Growth and productivity assessment of four cowpea genotypes under variable phosphorus levels and soil-moisture conditions.

Abstract

Cowpea (Vigna unguiculata L. Walp) is regrettably observed to have low yields, which are often attributed to, among others, low phosphorus (P) availability and moisture stress conditions. A greenhouse study was initiated to assess the possible effect of integrated soil available P and moisture management strategy in promoting improved growth, productivity, and grain quality attributes of four selected cowpea genotypes. The factorial trial comprised variable P application rates and soil moisture regimes imposed on the four cowpea genotypes as treatment factors. The cowpea genotypes, G (CV17I, CV17F, CV17B, and CV18–1A), P fertilizer levels (0, 30, 60, and 90 kg ha– 1) using single super phosphate (8.3% P) applied at planting, and three irrigation regimes (M) were combined to obtain 48 treatment combinations each replicated four times and fitted into a nested design. The soil-moisture levels comprised 15, 50, and 75 kPa water potential imposed at the reproductive stage for 20 days. Growth data were collected 21 days after moisture stress imposition, yield data at harvest, and plant tissue analyses (i.e., mineral, nutritional, and secondary metabolites contents) were essayed after crop harvest. Statistical analysis and mean separation were done using Duncan Multiple Range Test (DMRT) at 5% probability level. The results revealed a differential response of the cowpea genotypes to moisture stress and P levels. A significant (p≤0.05) GxPxM interaction effect on stomatal conductance (SC), leaf area (LA) and stem diameter (SD), and inconsequential effects were recorded on all measured yield attributes with treatment interaction. The highest recorded SC of 249.92 mmol m−2 s−1 was from CV17B genotype in adequate soil moisture-filled pot without P addition. Soil moisture stress severity decreased seed yield plant-1 by 22 to 37.5% while extreme moisture stress interestingly, had the least effect on a hundred seed weight (12.54 g) in the CV17I genotype. Plant height and chlorophyll content benefitted significantly from P application, with the tallest plants (35.19 cm) obtained at 60 kg P ha-1. Adequate irrigation at 15 kPa significantly increased the number of pods and pod length per plant. The significant GxP interaction effect on the total biological yield and WUE at harvest with CV17I genotype fertilized at 60 kg P ha-1 and CV17B without P fertilization attributed to 9.66 g/plant and 1.039 g mm-1, respectively. Furthermore, the results revealed that genotypes and P application exerted a significant (p≤0.05) effect on the flavonoid, anthocyanin, grain P, Iron (Fe), and zinc (Zn) content with an inconsequential (p>0.05) effect on the total soluble solids and protein content. The second-order GxPxM interaction also exerted a significant (p≤0.01) effect on all assessed quality parameters except for the protein content. Thus, introducing severe soil moisture stress at 75 kPa significantly reduced flavonoid content by about 10.96% compared to the well-watered regime at 15 kPa, albeit severe soil moisture stress increased the TSS. Likewise, CV18-1A genotype without P addition had cowpea grains with the highest mineral P and Zn content, while CV17I genotype fertilized at 60 kg P ha-1 had grains with the highest mineral Fe content. Notably, the principal components (PCs) recorded the highest variability of 32.595%, with the most important traits’ loads on PC1 being the number of trifoliate leaves (0.8411), number of branches (0.6539), and chlorophyll content (0.7234). The mean number of seeds per pod displayed a negatively low but significant (p≤0.05) correlation with the number of pods per plant while showing a very highly significant (p=0.000) and positive correlation with pod length. The flavonoid content had a substantially low and negative association with PC1, PC2, and PC3. Soil moisture effects and P management are important for optimizing cowpea productivity.