Phosphorus use efficiency and nitrogen balance of cowpea breeding lines ina low P soil of the derived savanna zone in West Africa

Differences in growth, nodulation and arbuscular mycorrhizal fungi (AMF) ro ot infection among recent cowpea breeding lines from IITA were examined at low and high P levels in pot (94 lines) and field experiments (43 lines) at Fashola in the derived savanna zone of Nigeria. Based on their growth perf ormance, these lines were subdivided into 5 groups: (i) poor performance un der low and high P conditions; (ii) good performance under low P and poor p erformance under high P; (iii) intermediate performance under high and low P; (iv) good performance under high and low P conditions; and (v) good perf ormance under high P and poor performance under low P. About 42% of the bre eding lines (18 out of 43 lines tested) had the same grouping for the field and pot experiments. Eight cowpea lines (4 P-responders and 4 non-P-respon ders) were selected from the first experiment for subsequent studies on the effect of P supply (0, 20, 40 and 60 kg P ha(-1)) on P uptake, P use effic iency, dry matter production, N-fixation, AMF infection and N balance. Dry matter production, shoot/root ratio, total shoot N, and total N-fixed of th e non-P-responder line, IT81D-715, were strongly related to P uptake effici ency. The P-responder IT81D-849 had a significant (95%) correlation between AMF and P-use efficiency. The cowpea lines fixed on average 22 kg N ha(-1) , which was 70% of the plant total N. The N balance based on the difference between the amount of N-2 fixed and N exported through the harvest, ranged between -10.6 kg N ha(-1) and +7.7 kg N ha(-1). Based on its adaptability to grow in low P soils and overall positive N balance, the cowpea line IT81 D-715 should be recommended for cultivation when P is the limiting factor.