The Variability of Grain Yield of Some Cowpea Genotypes in Response to Phosphorus and Water Stress under Field Conditions

The objective of this study is to identify cowpea genotypes that are tolerant to both phosphorous and drought stresses on highly weathered soil. It is hypothesized that (1) genotypes that have the highest grain yield under optimum conditions do not perform best under P or water stress and (2) genotypes that have the highest grain yield under P stress conditions also perform well under water or combined water and P stress. An experiment was conducted in the humid forest zone of Ghana during two dry seasons (2017 and 2018). Ten cowpea genotypes were evaluated in response to four combinations of P fertilizer and drought treatments. The treatments included 0 kg P ha−1 + water stress (0P + WS; control treatment); 60 kg P ha−1 + water stress (60P + WS); 0 kg P ha−1 + no water stress (0P + NWS); 60 kg P ha−1 + no water stress (60P + NWS; optimum condition) in both field experiments. The experiment was laid out in a split plot arrangement with three replications. The grain yield of the cowpea genotypes during 2017 growing cycle ranged between 1094 and 3600 kg ha−1, and in 2018 between 928 and 3125 kg ha−1. In both growing cycles, genotypes Asontem and GH5344 had the highest grain yield under optimum conditions (60 kg P ha−1 + water). Under combined P and water stress, Hans adua, GH6060 and Asontem were the best three genotypes with grain yield ranging between 1678 and 1478 kg ha−1 and this observation was made during both growing cycles. In conclusion, the genotypes showed a variable response to the different treatments in this study. Hypothesis 1 (genotypes that have the highest grain yield under optimum conditions do not perform best under water or P stress conditions) was not confirmed as the genotypes GH2309 and GH6060 (ranking 3rd and 4th under optimum conditions) were among the three best cultivars both under water or P stress conditions. Hypothesis 2.1 (genotypes that have the highest grain yield under P stress conditions perform well under water stress conditions) was confirmed for all genotypes studied except for the genotype Asontem. Hypothesis 2.2 (genotypes that have the highest grain yield under P stress conditions perform also well under combined water and P stress) was true since the best four genotypes under P stress where the best four genotypes under combined water and P stress (0P + WS). GH6060, Hans adua and Asontem are most adapted to combined water and P stress and need to be further explored to ascertain their potential as drought and phosphorus deficiency-tolerant genotypes.