Effects of Phosphorus and Bradyrhizobium japonicum on Growth and Chlorophyll Content of Cowpea (Vigna unguiculata (L) Walp)

The field and screen house experiment was conducted at Tanzania Coffee Research Institute and Seliani Agricultural Research Institute respectively in Tanzania, between March-July 2013, to assess the effects of Bradyrhizobium japonicum and phosphorus on growth and total leaf chlorophyll content of cowpea. The experiment was out down in split-plot design whereby the main plots comprised two inoculation treatments (with and without B. japonicum), while the sub-plots contained phosphorus (TSP) application at four different levels (0 kg/ha, 20 kg/ha, 40 kg/ha, 80 kg/ha). Both experiments were replicated four times. We measured plant growth parameters such as plant height; number of leaves per plant at different stages of plant growth, the stem girth was measured by vernier caliper at physiological maturity. The chlorophyll content was determined at 3, 5, and 7 weeks after planting (WAP). The chlorophyll was extracted by using dimethylsulphoxide (DMSO) and absorbance was determined at 645 and 663nm using UV/Visible spectrophotometer. B. japonicum inoculation significantly increased the plant height, number of leaves per plants, and stem girth above the control. The height was increased by 11.23, 10.43 and 8.99% (screen-house) and by 8.11, 24.05, 9.29% (field) in the measurements taken at 4, 6, and 8 WAP respectively. Number of leaves per plant counted at 6 and 8 WAP increased by 14 and 10.8% (screen-house) and 14 and 11.6% (field) respectively. B. japonicum also significantly increased leaf chlorophyll content of cowpea by 26% (3 WAP) in screen-house and 52, 37.9 and 13.3% (3, 5 and 7 WAP) respectively on the field. Phosphorus also significantly increased different plant growth parameters and leaf chlorophyll content relative to control. B. japonicum inoculation can be as beneficial as inorganic N fertilizers. Therefore, their combined use with phosphorus boosts plant growth and chlorophyll content of the legumes and eventually legume production is increased.