Wednesday, November 15, 2006

Symbiotic Competence of Bradyrhizobium spp on Diverse Cowpea Genotypes.

Fawzy M. Hashem1, Bessie M. Green1, Robert Dadson2, Iqbal Javaid3, and Thomas Devine4. (1) Univ of Maryland Eastern Shore, Dept of Agriculture, Trigg Hall, Princess Anne, MA 21853, (2) 30921 Back Bone Rd, University of Maryland-Eastern Shore, University of Maryland-Eastern Shore, Department of Agriculture, Princess Anne, MD 21853-1299, (3) Dept of Agriculture, Univ of Maryland, Princess Anne, MD 21853, (4) USDA-ARS, Beltsville, MD 20705

Cowpea is a new potential crop in the Delaware, Maryland and Virginia (Delmarva) region of the United States which is now being used as an assurance crop for the loss of soybean and corn farms’ income due to drought and heat stresses.  Information on the symbiotic response of this important crop to Bradyrhizobium spp is lacking in this region.  Field experiments, therefore, were conducted during the 2001-2005 growing seasons to study the effect of various Bradyrhizobium spp strains and commercial inoculants on nodulation, growth and yield of 27 diverse genotypes of cowpea, Vigna unguiculata L. Walp.  Earlier in 2001 and 2002 growing seasons surface sterilized cowpea seeds of these 27 genotypes were coated with commercial multi-bradyrhizobia strains legume inocula and planted in a Randomized Complete Block Design.  Initial results indicated that most cowpea genotypes had poor nodulation and low symbiotic competence with the commercial cowpea Bradyrhizobium inoculants.  Some genotypes, however, had few very small effective nodules.  Thus, bradyrhizobia isolates were obtained from these nodules and phenotypically and genetically characterized using 16s RNA and fatty acid analysis.  Their symbiotic competence was also evaluated in greenhouse and field experimentations.  Cowpea nodulation and nitrogenase activity were determined eight weeks after planting while cowpea seed and biological yields were determined at maturity.  Nodulation, nitrogenase activity, seed and biological yields varied significantly, and this variation depended on the cowpea genotype tested and the bacterial inoculant applied.  Results showed a significant overall increase in seed and biological yield in cowpea genotypes which had received Bradyrhizobium inoculation, where there was about 25% increase in biological yields over uninoculated control treatments.  It is concluded from this study that the use of effective bradyrhizobia inoculants could play a significant role in enhancing cowpea growth and yield in the Delmarva region.