Morpho-Physiological Characteristics and Grain Yield Attributes of Selected Cowpea Genotypes Under Phosphorus and Moisture Stress Conditions on a South Africa Typic Ustipsamment.

Cowpea production in South Africa is currently at subsistence level with limited commercial production solely for fodder. Its productivity as pulses is however, constrained by drought and widespread phosphorus (P) deficiency problems. An agronomic field trial was planted during 2012/13-summer growing season to assess the response of 8 selected genotypes to low soil available P and moisture stress conditions so as to identify potential genotypes that adapt well to South African field condition. Treatments evaluated comprised of two levels each for soil P (low and high) and moisture status (water stress and well-watered); and eight cowpea genotypes (Tvu 4632, Tvu 6365, Tvu 9848, Tvu 15445, Tvu 16408, Tvu 15143, Oloyin and IT00K-1217). Low soil-P level implied available P measured in situ, which was less than 8 mg kg^-1 while the high P level entailed 40 kg P ha ^-1 application rate. All treatment factors were combined as split-split plot arrangement fitted into randomized complete block design; with four replicates. Morpho-physiological parameters such as growth attributes, root traits, and photosynthetic parameters as well as grain yield were collected from the trial. Results revealed that cowpea plant height, number of branches, number of trifoliate leaves per plant and grain yield under the two P rates differed significantly (P<0.05) among the cowpea genotypes. Among the morpho-physiological traits, stem diameter, tap root diameter up to 15 cm and basal roots had significant and positive correlation with grain yield. Significant P rates × moisture status interaction effects were observed on the mean number of trifoliate leaves, lateral root density and grain yield. This suggests that elevated level of soil available P can help mitigate the negative effect of moisture stress through enhanced root growth and development. Such improvement could ultimately lead to enhanced water and nutrients uptake, plant growth and grain yields.