Symbiotic Performance of Diverse Cowpea Genotypes on the Delmarva Peninsula

Cowpea (Vigna unguiculata L. Walp) is a promiscuous legume which has the ability to fix atmospheric nitrogen by establishing an effective symbiotic relationship with Bradyrhizobium spp. in a process called Biological nitrogen fixation. This symbiotic relationship has the potential to maintain soil fertility and reduce soil amendments with inorganic nitrogen fertilizers. This study was conducted to identify 1) high performance genotypes of cowpea that can be grown as an alternative crop on the Delmarva Peninsula and 2) efficient Bradyrhizobium spp. strains that are very effective with cowpea genotypes. This study evaluated the symbiotic performance of twenty-seven diverse cowpea genotypes evaluated in the field, at the University of Maryland  Eastern Shore Agricultural Experiment Station, in a split randomized complete-block design with twenty-seven treatments and four replications each. The genotypes were assessed using the following parameters; nodulation, yield, and maturity. At the flowering stage, five plants were randomly selected from each replicate; nodule numbers and weight were recorded. Rhizobia (n=106) were isolated from the nodules and phenotypically and genetically characterized.  Confirmation test using growth pouches and cowpea plants in nitrogen- free nutrient solution under greenhouse condition showed that 104 out of the 108 isolates obtained were effective in nodulating cowpea plants. Ten additional nodules were randomly selected from each replicate for visual nitrogen fixing activity, of which 85% were effective in fixing nitrogen. Genotypes 524B, IT85F-867-5, and cv.CB46 exhibited the highest number of nodules per plant producing 31, 25, and 24, respectively. The 50% date to flowering, podding, and days to mature were also determined. Results show variability among diverse cowpea genotypes in their seed yield, plant biomass, harvest Index, and days to maturity. Genotypes IT85F-867-5, TX128BE and IFE Brown produced the highest biomass 5.64, 4.35, and 4.14 Kg m^-2, respectively. Genotype 524B produced the highest yield and harvest index (1.272.6 Kg/m^-2 and 25.71%) compared to cv. TX2015-2-1-1 (0.341Kg/m^-2 and 12.9%). In conclusion, cowpea genotypes varied in nodulation, yield, and maturity when grown on the Delmarva Peninsula, and the elite genotypes could be planted as alternative crops in this region whenever severe drought and elevated temperature are expected in the growing seasons.