Growth Analysis and Responses of Cowpea (Vigna Sinensis (L.) Savi Ex Hassk.) and Redroot Pigweed (Amaranthus retroflexus L.), Grown in Pure and Mixed Stands, to Density and Water Stresses

The effects of water stress and planting density on the competitive relationships, yield performance, and dynamics in canopy dominance of cowpea (Vigna sinensis (L.) Savi ex Hassk) and redroot pigweed (Amaranthus retroflexus L.) grown in pure and mixed stands were investigated under glasshouse and field conditions. Results showed that water stress at early growth stages reduced plant height, shoot dry weight, leaf area, leaf dry weight and lowered pod dry weight of cowpea, and inflorescence dry weight of A. retroflexus. The effect on both species was more pronounced at high planting densities as a result of severe intraspecific competition. In pure stands, cowpea was affected more than A. retroflexus and the effect was more pronounced on reproductive organs development. Growth analysis of both species grown in pure stands for different periods indicated that leaf area was the most descriptive variable in shoot dry weight and total dry weight of both species at early growth stage. A. retroflexus grew at a faster rate with higher net assimilation rate per unit leaf area and allocated more resources to leaves and roots than did cowpea. Results showed that A. retroflexus was stronger competitor than cowpea. Competition reduced growth and competitive abilities of both species mainly by reducing leaf area early in growth but the effect was more pronounced on cowpea. Keyword: A. retroflexus, Vigna sinensis, development, inter-specific competition, intra-specific competition, stress responses.