Impacts of Rhizobium Strain Ar02 on the Nodulation, Growth, Nitrogen (N₂) Fixation Rate and ion Accumulation in Phaseolus vulgaris L. under Salt Stress

As seen in Fig 1A and B, in general, as salinity increased, biomass and the number of nodules declined. However, there were higher biomass and nodule numbers at 75 mM NaCl, indicating that the Ar02 strain of Rhizobium had a higher tolerance for higher than moderate salinity. These results are different from those obtained in experiments with chick-pea plants, in which 50 mM NaCl concentration led to a 60% decline in nodulation (Elsheikh and Wood, 1990). Furthermore, chick-pea plant nodulation under exposure to the highest salinity concentration was completely depressed after four weeks. Hafeez et al., (1988) revealed a 50% nodulation reduction when Vigna radiata plants (mung beans) were exposed to plants with salinity >50 mM. Reduction in nodule formation at low salinity levels could result from a strong salinity influence on the initiation of the nodulation process. Nodule initiation is an event within Rhizobium-legume symbiosis and demonstrates high sensitivity to osmotic stress. According to Bhardwaj (1975), a higher salinity tolerance was demonstrated by Bradyrhizobium and Rhizobium than the legume host, indicating successful symbiosis depending on the level of tolerance of the host plants.
 

       
Root and shoot growth and biomass increased in Ar02 rhizobia-inoculated plants (with and without added salt) compared to non-inoculated plants. A gradual reduction in the root and shoot growth and biomass was detected with NaCl addition within the inoculated plants; however, the growth of root and shoot and biomass increased in the 50 mM NaCl inoculated plants. Similarly, salinity led to a decline in the weight of shoots and roots within the plants of the legumes green bean (Phaseolus vulgaris) (Gulmezoglu, 2016), soybean (Glycine max) (Grattan and Maas, 1988) and faba bean (Yousef and Sprent, 1983). Cowpea plants (Vigna radiata) survived (matured) for up to 65 days at 90 mM NaCl (Hafeez et al., 1988), whereas chickpea plants (Cicer arietinum) lived for 30 days (Elsheikh and Wood, 1990).
       
Increases in the roots and shoots of the common beans inoculated with Ar02 rhizobia strain and exposed to salinity are shown in Fig 1C-F with the results of non-inoculated plants. As seen in Table 1, there was no improvement in the overall content of shoot nitrogen to affect inoculation at different salt concentrations. With an extreme decline in the nodule number and biomass, the results of this study indicate that common beans inoculated with rhizobia Ar02 grew under salinity, which led to the accumulation of limited N₂ in shoots. The findings contrast with Singleton (1983) and Weil and Khalil (1986), who detected N₂ concentration declines.
 

       
Na⁺ shoot content increased significantly as the salt dose increased with and without inoculation. As seen in Table 1, increased Na⁺ did not significantly hinder plant growth in the shoots of plants inoculated with Ar02 compared to the plants not challenged with salinity (Table 1). It is very difficult to alleviate the adverse impacts of salt in hydro-aeroponic cultures, specifically in salt-tolerant species. Our results indicate absorbed Na⁺ ions significantly impacted growth.
       
The uptake of Ca²⁺ and K⁺ was adversely influenced by salinity, as shown in Table 1, which is consistent with the results of Ushakova et al., (2005) and L’taiefet_al(2020). Competition between K⁺ and Na⁺ at the absorption sites influence K⁺ intake through the common transport system (Khan, 2000). There is high selectivity for K⁺ within the transport system, which assists in Na⁺ leakage and any drastic decline in the accumulation of K⁺ in response to varying halophytes. A decrease in the efficiency of K⁺ uptake in plants resulted from direct competition within root transporters to uptake K⁺ and Na⁺. Due to similarity in the physiochemical properties of Na⁺ and K⁺, higher Na⁺ concentrations lead to reduced K⁺ uptake by the roots.

The tolerance to salinity stress in common beans (Coco Blanc variety) inoculated with rhizobial strain Ar02 was characterised by excluding Na⁺ from the aerial portion of the plant. The nodulation process of common beans was exposed to ≤ 5 mM NaCl or less and the growth of common bean plants was significantly improved. In addition, the salt tolerance of this bean variety inoculated with Rhizobium was associated with a decrease in the concentration of K⁺ and Ca²⁺ in shoots. Microorganisms found within the soil are closely interconnected with plant roots, thereby playing a critical role in plant growth under stress. These findings requiring agronomic validation depict that the impact of inoculation in enhancing the growth of the aerial plants, nodulation and the Na+ concentration in the salt-stress conditions.

The authors have declared no conflicts of interest.