Genotypic variability in P use efficiency for symbiotic nitrogen fixation is associated with variation of proton efflux in cowpea rhizosphere

Vigna unguiculata sp., or cowpea, varieties vary in their adaptation to low-P soils. In order to investigate to what extent this variation may be related to P use efficiency and proton efflux by nodulated roots, three genotypes, 26-73, Danila and Melakh, inoculated with Bradyrhizobium sp. Vigna CB756 were grown in hydroaeroponic culture in a glasshouse at two levels of phosphorus supply corresponding to P sufficiency or P deficiency. After 4 weeks, individual symbiotic-plants were transferred to a reference-soil layer in a rhizotron, and harvested after 2 further weeks. Nodule and shoot biomass were less when P was deficient. The effect of P deficiency on biomass production followed the trend Danila > 26-73 > Melakh. Under P deficiency, the proton efflux for the P-efficient genotype 26-73 was 43% and 60% greater than for the P-inefficient Danila in hydroaeroponics and in soil, respectively. This increase in proton efflux was associated with an increase in nodule specific respiration that was 115% greater for Danila than for 26-73. It is concluded that the genotypic variability in P use efficiency for symbiotic nitrogen fixation is associated with a variation in nodulated-root proton efflux and respiration in cowpea rhizosphere, and that these parameters should be measured for more contrasting genotypes in order to test whether they correlate with the adaptation of N2-dependent legumes to low-P soils.