Limiting-stress-elimination hypothesis: approach to increase savanna cowpea productivity by stress reduction

We propose and test the Limiting-Stress-Elimination Hypothesis (LSEH), as a decision axiom to guide in determining the optimal intervention strategy towards yield allocation in a savanna legume. We considered osmotic stress and soil nitrogen (N) limitation, which both characterize Guinea savanna agro-ecozone. We hypothesized that biomass allocation will increase when the limiting stress is eliminated at least until the next limiting stress impacts yield. We assessed responses of Vigna unguiculata (L.) Walp (cowpea) to osmotic stress treatments (non-hormonal biostimulant and exogenous metabolite) and N input by leaf level physiology, N-fixing capacity and biomass. The relative increase in biomass (%), and pod yields reveal that osmotic stress (45%) more than nitrogen (13%) is limiting to cowpea growth under Guinea savanna conditions, although N fertilization increased nodulation and maximized PSII quantum yield. In the Sprengel-Liebigs decrement from the maximum concept, the decrement from the maximum for each stressor must be minimized in order to produce the absolute maximum production. However, this may not be economically feasible in many situations. Conversely, LSEH demonstrates that significant productivity is attainable by eliminating a relatively more limiting stress, osmotic stress, regardless of the limitation and natural demand for the relatively less limiting N in leguminous cowpea in the savanna.