The current study depicted that Cd treatment impaired the growth of wheat plants with respect to all morphological traits and Cd had severe impact at highest level of Cd-50 ppm (Figure.1 and 2). In contrast, the seed inoculation with B.s positively affected the plant growth profile exposed to Cd stress. The detailed molecular mechanism of Cd toxicity is poorly understood yet however, few researchers explained the damaging effects that Cd may destroy the soil microbial communities, reduce the water and nutrients uptake, and impair the cell division and elongation process ultimately decrease the crop growth (Khanna et al. 2019). Our results reaffirm the findings of (Ahmad et al. 2015), who described a significant reduction in root and shoot length of B.juncea exposed to Cd stress. In addition, the limited growth of root and shoot and leaf chlorosis on exposure to heavy metals has been suggested in previous studies (Hussain et al. 2019). The decline in root and shoot elongation is directly associated with the inhibition of root and shoot metabolism and ultimately affected the overall plant growth (Khanna et al. 2019). Seed inoculation with microbial strain (PGPR) considerably improved the wheat growth in the current study (Figure.1). Microorganisms facilitate the plant growth and development, and increase the supply of phosphate through siderophores formation, root hairs growth, and hormonal stimulation that reduce the heavy metal translocation (Gupta et al. 2018). PGPR induces changes in metabolic activities involve in solubilization and mineralization of organic phosphorous. These metabolic activities helps in the efflux of proton and other various anions, and then phosphatase enzymes release that enables the hydrolysis and mineralization of phosphorus (Ahemad and Kibret 2014). (Liu et al. 2018) reported the microbial treatment enhanced the growth of maize plant under Cd stress.
Moreover, the Cd stress declined the biomass (fresh and dry) accumulation in both wheat varieties as shown in (Figure.3). Similar findings were achieved by (Verma et al. 2008) in B. juncea under Cd stress. A significant decrease in the plant biomass under Cd stress could be due to its harmful impact on root and root hair development, essential nutrient uptake via roots, chlorophyll biosynthesis in leaf, photosynthesis, less water and more Cd accumulation in different organs of plants (Qadir et al. 2014). The total biomass accumulation and distribution were observed to be decreased in Russian knapweed (Rasouli-Sadaghiani et al. 2019) and M lupulina (Jian et al. 2019) due to severe oxidative stress and root damage caused by Cd stress. On contrary, our results showed that B.s enhanced the total biomass in both wheat varieties. Therefore, the increased plant growth and biomass production, and distribution is directly correlated with PGPR applications in Eruca sativa under Cd stress (Kamran et al. 2015). (Treesubsuntorn et al. 2018) reaffirms our study, who described that B. subtilis and B. cereus increased root and shoot biomass when inoculated to O. sativa exposed to Cd toxicity. The possible explanation for increasing biomass accumulation and distribution could be the solubilization of organic minerals from soil towards plants organs, phytoremediation of heavy metals, metal resistance ability of PGPR and the regulation of hormonal production require for plant resistant to heavy metals (Jian et al. 2019; Khanna et al. 2019). Therefore, the PGPR have widely being used to improve the plant growth under various types of environmental stresses.
Chlorophyll as a major component of chloroplast is efficiently associated with plant photosynthetic ability whereas, Cd and rest of the heavy metals negatively affected the chlorophylls and caused chlorosis in leaves (Rizwan et al. 2016a). Several types of heavy metals such as Cd, Zn, Cu, Hg and Pb induce toxicity to cell wall and thylakoid membrane integrity which lead to the inhibition of enzymes i.e Rubisco, chlorophyll synthase, involved in the synthesis of chlorophyll and resulting in the degradation of chlorophylls (Hashem 2014). (Rascio et al. 2008) stayed with our results who reported that Cd reduced the Chl a, b and a + b content in rice. The photosynthates produced by plants with the help of chlorophyll directly linked with increase in plant biomass production whereas, decline in chlorophylls leads to the lower biomass production influenced by Cd stress (Khanna et al. 2019). The increase in nitrogen content an important molecule of chlorophyll structure, was observed in PGPR inoculated M.lupulina which is associated with more production of plant biomass under heavy metal stress (Jian et al. 2019). Moreover, the Cd stress severely affected the membrane permeability and enhanced the protein degradation in B.juncea (Ahmad et al. 2015). However, our results are in line with (Pramanik et al. 2018) who revealed that Enterobacter species has stimulated the growth and improved the chlorophyll content in Oryza sativa seedlings by reducing the toxicity of Cd stress.
In our study, the Cd treatment significantly decreased the water potential, osmotic potential and LRWC in both wheat varieties while the highest values for these parameters was observed under PGPR application. Cd stress in the soil decreased the microbial community and damaged the root tips to reduce the uptake of water and disturb the water balance of cells in leaf resulting in the reduction of stomatal conductance and transpiration rate (Qadir et al. 2014). Consequently, this is directly linked with decline in chloroplast amount as well as cell enlargement and ultimately reduced the plant growth and biomass formation (Rucińska-Sobkowiak 2016). In addition, Cd reduced the surface area of cells that absorb water indicating the disturbance of water balance (Sun et al. 2016). However, the PGPR improves the LRWC and water potential in different plant species exposed to different types of environmental stresses (Naveed et al. 2014). It is reported that PGPR improves the stomatal aperture to uptake more water via roots and enhances the stomatal conductance as compared to non-PGPR inoculated plants (Vejan et al. 2016). (Ahmad et al. 2016) described that PGPR enhanced the water uptake, RWC and membrane stability in the leaf of maize plant under Cd stress which support our study. Moreover, PGPR efficiently improved the tolerance ability of plants exposed to various environmental stresses including heavy metals and increased the yield of plant (Enebe and Babalola 2018).
It is depicted that the effect of Cd is dose dependent that varies with its concentration, duration of exposure and the nature of plant species at different growth stages (Hussain et al. 2019) thus, the response of NARC-2009 was found better than NARC-2011 on exposure to different levels of Cd. Besides, the PGPR enhanced the plant efficiently alone or with Cd treatment as shown in (Figure.8). Overall, it is estimated that Cd stress at all levels severely impacted the growth of both wheat varieties while the inoculation with PGPR reduced the Cd toxicity and improved plant growth attributes. However, further studies are needed to find out the actual mechanism of Cd toxicity in plants at molecular level with the application of PGPR.