DROUGHT STRESS Evaluation of Common Bean for Drought Tolerance in Juana Diaz, Puerto Rico

Common bean (Phaseolus vulgaris L.) is the most important food legume (Broughton et al. 2003); however, drought stress (DS) results in significant seed yield reductions in 60 % of global bean production areas (White and Singh 1991). Increasing competition for production area has resulted in a move of bean acreage to more marginal zones often associated with increased abiotic stress (Porch et al. 2008). The majority of common beans are produced in the Midwestern and Western areas of the US, while DS impacts over 50 % of the cropped area in the Western US (Cook et al. 2004). Considering mounting pressure on limited water resources, and additional factors such as climate change and increasing household water use, water threatens to become a progressively scarce resource. Given the high consumption rates of water by agriculture, constraints on water resources can be mitigated through the genetic improvement of DS tolerance in crop species. Genetic improvement, based on using existing genetic variability in the species, is founded on the evaluation and selection of drought tolerant germplasm and on the understanding of the physiological and genetic responses to abiotic stress. The duration and severity of DS in common bean determines the level of seed yield reduction (Singh 1995). A number of indirect techniques have been used for the evaluation of drought tolerance; however, seed yield is the most reliable indicator because it directly represents the harvestable product (White et al. 1994a,b, Ramirez-Vallejo and Kelly 1998). Seed yield reductions due to drought are further increased as a result of interactions with other sources of stress, such as high temperature, disease and soil conditions (Ramirez-Vallejo and Kelly 1998). The effects of diseases are important under DS, such as root rot caused by Fusarium solani, Pythium ultimum and Rhizoctonia solani, as well as ashy stem blight, caused by Macrophomina phaseolina. Other abiotic stresses, such as high temperature