Effect of inoculation with rhizobia and reduced water supply in yield and biological nitrogen fixing activity of cowpea

Cowpea (Vigna unguiculata (L.) Walp) is known for its drought tolerance as well as its high biological nitrogen fixing activity. Deficit and sufficient irrigation levels were applied to hydroponic cultivation of cowpea to investigate whether the reduced water supply affects the yield and the total amount of biological fixed nitrogen in the cowpea. To do so, plants were either inoculated with Bradyrhizobium pachyrhizi PAC48 or Bradyrhizobium sp. VULI11 or Bradyrhizobium sp. VULI11 + Enterobacter sp. to assess their contribution in both nitrogen fixing activity and plants yield as result of two different irrigation levels, namely full irrigation and 50% of the total nutrient solution supply. To identify the effects of the above factors in N fixing activity and water stress tolerance of cowpea, δ¹⁵Ν and δ¹³C were determined using the natural method of abundance of isotopes ¹⁵N and ¹³C, respectively. Both yield and biological nitrogen fixing activity were significantly enhanced when inoculated plants were grown under sufficient-watered conditions. In contrast, both yield and biological nitrogen fixing activity were restricted under water stress conditions. Furthermore, the stress on cowpea plants caused by drought condition was confirmed by δ¹³C results. The conclusion of this study was, despite cowpea being the grain legume most tolerant to drought conditions, the advantages of inoculation with rhizobia in N fixing activity, for the growth and yield of cowpea were eliminated when inoculated plant with strains of Bradyrhizobium sp. VULI11 + Enterobacter sp. and grown under water stress conditions.