Two greenhouse studies were conducted to investigate the relationship between water stress and N2 fixation among drought-resistant and susceptible cowpea [Vigna unguiculata (L.) Walp.] genotypes. In both experiments, seeds were planted in 7.6-liter black polyethylene pots containing composted sawdust medium and were inoculated with Rhizobium. Throughout the experiments, flowers were removed to maintain vegetative growth. Water stress treatments were imposed by withholding water, while the control plants were watered as needed. The treatments were applied 58 and 56 days after planting (DAP) in the first and 2nd experiments, respectively. In both experiments, leaf water potential (LWP), shoot fresh weight (SFW), shoot dry weight (SDW), root fresh weight (RFW), nodule fresh weight (NW), nodule number (NN), and plant specific activity (PSA) by both in situ and destructive acetylene reduction methods were measured. Repeated observations of in situ acetylene reduction were made 58, 63, and 71 DAP in the first experiment. All other variables were measured 77 to 78 DAP in the first experiment. Single observations of all variables, including in situ and destructive acetylene reduction were made 56, 67, and 81 DAP in the 2nd experiment. Results suggested that resistant genotypes are capable of maintaining LWP and biomass production (as measured by SDW and SFW) during water stress. In addition, the effect of water stress on N2 fixation was far greater than the influence of genotype when genotypes were selected for relative drought resistance. Path analysis revealed that LWP is correlated to N2 fixation in water-stressed plants, and improvement of plant water status via drought resistance should increase N2 fixation potential under drought conditions. Therefore, breeding for drought resistance in conjunction with N2 fixation may be more beneficial than breeding strictly for N2 fixation potential without regard for environmental adaptation. The in situ method of acetylene reduction was found to be useful for detecting physiological changes due to water stress and estimating its genotypic N2 fixation potential.
Cuticular resistance to water loss was estimated for drought resistant and susceptible cowpea [vigna unguiculata (L.) Walp.] genotypes in a series of field and greenhouse experiments. The procedure consisted of harvesting the youngest, fully-expanded middle trifoliate leaf of a well-watered plant. The detached leaf was weighed immediately and then allowed to dry in an air-conditioned laboratory (about 25°C). Leaves again were weighed 24 and 48 hr after detachment and then oven dried at 70° for 24 hr. Oven dry weight was used to determine leaf water content (LWC) at each sampling time. Specific drought resistant and susceptible genotypes consistently expressed increased or reduced LWC, respectively, 48 hr after detachment. Interestingly, named cultivars generally had even higher LWC values after drying than did the selected resistant genotypes. Intraspecific variability for the trait appears to exist and may be related to drought adaptation in cowpea.
Two cowpea cultivars, Pinkeye Purple Hull and Royal Blackeye, were evaluated for their ability to produce a ratoon crop. Dry weight and pod yield were measured following harvest from two different cutting heights (second and fourth node), and stages of pod maturity (green and dry). The cultivar Royal Blackeye produced more green manure or returned biomass following ratooning than did Pinkeye Purple Hull. Cutting height and sampling at different pod maturities influenced ratooning potential. These results suggest that cowpea ratooning appears to be economically feasible and that further screening of cowpea cultivars for ratooning ability is warranted.