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
To assess genetic and environmental variation for outcrossing in common bean (Phaseolus vulgaris L.), a bulk of nine purplehypocotyled (dominant), black-seeded, common bean lines (pollen parents) and six green-hypocotyled (recessive), white-seeded, common bean lines (seed parents) representing Mexican germplasm were used to estimate outcrossing rate. The experiment was conducted at Irvine and Riverside, CA, in mid-May and mid-July of 1989 and 1991. Outcrossing estimates were based on the proportion of purplehypocotyled progeny from green-hypocotyled parents. Approximately 1400 single-plant families were progeny tested in which 120 278 seedlings were scored for the presence of anthocyanin in the hypocotyl. Of these, 113 246 were classified as selfed progeny and 7031 as hybrid progeny, giving rise to a weighted mean outcrossing rate of 6.9%. Entire progenies from each maternal line were grown and scored, and the minimum and maximum rates of outcrossing observed ranged between 0.0 and 78.0%. Mean outcrossing rate for the six white-seeded lines ranged from 4.41 to 10.16%. However, differences in outcrossing rate among these lines were not significant, nor were differences between dates of planting, locations, or years. The combined analysis of variance showed significant location x planting date x line interaction and year x location x line interaction, indicating that environmental factors had a strong effect on rate of outcrossing in the lines examined. K NOWLEDGE about the breeding system of a crop is important to design effective breeding schemes to improve that crop. Common bean was listed as a predomi
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