Abstract
Soybean is the most widely grown legume worldwide, but it is a glycophyte and salinity stress can decrease its yield potential up to 50%. Plant growth promoting rhizobacteria (PGPR) are known to enhance growth and induce tolerance to abiotic stresses including salinity. The aim of this study was to isolate such PGPR from the root nodules of Amphicarpaea bracteata, a North American relative of soybean. Isolated strains were identified, and 15 strains were screened for potential utilization as PGPR of soybean through a series of greenhouse trials. Four isolates that greatly improved shoot and root growth were further selected and screened under a range of salt concentrations. Two of the most promising strains, Rhizobium sp. SL42 and Hydrogenophaga sp. SL48 were ascertained to exert the greatest beneficial effects on soybean growth and salinity tolerance. They were co-inoculated with Bradyrhizobium japonicum 532C (Bj) and the plants were grown up to the harvest stage. The treatment of Bj+SL42 resulted in higher shoot biomass than the control, 18% at the vegetative stage, 16% at flowering, 7.5% at pod-filling, and 4.6% at harvest and seed weight was increased by 4.3% under salt stress (ECe = 7.4 ds/m). Grain yield was raised under optimal conditions by 7.4 and 8.1% with treatments Bj+SL48 and Bj+SL42+SL48, respectively. Nitrogen assimilation and shoot K+/Na+ ratio were also higher in the co-inoculation treatments. This study suggested that inoculation with bacteria from an indigenous legume can induce stress tolerance, improve growth and yield to support sustainability, and encourage ecological adaptability of soybean.
Highlights
Since Bradyrhizobium takes nearly a week to grow on yeast extract mannitol (YEM) plates, colonies that grew on the agar were selected after 24 to 96 h incubations
The phylogenetic tree revealed that Hydrogenophaga and Variovorax genera are in the same cluster whereas the Bacillus subtilis, Gemmobacter sp., Flavobacterium sp., Rhizobium sp., and Devosia sp., are in the subsequent nodes of divergence, distant from one another
The results indicated that the strains SL42 and SL48 have greatly improved soybean growth under a range of salt stress conditions (Figure 5)
Summary
Salinity is a major threat to agricultural sustainability and undermining the crop productivity on arable lands worldwide of which more than 50% is predicted to be affected by 2050 (Ashraf, 1994). The Canadian Prairies are susceptible to salinity, due to their soil type, moisture loss, high evapotranspiration rates and mineral salts in groundwater (Wiebe et al, 2007). Salinity stress is mainly caused by uptake of NaCl, the dominant salt in nature, which creates both osmotic and ionic imbalances in plants. These lead to physiological dysfunctions that inhibit plant growth and development, thereby declining crop yield (Munns and Tester, 2008). Even though salinization risk has been lowered in the Prairies through better land-soil-water management practices, it persists to be a localized issue (AAFC, 2020)
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