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Breeding for Drought Resistance in Common Bean: In Vitro Assay for Root Osmotic Potential

ABSTRACT Initial and adjusted osmotic potentials of separate leaf- and root-derived calli were assessed in vitro for two teparies (Phaseolus acutifolius A. Czray) and a common bean (P. vulgaris L.) cultivar differing in physiological and morpho-physiological reactions of intact plants to water stress. Unstressed leaf- and root-derived calli of common bean (CB) had higher preexisting osmotic potential (ψs) and relative growth rate (RGR) but lower dry-matter percent (DM) than those of tepary bean (TB) lines when grown on basal Murashige and Skoog medium (–0.30 MPa). Exposure to water stress (−0.58 MPa) imposed by adding polyethylene glycol (10% [w/v], PEG-10,000) did not affect these parameters for leaf-derived callus of CB and both leaf- and root-derived calli of TB. However, root-derived callus of CB showed decreased ψs that was maintained after transferring stressed callus onto medium lacking PEG for recovery from water stress. Nevertheless, its RGR decreased and DM increased. Stressed plants of both CB and TB grown in the greenhouse manifested significant reduction in leaf, stem, and root dry mass and leaf area. However, the magnitude of mass reduction was especially high for CB stem, leaf, and root. While TB root system penetrated deeper in the soil profile, CB root depth did not significantly change in response to drought. In addition, substantial CB root mass was detected in the top 10 to 20 cm of the soil profile. Stomata conductance and transpiration rate were higher for CB than TB under both well-watered and water-stress conditions. Significant relative water content reduction was detected only in CB. In spite of water stress, soil moisture was high around roots of CB compared with TB, suggesting lesser efficiency of CB root system to extract water. Stressed CB grown in the production field did not produce seed yield whereas TB gave 60 to 64% of its potential dry seed yield. In vitro assay for root osmotic potential should be useful in improving drought resistance in common bean.