Carbon assimilation and water-use efficiency in cowpea varieties inoculated with Bradyrhizobium, measured using 13C natural abundance

ABSTRACT Moisture stress is one of the most important constraints for crop production in arid regions. Cowpea is a vital food legume that has been cultivated in tropical and sub-tropical regions where water is scarce. Rhizobia inoculation confers resistance to water stress legumes. Two-year field experiments were conducted to assess the carbon assimilation and water use efficiencies of inoculated cowpea varieties at three sites. The treatments consist of four varieties and three levels of Bradyrhizobium inoculation arranged in a factorial randomized complete block design with four replications. The nitrogen (% N) and carbon (% C) concentrations in plant shoots were obtained directly from the mass spectrometric analysis. The results revealed considerable variation for shoot % N and % C, shoot growth, and δ13C among the varieties. Similarly, Bradyrhizobium significantly affected plant growth, % C, C and N contents, C/N ratio, and carbon isotope discrimination of the shoot. For instance, C and N contents and the C/N ratio increased by 28, 24, and 31%, respectively, due to Bradyrhizobium inoculation. In general, these results indicated that physiological performances such as carbon assimilation and water use efficiency of the crop could be, significantly improved when effective Bradyrhizobium strains and the best performing varieties are selected.