Improvement of rhizobium-soybean symbiosis and nitrogen fixation under drought

Abstract

The symbiotic interaction between soybean plants and rhizobacteria can be severely affected by drought, which results in a reduction in symbiotic nitrogen fixation and ultimately decreased yields. The aim of our research was to determine whether symbiotically efficient rhizobia that can better tolerate soil water deficits can improve nodule performance in plants subjected to drought. Firstly, rhizobial strains were selected that exhibited differences in tolerance to salt (NaCl) or water deficit (PEG 6000). Sinorhizobium fredii strain SMH12 showed the highest tolerance to these treatments while Bradyrhizobium diazoefficiens strain WB74-1 showed the lowest tolerance. Greenhouse-grown Prima 2000 soybean plants were then inoculated with either SMH12 or WB74-1 and subjected to two water deficit regimes. Different nodule and plant growth traits were determined, including nodule number, dry weight, water potential, and the accumulation of malondialdehyde and ureide. Plants inoculated with SMH12 had significantly more nodules under water deficit conditions than those inoculated WB74-1, despite having lower root and shoot biomass. SMH12-inoculated plants had higher nodule water potentials and lower malondialdehyde contents than the WB74-1-inoculated plants. These results demonstrate that inoculation of soybean plants with the more water deficit-tolerant S. fredii strain improved nodule characteristics when plants were grown under water deficit conditions. However, these improved nodule characteristics do not always directly translate into better plant growth.