abbreviations: Al, aluminum; ARD, average root diameter; CIAT, Centro Internacional de Agricultura Tropical; QTL, quantitative trait loci; RB, root biomass; RER, root elongation rate; TRL, total root length; SRL, specific root length. abstract Aluminum (Al) toxicity is the principal abiotic constraint in acid soils of the tropics. Common bean, meanwhile, is for the most part very sensitive to this stress although certain genotypes within the species show some level of resistance, justifying screening of germplasm from both the Andean and Mesoamerican genepools to identify better performing sources. The objective of this study was to evaluate 36 genotypes of common bean under hydroponic conditions to identify root morphological traits that could be associated with Al resistance. A total of five root traits (elongation rate of the primary root, total root length, root biomass, average root diameter and specific root length) were measured using a simple nutrient solution with or without Al (20 µM) over a 48 hour growth period with the experiments conducted in five replicates. We found that genotypes from the Andean gene pool were more resistant to Al than Mesoamerican genotypes under these conditions, based on a smaller decrease in the elongation rate of the primary root, total root length, specific root length and a lesser increase in root diameter in the presence of Al in nutrient solution. These root traits but not root biomass can serve as selection criteria to distinguish between Al-resistant and Al-sensitive genotypes.

Aluminum (Al) toxicity is a major limiting factor of crop production in acid soils, which are found mostly in developing countries of the tropics and sub-tropics. Common bean (Phaseolus vulgaris L.) is particularly sensitive to Al toxicity; and development of genotypes with better root growth in Al-toxic soils is a priority. The objectives of the present study were to physiologically assess root architectural traits in a recombinant inbred line (RIL) population of common bean that contrasts for Al resistance (DOR364 x G19833) and to identify quantitative trait loci (QTL) controlling root growth under two nutrient solutions, one with 20 microM Al concentration and the other without Al, both at pH 4.5. A total of 24 QTL were found through composite interval mapping analysis, 9 for traits under Al treatment, 8 for traits under control treatment, and 7 for relative traits. Root characteristics expressed under Al treatment were found to be under polygenic control, and some QTL were identified at the same location as QTL for tolerance to low phosphorous stress, thus, suggesting cross-links in genetic control of adaptation of common bean to different abiotic stresses.