Abstract
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 N2 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 N2 fixation activity should prioritize traits related to the root system and to N2 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.
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