Variation in Root Traits Among Wild and Domesticated Common Beans (Phaseolus vulgaris L.) of the Mesoamerican Gene Pool.

As a result of genetic bottlenecks during domestication and ongoing post-domestication natural selection and isolation, useful variation in the wild germplasm has not been integrated into the domesticated gene pool. Transgressive segregation for higher yield has been found in wild by domesticated bi-parental recombinants beans suggesting its potential in a domesticated background. Mesoamerican wild are currently distributed from Colombia to northern Mexico, from 500 to 1,900 meters above sea level and with 500 to 2,000 mm of annual rainfall. With the exception of the populations in Colombia and parts of Central America, wild common beans receive relatively low and erratic rainfall throughout their developmental cycle and experience drought stress during seed maturation. Hence, drought adaptation could be a driver in local adaptation in wild beans.  To assess the variation in drought adaptation in wild beans, 115 accessions from the range of its Mesoamerican distribution and 11 domesticated cultivars were planted in soil:sand-filled clear plastic tubes and under full irrigation vs. continuous soil drying in greenhouse conditions for 30 days. Using a randomized complete block design with 3 repetitions, differences in root architecture, dry matter productivity, leaf area, specific leaf area, and water use were measured. Compared to domesticated beans, wild beans had lower root depth, water use efficiency and proportion of roots to total biomass (PRTB), but higher specific leaf area. For wild beans, genotype and treatment effect for final root depth were significant but not their interaction, but interaction was significant for PRTB. A comparison with the annual rainfall of the origin of the accessions showed there was a significant negative relationship with root depth but positive with PRTB. Overall, these results suggest that wild beans from drier areas spend relatively less biomass in roots, but have a deeper root system.