Variability of N2 fixation in cowpea under phosphorus deficiency is related to phosphorus use efficiency

Low availability of phosphorus (P) is a major constraint to legumeproduction, and efforts are being made to identify legume genotypeswith tolerance to low P and greater P use efficiency (PUE). Cowpea(Vigna unguiculata L.) genotypes vary in their adaptation to low-Psoils. In order to investigate to what extent this variation may berelated to PUE for symbiotic nitrogen fixation (SNF), five genotypes,Adrar, El Kala, Djenet, Tizi Ouzou and Bejaia, were grown in semihydroponicculture with sufficient versus deficient P supply. At theflowering stage, the biomass of plants and nodules and their Pcontents was determined after measuring the quantity of N2 fixed.The results showed that nodule and shoot biomass were less when Pwas deficient. The genotypes that presented the maximum growthduring the experiment presented a high efficiency in use of therhizobial symbiosis calculated as the slope of plant biomass regressionas a function of nodulation. Under P-deficiency, efficiency in use of Pfor SNF was significantly increased in the genotypes El Kala, Djenetand Adrar and accompanied with an increase of the efficiency in useof the rhizobial symbiosis. Hence the large differences show thattraits for more P uptake-efficient plants exist in the tested cowpeagenotypes. This opens the possibility to breed for more P uptakeefficientvarieties as a way to bring more sparingly soluble soil P intocycling in crop production and obtain capitalisation of soil P reserves