Soil salinity is becoming more serious problem each passed year in agricultural production of Uzbekistan. Moreover, drying up of Aral Sea is exacerbating current already deteriorated soil as well ecological condition. Although, cotton production significantly decreased in during last decades, it is still counts as the main cropcovering 1.3 mln ha of arable field almost same amount of area as wheat.

Plants grown in the field are surrounded by various microorganisms such as bacteria and fungi that help and improve the plant growth and yield under various stress conditions (Egamberdieva et al., 2007). There have been many reports describing potential benefit of AMF on nutrient uptake of cotton in soil salinity condition, thereby increasing growth and development of cotton plant in harsh environmental condition (Linderman, 1992; Egamberdieva et al., 2007). However, not all plant species form AM associations, and not all AM plant species show clear nutritional benefits from colonization by mycorrhizal fungi under all growth conditions (Connor et al., 2002). Since the mycorrhizosphere is greatly influenced by the host plant genotype (Linderman, 1992), to date there is no any study describing AMF symbioses with new cotton varieties grown in harsh soil salinated condition of Uzbekistan. The objectives of our research were to study the response of two different cotton varieties (Gossipium hirsutum L.) grown on salinated serozem soil with treatment AMF under arid climatic conditions.

Material and Methods
The field experiment was established in three replications on salinated serozem soil of Oq Oltin district Sirdarya region.  Electrical conductivity (EC) values of saline soil were 6.2 dS/m. According to the FAO classification, the soils are calcareous calcisol with low nutrient contents. Cotton seed Gossipium hirsutum L. variety AN-Bayaut-2 and S-6524 were obtained from Seed laboratory of Tashkent State Agrarian University.

The cotton seeds were soaked during 6 hours and inoculated AMF Glomus mosseae hyphae/spores before planting them on experimental plot. For the inoculation with mycorrhiza ~50 g of Glomus mosseae inoculum (BEG 12; International Bank of Glomeromycota; http://www.kent.ac.uk/bio/beg) was added to 1 kg cotton seeds and planted.

The criteria for growth promotion were studied in root and shoot dry matter, N and P content of plants were analysed. After 4~8 weeks, shoots and roots were separated and dried. The total N content in plant tissues was determined following the semi-micro Kjedahl procedure. The P content was determined spectrophotometrically using classic methods of P measurement. The roots were carefully washed free of soil and the mycorrhizal colonisation of the roots was assessed using the methods of Vierheiling et al. (1998). Plant samples from field were also collected for mycorrhizal assessment and to compare with root box data.  At the end of growing season the two center rows of each plot were harvested, weighed and ginned and lint yield and percent lint were determined. Statistical analysis

Data were tested for statistical significance using the analysis of variance package included in Microsoft Excel 98 and comparison was done using a Student’s t-test. Mean comparisons were conducted using a least significant difference (LSD) test (P=0,05). Standard error and a LSD result were calculated.

Results and Discussion
Salinity decreases shoot/root ratio of plant because shoots are sensitive than roots to salts stress (Nazarov, 2007). In another study Lin et al. (1997) observed that increasing salinity (NaCl) decreased the protein content in the seedlings and increased the enzyme activity in both salt tolerant and salt sensitive cotton cultivars. Growth and biomass inhibition of cotton varieties under salt stress are caused due to high accumulation of NaCl salt.

Cotton is a mycotrophic plant in which growth and nutrient uptake is usually increased by AM colonisation. It is known that most terrestrial plants live in symbiosis with AMF (Hoffmann, 2009), and in most cases AMF symbiosis improves the nutritional status of the host plant and thus, its overall fitness (Read, 1998). However, it is uncertain whether slow AM colonisation in the cotton growth disorder is a symptom or the cause of reduced plant growth (Nehl et al. 1998). Some authors declared that the highest growth stimulation of plant by beneficial bacteria and fungi occurred when plant have encountered stressful conditions, whereas non-treated plants by comparison performed poorly under such conditions (Egamberdieva, 2013; Nehlet al. 1998).

In our experiments inoculation of cotton varieties with AMF caused increase in plant height, number of bolls per plant, and overall cotton yield compared uninoculated plants in saline soil condition.

In AN-Bayaut-2 variety, inoculation with AMF resulted in increased shoot and root development, the fresh and the dry weight of the shoot and root (Table 1), the number of capsules, and the shoot P and N content (Table 2). Also, in S-6524 variety were observed the significant increase of above indicated parameters of growth but slightly lower than AN-Bayaut-2 (Table 3).  
 

Table 2 Mycorrhizal colonization and P and N content of plant

At 8 weeks, root and shoot dry weights were significantly greater at the AMF symbioses cotton plants (Table 1). Also, significant differences determined between cotton varieties in root and shoot weight at this stage. Also, shoot and root fresh weights (Table 1) were significantly increased by the mycorrhizal associations, although no major effects on shoot:root ratios were apparent.

In AN-Bayaut-2 variety, inoculation with AMF resulted in increased shoot and root development, the fresh and the dry weight of the shoot and root (Table 1), the number of capsules, and the shoot P and N content (Table 2). Also,

Plants when colonized by AM fungi show high degree of protection by accumulating more and more solute as it has been indicated in many plants (Evelin et al., 2009). Similar results were observed by Patale and Shinde (2012) where they found salinity affected the plant growth at higher salt concentrations and inoculated plants were healthier at low to medium salt concentrations while could obtained necessary nutrients from the soil.

Accordingly, to combat the poor supply of nutrients from the soil, mycorrhizal fungi help their host plant to restore the uptake of mineral nutrients and hence plant growth (Sharifiet et al., 2007). AM fungal symbiosis plays a key role in supplying the poor mobility nutrients like phosphorus to the host plant by the roots and hence suppress the negative effects of salt (Feng et al., 2002).

The root and shoot dry weight showed substantial differences with approximately 1.2-1.3 times greater than non-inoculated one in both cotton varieties. Mycorrhiza inoculated plants in medium salinated soil of Sirdarya region had higher root mass and colonization compared to non-inoculated cotton plants. The percentage of root colonization in AN-Bayaut-2 was almost two times more than S-6524; 36.7 % and 19.1 respectively (Table 2).

No any AMF root colonization was found in root system of control plants, we suppose there were no any ubiquitous AMF on the soil. According observations Linderman (1992), the fungi and hosts of the various types of mycorrhizae are so different, it is important to recognise that aspects of the symbiosis may also be very different.

P and root N content of plants biomass (Table 2) were considerably increased by the mycorrhizal associ- ations, the highest P and N recorded on AN- Bayaut-2 variety. Similar results were observed by Ortas (2012) where he revealed mycorrhiza inoculated plants had a higher nutrient content than non-inoculated plants and inoculation effectiveness was higher under stress condition than normal one.

As AN-Bayaut-2 is responsive variety tended to have higher levels of infection and S-6524 is less responsive variety tended to have lower levels of AMF infection in salinated soil condition. AM fungi play a key role in alleviating the toxicity induced by salt stress, thus normalizing the uptake mechanism in plants by supplying the essential nutrients (Carretero et al., 2008; Porcel et al., 2012).

Microorganisms such as bacteria and fungi increase plant growth and yield under adverse environmental conditions as they have the tendency to resist the damage and hence develop resistance against harmful effects of salinity stress (Asiya et al., 2014).

The response within the varieties to AMF inoculation significantly differed almost in all growth parameters; AN-Bayaut-2 variety showed greater response from beginning of growth period; root and shoot dry weight and P content in shoot was higher than the S-6524 variety. And at the end of growing season cotton yield was significantly higher in AN-Bayaut-2 cotton variety comparing to S-6524. Also, inoculation with AMF resulted in a 21% and 17% increase in the weight of bolls plant−1 and cotton fibre qualities, respectively, as compared with the uninoculated control (data not shown).

According to (Gargand Baher, 2013) arbuscular mycorrhizal (AM) fungi are considered to be bioameliorators of salinity stress due to their capability to live in contaminated soils and their role in modulation of biochemical processes. The role of the AM fungi as P suppliers to the plant appears to be of great relevance, and AMfungi may solubilize P from surrounding areas and make it available to roots (Behl et al., 2003). Thus, in both varieties AN-Bayaut-2 and S-6524, inoculation with AMF produced the greatest effect on both plant growth and nutrient uptake, together with a noticeable increase in mycorrhizal root colonization.

Conclusions
We observed that cotton growth and development stimulation by symbioses AMF was higher in soil salinated condition compared to without symbioses. It can be concluded that AMF is important under soil salinity stress condition to help the plant to get more nutrients from the soil and the importance of AMF towards cotton varieties is variable. We revealed there was a significant relationship between AMF colonization among the cotton varieties. In conclusion, the results of the present study suggest that the inoculation of AM fungi in cotton varieties in soil salinated condition improved the plant growth, development and yield, thus suggesting that it can provide an alternative to chemical fertilizers.

Acknowledgements   
This research was funded by the OEAD and CASIA post doctoral fellowships.

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