Gas exchange and carbon assimilation of Bambara groundnut landraces co-inoculated with Bradyrhizobium japonicum and Bacillus subtilis under simulated drought stress

Abstract

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.