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Currently, the world experiences unpredictable and severe climatic events that have led to a decline in the yield of especially crops grown in low-nutrients soils under rainfed conditions. Studies have shown that the growth, photosynthetic performance and yield of crops established in unfavourable conditions can be enhanced by inoculation with plant growth-promoting rhizobacteria. In particular, some authors have reported that the inoculation of crops with PGPR strains under drought conditions enhance their growth, yield and photosynthetic performance.This study assessed the effect of co-inoculation with Bradyrhizobium japonicum and Bacillus subtilis on the gas exchange measurements, shoot biomass and carbon assimilation of two Bambara groundnut (Vigna subterranea (L.) Verdc) landraces grown under simulated drought stress. The experiment was laid out in a 5 × 3 × 2 factorial arrangement of completely randomized design (CRD) with four replicates. Treatments included Bacillus subtilis BD234 strain (BA), Bradyrhizobium japonicum (BR), co-inoculation of B. subtilis BD234 and B. japonicum strain, ammonium nitrate (NH4-NO3) (positive control), and no inoculation (negative control). The simulated drought stress was divided into three levels namely 100% pot capacity (well-watered), 50% pot capacity (moderate stress), and 25% pot capacity (severe stress). Two landraces of Bambara groundnut, characterized by different seed colour (red and cream), were used. Significant variations were observed between the landraces and across the inoculation treatments. When grown under the drought stress levels and co-inoculated with B. japonicum and B. subtilis, the landraces of Bambara groundnut exhibited enhanced photosynthetic rate, transpiration rate, and intercellular CO2. Also, dual inoculation of the food crop grown under drought increased carbon accumulation. The red landrace revealed markedly improved growth and photosynthetic performance compared to its cream counterpart. Overall, co-inoculation of Bambara groundnut with B. japonicum and B. subtilis alleviated adverse effects under moderate and severe drought through improving the growth, photosynthetic performance, and carbon assimilation. Therefore, co-inoculation with B. japonicum and B. subtilis has a potential to combat negative effects of drought in Bambara groundnut.

Research efforts to develop alternatives to chemical-based fertilizers for sustainable crop production has led to renewed interest in beneficial soil microbes such as rhizobia and plant growth promoting biostimulants such as the seaweed (Ecklonia maxima). This study assessed the interactive effect of the co-application of seaweed extract with two Bradyrhizobium strains (Inoculant 1 and Inoculant 2) on the growth, symbiotic performance and nutritional composition of three cowpea (Vigna unguiculata (L.) Walp.) genotypes (IT97K-390-2, Songotra and TVU13998) grown under glasshouse conditions. The response of cowpea to the treatments was genotype dependent, such that the combined application of inoculant 2 plus seaweed extract increased nodule dry matter in genotype Songotra, and together with sole inoculant 2 increased the parameter in genotype IT97K-390-2, just as the inoculation plus seaweed extract treatments increased the parameter in genotype TVU13998 when compared to their respective counterparts receiving other treatments. Sole inoculation or inoculation plus seaweed extract treatments increased shoot dry matter in all varieties (2.0 to 7.2 g.plant-1) relative to the control plants receiving sole nitrate (0.5 to 1.2 g.plant-1), sole seaweed extract (0.3 g.plant-1), nitrate plus seaweed extract (1.2 to 1.6 g.plant-1) or the absolute control (0.2 g.plant-1). Due to N2 fixation in the inoculated plants, their leaf δ15N (-2.66‰ to -1.20‰) were markedly lower (p≤0.001) than values recorded by the control plants (+3.30‰ to +510‰) which had no nodules; consequently, leaf N accumulation was greater in the inoculation-based treatments (41.2 to 258.2 mg.plant-1) relative to the uninoculated controls (1.7 to 24.7 mg.plant-1). In most instances, the sole inoculation and inoculation plus seaweed extract treatments increased leaf photosynthetic rates (except for genotype TVU13998 treated with inoculant 1 + seaweed extract), water use efficiency (δ13C) (except in genotype TVU13998) and the concentrations of macro and micronutrients in leaves (except for K in Songotra treated with inoculant 1 or inoculant 1 + seaweed extract as well as Mn in TVU13998 treated with inoculant 1 among others) of the cowpeas relative to the controls. We highlight the potential benefits of the synergistic interactions between rhizobia and seaweed extract for enhancing plant growth and nutrient accumulation in cowpea leaves.