Variability of cowpea breeding lines to low phosphorus tolerance and response to external application of Phosphorus

Abstract

Cowpea is the most important legume crop in the dry areas of West Africa and features in several cereal-based cropping systems of Sudan and Sahel environments. Low availability of phosphorus is a major constraint to crop production, and efforts are being made to identify crop genotypes with tolerance to low P and greater P use efficiency. Two hundred cowpea genotypes differing in plant type and maturity were evaluated at 3 P levels (0 P, 90 kg P ha(-1) as rock phosphate, and 30 kg P ha(-1) as SSP) in 2002 at Minjibir, Nigeria and at Toumnia, Niger. The results showed large variations in grain and fodder yields was found among the 200 genotypes. Grain yield response to RP and SSP ranged from I to 160%. Based on the performance at low (0 P) and high phosphorus levels (SSP), genotypes were classified into the following groups: i) inherently low yielding at low (0 P) and high P, ii) inherently high yielding at low and high P, iii) low yielding at low P but high yielding at high P, iv) high yielding at low P, but low yielding at high P, and v) inherently moderate yielding. Fifteen genotypes were selected from the different P use and response groups and further evaluated in the greenhouse and field studies for tolerance to low P and response to applied P. In general, application of P increased grain yield, shoot-root ratio but decreased AMF colonization of cowpea roots. There were large differences in P use efficiency and the values ranged from -11 to 3 8 kg grain (kg P)(-1) applied. AMF infection was reduced by at least 50%, while shoot-to-root ratio was significantly increased with P application. Variation between genotypes was significant for certain paired means but not consistent for all parameters measured, and the locations. The cowpea genotypes appeared to differ in AMF colonization, shoot growth relative root development, and relative P use efficiency for tolerance to low P soils and response to external application of P.