Symbiotic N2 Fixation, Leaf Photosynthesis, and Abiotic Stress Tolerance of Rhizobia Isolated from Soybean at Da, Upper West Region, Ghana

Soybean is an important source of protein and is gaining popularity in Ghana due to a rising demand for its use in the poultry industry. However, the grain yield of soybean is relatively low in the Upper West Region due to infertile soil and climate change. This study assessed the symbiotic efficiency of rhizobia isolated from root nodules of soybean via evaluating their effect on photosynthetic functioning in the homologous host. The study also assessed the tolerance of the rhizobial isolates to temperature, drought, salinity and varying pH levels in the laboratory. The infra-red gas analyser, 15N and 13C natural abundance were used to assess photosynthetic activity, N2 fixation, and water-use efficiency, respectively. The test isolates that induced greater photosynthetic rates from higher stomatal conductance, also led to increased water loss via leaf transpiration in soybean plants. Isolates TUTGMGH9 and TUTGMGH19 elicited greater shoot δ13C in the host soybean and also promoted high shoot biomass, C accumulation, relative symbiotic effectiveness, and symbiotic N2 fixation relative to Bradyrhizobium strain WB74 and 5 mM nitrate which were used as positive controls. Although isolate TUTGMGH9, did not grow at 40℃, it showed a growth at 5% of PEG-6000, NaCl, and low pH, while also producing moderate IAA. However, for a better utilization of these rhizobial isolates as bioinoculants, their growth performance needs to be assessed under field conditions to ascertain their competitiveness and symbiotic efficacy.