in fixed nitrogen content of soil, and microbial secretion of stimulating hormones like gibberellins, auxins, and cytokinins (Penrose and Glick, 2003; Radwan, 1998).

The main component for producing of cotton is for fibre and secondly for oil production purpose. In our research we revealed that at saline condition the cotton yield was lower than non-saline condition. But bacterial fertilizers could increase the yield of cotton significantly in saline soil condition and also, non saline soil conditions. Accordingly, promising results were achieved by the use of bacterial fertilizers Subtin and Fitobisol that could increase the cotton yield up to 31.2 and 29.7 dt ha^-1 while in control the yield was just 20.5 dt ha^-1 in non saline soil condition. The greatest effects of inoculation were seen in salinated soil where the maximum yield was observed after inoculation with Subtin (24.7 dt ha^-1) followed by BIST (24.6 dt ha^-1) and in control the yield was 20.5 dt ha^-1. The other bacterial fertilizers also showed some increase in total cotton yield as compared to the control. These results evaluated BIST and Subtin as a best growth promoting biofertilizers as they significantly increased germination, plant biomass and cotton yield in both salinated and non-saline conditions (Figure 3).

These results indicate that bacterial fertilizers can be a good source to increase cotton yield in saline and not saline arid regions of Uzbekistan.

 

 
2 Discussion
This work demonstrated that bacterial fertilizers produced on the base of indigenous beneficial soil bacterias are able to increase the plant growth and yield on both moderate saline and favorable soil conditions. All inoculated treatments increased yield of cotton as compared to control plants. The use of these microbial inoculants may increase the cotton yield with saving chemical fertilizers. We conclude that salinization of soils due to prolonged irrigation and continous use of high amounts of fertilizers in cotton monoculture reduced cotton yield, and it is important to inoculate of cotton seeds with bacterial fertilizers to overcome these challenges in cotton production of Uzbekistan.

The aim of this research was to evaluate the application of bacterial fertilizers to the development and yield of cotton under saline arid conditions. Further research is needed to assess the ability of bacterial fertilizers for helping plant uptake nutrients both saline and non saline conditions.

3 Materials and Methods
3.1 Plant seeds
The seeds of cotton (Gossypium hirsutum L.) variety C-6524 were obtained from the Department of Plant Science, Tashkent State Agrarian University, Uzbekistan and used in this study. Seeds were sorted to eliminate broken, small and infected seeds. Seeds of cotton were surface-sterilized for 5 min with concentrated sulfuric acid followed by 70% ethanol for 3 min and rinsed 5 times with sterile, distilled water.

3.2 Bacterial fertilizers
Bacterial fertilizers BIST (prepared on the base of Pseudomonas putida consists 1×10⁷ CFU/mL), Er Malxami (Nitrogen fixing bacterial fertilizer Azotobater Chroococcum consists 5×10⁹ CFU/mL), Subtin (Bacillus Subtilis consists 1×10⁷ CFU/mL) and Fitobisol (consortium of beneficial soil bacteria) were provided by the culture collection of the Department of Microbiology and Biotechnology, National University of Uzbekistan.

3.3 Study sites
The climate in both sites is continental with a yearly average rainfall of (200±36) mm and more than 90% of the total rain falling between October to May. The average minimum monthly air temperature is 0°C in January, the maximum of 37°C in July, and the soil temperature ranges between −2°C and 35°C. The average highest relative humidity is slightly more than 80% in January and the minimum is less than 45% in June. The combination of high temperatures and low rainfall under continental climate makes irrigation essential for crop production.

The field experiment was carried out at irrigated agricultural site located in Syrdarya Province (41°00′N, 64°00′E,) in north-eastern Uzbekistan. In these soils, cotton has been grown for the long years under a continuous monoculture production system and under flood irrigation without proper drainage facilities but using a natural flow system. According to the WRB-FAO (2006) classification (www.fao.org/soils-portal/soil-survey/soil-classification/world-reference-base/en/), the soils of selected fields were identified as Calcisol (silt loam serozem) having a calcic horizon within 50 cm of the surface. The surface soil horizon was calcareous saline whereas the deeper soil horizons were only mildly alkaline. The orchic horizon is low in organic matter. The selected field (0.5 ha each) was categorized salinity levels based on electrical conductivity (EC): moderately saline ((5.6±0.6) dS m⁻¹) soils.

The second experiment was carried at the field site of experimental station Tashkent State Agrarian University, Tashkent region in Uzbekistan. Soil is typical serozem (1% organic matter, 0.6 mg N 100 g^-1 soil; 3.0 mg P 100 g^–1; 12 mg K 100 g^–1; 6 mg Mg 100 g^–1 soil; pH 7.4) having a calcic horizon within 50 cm of the surface. The orchic horizon is low in organic matter. The experiment was in randomized complete-block design. There were 5 treatments including 4 bacterial fertilizers and control variation in 3 replications, 15 plots in total. Each plot was 10 m².

3.4 Soil sampling
Soil samples were collected from soils twelve conventionally tilled ((0~40) cm depth) irrigated cotton fields both saline and non saline conditions. According to the WRB-FAO 2006 (www.fao.org/soils-portal/soil-survey/soil-classification/world-reference-base) classification, the soils of all the selected fields were identified as calcisol (silt loam serozem) and were formed from loess, eluvial, and proluvial parent materials. The soils have been cropped to cotton monoculture for the last (50~60) years under flood irrigation without proper drainage facilities using natural flow system.

On average, the soil contained (43±9) g sand kg⁻¹, (708±12) g silt kg⁻¹, and (250±13) g clay kg⁻¹, had the cation exchange capacity of (23.6±1) cmol kg⁻¹, with exchangeable Na percentage of 4.41, and Na absorption ratio of 0.32.

The conventional tillage consisted of moldboard plowing to 30 cm depth after harvest and offset disking, to a depth of 10 cm, prior to planting in the spring. Soil samples of (0~30) cm depth were taken with a soil corer (3.5 cm diameter). Samples were collected at the beginning March (spring), and end of the experiments October (autumn). The cores were pooled; field-moist soils were sieved (<2 mm) directly after collection. The soil samples were kept in black polyethylene bags and stored at 4°C. These “fresh” field-moist, sieved samples were used for the incubation study.

3.5 Germination of seeds
The seeds of chickpea were first sorted to eliminate broken, small seeds and then they were surface-sterilized with a solution of 75 mL chloride +25 mL water for (2~3) min, rinsed thoroughly with distilled water. Surface-sterilized seeds were transferred on paper tissue towels soaked in 0.5 mM CaSO₄ and germinated for 18 hours in a dark room at 25°C. During germination process the appropriate bacterial fertilizers were applied to the cotton seeds and planted to the appropriate field plots.

3.6 Field experiment
Local cotton variety C-6524 was used in this experiment. The planting density was 60 cm×15 cm. Irrigation, pest and weed control were carried out on the conventional management for cotton. Conventional mineral fertilizers N, P, K input rates range from 200; 140; 70 kg·ha^-1·yr^-1 respectively in all plots of the experiment. The parameters of seed germination, shoot/root fresh and dry weight, plant height, the number of bolls, the cotton yield were analyzed. The plants were harvested according to their maturity time, and then were used to estimate the yield based on the plot yield.

3.7 Statistical procedures
Data were tested for statistical significance using the analysis of variance package included in Microsoft Excel 97. Comparisons were done using Student’s t-test. Mean comparisons were conducted using a least significant difference (LSD) test (P=0.05).

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