Variability of N-2-fixation in common bean (Phaseolus vulgaris L.) under Pdeficiency is related to P use efficiency - N-2-fixation tolerance to P deficiency

Common bean often faces P deficiency in soils where it is generally grown. Such a deficiency is a major limitation to grain yield improvement, especia lly when common bean relies on N-2 fixation. Screening for symbiotic Nz fix ation (SNF) under P deficiency (72 mu mol P plant(-1) week(-1)) was perform ed in a glasshouse with 220 lines originating from the Andean and Meso-Amer ican gene pools. Large variability in shoot dry weight and SNF under P defi ciency was found. SNF tolerance to P deficiency was mostly found among late flowering type TV lines, with the exception of three early flowering type III lines. The SNF tolerance to P deficiency was correlated with: (i) low n odule P concentration in all the growth types, and (ii) intense and early n odulation for type IV lines, or (iii) large root dry weight and high nodule nitrogenase for type Ill lines. Large genotypic variability for nitrogen u se efficiency, i.e. biomass per N concentration was also found. However, th e ratio plant N per plant P concentration was the most discriminating. We c oncluded that this later ratio, which is almost similar to plant N-fixed pe r plant P concentration, in glasshouse hydroponics culture might be: (i) an estimate of the P use efficiency for SNF, (ii) a consequence of low P conc entration in plant organs, especially nodules, and (iii) a determinant of g enotypic variability of SNF tolerance to P deficiency.