Elevated levels of acid and alkaline phosphatase activity in roots and rhizosphere of cowpea (Vigna unguiculata L. Walp.) genotypes grown in mixed culture and at different densities with sorghum (Sorghum bicolor L.)

Abstract

The aim of this study was to assess P acquisition efficiency in 5 cowpea genotypes in mixed culture and at different plant densities using assays of acid and alkaline phosphatase activity and measurement of P in organs. Five cowpea genotypes (2 improved cvv. ITH98-46 and TVu1509, and 3 farmer-selected varieties, namely Bensogla, Sanzie, and Omondaw) were grown in the field at 2 planting densities (83 333 and 166 666 plants/ha) under monoculture and mixed culture with sorghum during 2005 and 2006. Fresh plant roots and rhizosphere soils were collected during the 2 years of experimentation, and assayed for acid and alkaline phosphatase activity. P concentrations in root tissue and rhizosphere soil were also determined using inductively coupled plasma-mass spectrometry. The data for 2005 and 2006 were similar, and therefore pooled for statistical analysis. Our results showed that raising cowpea density from 83 333 to 166 666 plants/ha significantly increased both acid and alkaline phosphatase activity in the rhizosphere, just as mixed culture (or intercropping) also increased the acid and alkaline phosphatase activity in cowpea rhizosphere soil. High plant density and mixed culture (or intercropping) also raised the acid phosphatase activity in fresh roots of cowpea plants. The increased enzyme activity in roots and rhizosphere soil resulted in significantly improved P nutrition in cowpea, greater plant growth, and higher grain yield in the farmer-selected varieties, especially cv. Sanzie. This suggests that field-grown legumes can be screened for high P acquisition efficiency by assaying for acid and alkaline phosphatase activities.