Genotypic variability in phosphorus use efficiency for symbiotic N-2 fixation in common bean (Phaseolus vulgaris)

Three bean (Phaseolus vulgaris L.) genotypes (APN18, BAT271, G12168) were i noculated with a mixture of 3 rhizobia (R. etli CIAT632 and CIAT7115 and R. tropici CIAT899) and grown either with 10 mmol NO3-.week(-1) (NO3-fed) or with N-2 fixation (N-2-dependent) in an aerated nutrient solution under var ious P supplies. Phosphorous supplies for maximum plant growth varied betwe en genotypes, and were always lower with N-2 than with NO3. By contrast wit h NO3-fed plants, N-2-dependent plants accumulated highly contrasting amoun ts of N at low versus high P supplies (72 versus 360 mu mol P plant(-1).wee k(-1)). Under low P, N-2-dependent APN18 and BAT271 fixed respectively 2 an d 3 times more N-2 than G12168, although nodule dry weight was similar in t hese genotypes. The ratio N accumulation: plant P concentration (PUEN) show ed large genotypic contrast under low P supply. It is concluded that these genotypic differences depend on the efficiency in use of P for N-2 fixation , which is linked to the partitioning of P in plants, and that PUEN may be used to screen for genotypic variation of N-2-dependent growth of legumes u nder low P supply.