Early Metabolic and Photosynthetic Responses to Drought Stress in Common and Tepary Bean

Common bean (Phaseolus vulgaris L.) is an important food crop, especially in East Africa and Central and South America. Subsistence farmer yields in these areas are limited by the many common bean cultivars that are drought sensitive. Consequently, a major goal of breeding programs is to improve drought tolerance. Our objective was to identify and compare physiological components of drought stress among common bean genotypes varying in stress tolerance. We also included the closely related and highly stress-tolerant tepary bean (P. acutifolius A. Gray). Compositional parameters often related to drought tolerance revealed no drought effect on free proline, but malate, glucose, fructose, inositol, and raffinose all increased, sometimes enough to osmotically adjust leaf tissues. Abscisic acid (ABA) was especially drought responsive. Although genotypic differences were unrelated to tolerance, grafting revealed that shoot identity controlled ABA levels in stressed roots and that root identity had little or no effect on stomatal behavior. Photosynthetic parameters derived from photosynthesis versus intercellular CO2 concentration curves were not related to drought tolerance, but some traits were related to productivity. Stomatal control and low conductance were clearly associated with tolerance, conserving water during stress, and increasing water use efficiencies. This study should provide guidance in selection of traits associated with enhanced drought tolerance in common bean.