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Drought is the second major constraint to common bean (Phaseolus vulga~s L) production after disease. This study examined yield under drought, yield potential, drought susceptibility index, harvest index, and geometric mean as potential indicators of drought resistant genotypes. The performance of two common bean populations, consisting of 78 and 95 recombinant inbred lines, was examined under moisture stress and nonstress regimes. Experiments were conducted at seven locations (1990-1994) in Michigan and Mexico to identify effective selection criteria for drought resistance. Two genotypes from each population yielded in the top 10% under both stress and nonstress conditions. Heritability estimates for yield in the Sierra/AC1028 population, based on 5 yr of data, ranged from 0.55 to 0.59 for stress and nonstress, respectively, and from 0.20 to 0.19 for stress and nonstress, respectively, in the Sierra/Lef-2RB population. Heritability for plant biomass was 0.52 for stress and 0.55 for nonstress in the Sierra/ AC1028 population and 0.15 under stress and 0.05 under nonstress in the Sierra/Lef-2RB population. One-hundred seed weight was the most highly heritable trait in both populations with heritability estimates of 0.80 for the Sierra/AC1028 population and 0.65 for the Sierra/Lef-2RB population. The geometric mean of the two moisture regimes was the single strongest indicator of performance under stress and nonstress, and a breeding strategy that involves selection based first on the geometric mean, followed by selection based on yield under stress, was suggested as the most effective strategy to improve drought resistance in common bean. S IXTY PERCENT of common bean production worldwide is grown under water stress, making drought the second largest contributor to yield reduction after disease (Singh, 1995). One of the largest production areas the world is the Mexican highlands (1800-2200 masl), where more than one million hectares of common bean are planted annually. Ninety-eight percent of this region is subjected to intermittent rainfall, and much of this area does not receive sufficient moisture for optimum performance (annual precipitation 200-400 mm). Drought in this region is categorized as intermittent stress where rainfall and/or drought can occur at any time during the growing season. This type of stress is typical of the semiarid tropics (Ludlow and Muchow, 1990). However, drought stress in the semiarid highlands is not aggravated by high temperatures inherent to the tropics (Acosta-G and White, 1995). Common bean is well

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Common bean diversity has been classified into two major gene pools, Middle American and Andean. Each Landraces and bean (Phaseolus vulgaris L.) cultivars grown in pool can be further subdivided into three races (Singh Mexico are diverse, as are consumer preferences and agroecological et al., 1991b). The main races grown in Mexico are the production environments. Mexican common bean cultivars were analyzed using amplified fragment length polymorphism (AFLP) finger-Middle American: Mesoamerica, Jalisco, and Durango, printing to examine the genetic relationships within and among races, as well as the Nueva Granada race from the Andean based on the genotyping of 112 bred cultivars developed in Mexico.pool. This last race was probably introduced into Mexico Molecular analysis of dry bean germplasm will be useful to corroborate in pre-Columbian times. previous cultivar characterizations and establish the genetic basis of The knowledge of genetic diversity patterns can inimproved germplasm, to facilitate the use of that diversity, and to crease the efficiency for conservation, utilization, and implement the use of markers in selection. Germplasm included 111 genetic improvement of common bean (Beebe et al., cultivars belonging to Mesoamerica (25), Jalisco (39), Durango (28), 2000; Rosales-Serna et al., 2003). Different methodoloand Nueva Granada (19) races, which are commonly cultivated throughgies and traits, such as morphological (Cá rdenas, 1984; out the bean-producing areas of Mexico. A Mexican P. coccineus Singh et al., 1991a; Rosales-Serna et al., 2003), biochemispecies cultivar (Blanco Tlaxcala) was also included for comparison. Broad genetic diversity was found within bean races, and diversity cal (Singh et al., 1991a, 1991c), and molecular (Beebe values between races were similar. Most of the Nueva Granada germet al., 2000; Metais et al., 2000; Rosales-Serna et al., plasm was clearly different from that of all other races, whereas the 2003) have been suggested for the evaluation of genetic P. coccineus cultivar was distinct from all P. vulgaris cultivars. A diversity in common bean. Bean cultivars have also been dendrogram based on the AFLP analysis did not clearly match with classified by seed type, growth habit, morphology, phethat made on the basis of racial classification. This mismatch was nology, and reaction to photoperiod (Voysest, 2000; Roprobably due to genetic recombination between Andean (Nueva Grasales-Serna et al., 2003). Molecular markers are useful nada) and Mesoamerican (Jalisco, Durango, and Mesoamerica) genefor cultivar identification, due to the fact that they are pools. Utilization of contrasting parents for specific crosses has also not influenced by variable environmental conditions or contributed to broadening the genetic basis of common bean.

A core collection of the common bean (Phaseolus vulgaris L.), representing genetic diversity in the entire Mexican holding, is kept at the INIFAP (Instituto Nacional de Investigaciones Forestales, Agricolas y Pecuarias, Mexico) Germplasm Bank. After evaluation, the genetic structure of this collection (200 accessions) was compared with that of landraces from the states of Oaxaca, Chiapas and Veracruz (10 genotypes from each), as well as a further 10 cultivars, by means of four amplified fragment length polymorphisms (AFLP) +3/+3 primer combinations and seven simple sequence repeats (SSR) loci, in order to define genetic diversity, variability and mutual relationships. Data underwent cluster (UPGMA) and molecular variance (AMOVA) analyses. AFLP analysis produced 530 bands (88.5% polymorphic) while SSR primers amplified 174 alleles, all polymorphic (8.2 alleles per locus). AFLP indicated that the highest genetic diversity was to be found in ten commercial-seed classes from two major groups of accessions from Central Mexico and Chiapas, which seems to be an important center of diversity in the south. A third group included genotypes from Nueva Granada, Mesoamerica, Jalisco and Durango races. Here, SSR analysis indicated a reduced number of shared haplotypes among accessions, whereas the highest genetic components of AMOVA variation were found within accessions. Genetic diversity observed in the common-bean core collection represents an important sample of the total Phaseolus genetic variability at the main Germplasm Bank of INIFAP. Molecular marker strategies could contribute to a better understanding of the genetic structure of the core collection as well as to its improvement and validation.