Resilience to low phosphorus and water stress tolerances in new cowpea (vigna unguiculata [l] walp.) genotypes and associated morphological adjustments.

Cowpea (Vigna unguiculata L. Walp) is an important legume crop, but drought and soil infertility, especially Phosphorus (P) deficiency are primary constraints for its production. Three studies, a field, greenhouse and hydroponics experiments were carried out at the University of Zambia to determine the effect of low soil phosphorus and water stress on the performance of new selected mutation derived cowpea genotypes. In the field experiment, 23 genotypes were planted in a Randomized Completely Block Design (RCBD) to identify cowpea genotypes with dual tolerance to low phosphorus and water stress condition. The field that was used had suboptimal soil phosphorus, and the cowpea was subjected to terminal water stress during the growing period. The effect of genotype on the number of pods per plant, 100 seed grain weight, yield per hectare, number of seeds per pods and pod length were highly significant (p<0.001). The genotypes LT 11-3-3-12, BB 3-9-7-5-2, BB 8-1-5-2 and MS 1-8-1-4 were identified to be better performing, hence were selected for the greenhouse experiment. The greenhouse study separated the effect of P and water stress in order to determine the effect of low P and water stress on the morpho agronomic traits of cowpea. The greenhouse study was conducted using Completely Randomized Design (CRD) and the treatments replicated three times. It was a factorial experiment with three factors; P application at 2 levels, water application at 3 levels and 7 cowpea genotypes. Phosphorus (P2 O5) was applied at planting at the rate of 23 and 80 kg P2O5 ha -1 as low and high P levels, respectively. The water regime was imposed by irrigation at 40% and 60% (water stress) and 100% (no stress) based on Plant Available Water (PAW). Water stress was used as proxy for drought condition. Moisture stress significantly reduced shoot biomass, the number of branches, plant height, root biomass, pod and grain yield. P had a significant effect on shoot biomass, root biomass, grain yield and grain yield partitioning. The studies established that some mutation derived cowpea genotypes were resilient to both low soil phosphorus and water stress conditions, and can be directly released as varieties or used as parents in breeding for improved tolerance to low phosphorus and water stress. Therefore, they can be grown in drought-prone areas, and soils that are deficient in Phosphorus.
Key terms: Genotypes, phosphorus, mutation derived lines, water stress.