Drought and low soil fertility are primary constraints to food security in developing countries. Root traits have been identified that enhance water and phosphorus acquisition, but these traits may increase interplant competition. Multilines (genetic mixtures) of contrasting root architectures may have better drought tolerance, low fertility tolerance, and yield stability than would genetic monocultures. The overall goal of this project was to test the hypothesis that multilines of common bean genotypes (Phaseolus vulgaris) having shallow and deep root architectures are more tolerant of drought and low fertility than are genetic monocultures. Specifically, the following hypotheses were investigated: 1) multilines will be better able to exploit soil resources by occupying divergent soil niches (i.e. topsoil vs. subsoil) 2) multilines will be more resilient to stress treatments by yield compensation between drought-tolerant and low fertility-tolerant genotypes 3) in multilines, shallow-rooted genotypes will benefit from the proximity of deep-rooted neighbors through the phenomenon of ‘hydraulic lift', whereby deep roots transport water to the topsoil at night.

The main field study was conducted at Zamorano University, Honduras, with drought, low phosphorus (P), combined drought and low P, and unstressed plots. Six satellite plots in Honduras representing low fertility, drought-prone environments were also used. Genetic treatments included three combinations of a multiline of shallow and deep-rooted bean genotypes, and the corresponding single deep or shallow rooted lines. Shoot biomass, P uptake, shoot water status, soil moisture content with depth, and root distribution with depth were monitored over time in each treatment. Molecular markers were used to determine the success of each genotype of the multiline in terms of seed set and root length density.