No abstract

Common bean (Phaseolus vulgaris L.) is a nutritious crop grown around the world, a staple that provides high levels of protein and iron in the diets of Central and South Americans and East Africans. Heat stress negatively affects common bean seed yields and prevents cultivation in certain areas. Furthermore, under field conditions, heat stress often coincides with and exacerbates drought stress effects. Breeding more heat‐tolerant cultivars would stabilize seed yield and open new regions to field production. To support these efforts, we examined a variety of methods for screening large numbers of bean germplasm exposed to heat stress at the vegetative growth stage as opposed to the reproductive stage, which would prolong the screening process. Tepary bean (P. acutifolius A. Gray), a closely related species to common bean, was used as a heat‐stress‐tolerant check. Plants exposed to day/night temperatures of 45/36°C for 2 d showed measurable signs of heat stress, but tepary bean outperformed the common beans on all stress tolerance measures. Gas exchange, chlorophyll fluorescence, and oxidative stress were only affected by this high temperature and not by temperatures below 45/36°C. Heat stress measurements also correlated well with visual signs of leaf tissue damage. Gradually raising temperatures was useful for screening large number of entries for heat tolerance, but this heat tolerance was only partially related to drought tolerance in the field. Plant breeders can use some of these methods to supplement field data and to further characterize the stress tolerance of bean lines.

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.