Varietal Differences in Physiology of Drought Stressed Phaseolus

Common bean (Phaseolus vulgaris) is the world’s most important directly consumed food legume; however, in many areas of the world bean production is drought limited, and this is a particular food security problem in the developing world.  In an effort to improve the efficiencies of breeding for drought tolerance, we have compared various drought tolerance mechanisms in a broad range of Phaseolus genotypes from Andean, Mesoamerican, and Sonoran populations, from both P. vulgaris (common bean) and P. acutifolius (tepary bean), that are known to exhibit significant differences in drought tolerance in field trials. Mesoamerican and tepary bean genotypes are commonly recognized as generally more drought tolerant and more productive under moderate levels of stress when compared to Andean populations.  Although these differences have often been attributed to root architecture, our studies of gas exchange, fluorescence parameters, and growth data show that under root limited conditions there are major differences related to several shoot factors, including heat tolerance, photosynthetic capacity, stomatal behavior and conductance.  Some genotypes, for example, show evidence of osmoregulation (accumulation of several compatible solutes), particularly when water stressed allowing for the maintenance of lower leaf water potentials.  Other characteristics include more rapid stomatal closure when stressed and higher water use efficiencies, i.e., the ability to maintain photosynthesis as transpiration is decreasing.  A/Ci analyses also show that the higher photosynthetic rates under both stress and well watered conditions are primarily due to differences in the generation of ribulose bisphosphate for the Calvin-Benson cycle.  These results suggest several productive strategies to improving drought tolerance in common bean.