Ontogenetic variations on absorption and utilization of phosphorus in common bean cultivars under biological nitrogen fixation

An experiment was conducted to evaluate the effect of plant ontogeny on traits associated with absorption and utilization of P by common bean (Phaseolus vulgaris L.) cultivars under biological N-2 fixation through plant growth analysis. Eight cultivars were grown in pots at two levels of applied P (20 and 80 mg P kg(-1) soil) and harvested at three growth stages (third trifoliate expanded, plentiful flowering, pod setting). The development of nodulation differed among cultivars and depended on P supply; some cultivars showed an intense decline in nodule number after flowering. There were different patterns of root growth: Some cultivars increased root area after flowering mainly by increasing root mass, whereas others by decreasing root radius. Despite the overall drift of reducing P influx into roots over time, some cultivars maintained P influx almost stable after flowering. Cultivars with greater root area had lower influx, whereas cultivars with smaller root area had higher influx, denoting a compensatory mechanism between root growth and P influx, such mechanism presenting ontogenetic variations associated to patterns of root area production. The cultivars differed in specific P utilization rate at low but not at high soil P level; hence P utilization seems an important physiological component for screening bean genotypes under limited P supply. High correlation between net assimilation rate and specific P utilization rate suggests that P utilization is strongly related to photosynthetic activity. Despite, the wide variation among cultivars on root growth, nodulation and rates of P absorption and utilization, the genotypic variability for total dry mass and P content was narrow by pod setting. Owing to growth constraints caused by a possible inefficient symbiosis, the concomitant selection of bean genotypes for tolerance to low P and high N-2 fixation activity should prioritize traits related to the root system and to N-2 fixation over total dry matter. The ontogenetic variations of traits associated with P absorption and utilization in common bean cultivars require evaluations at different plant growth stages.