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
A major constraint to dry edible and snap bean (Phaseolus vulgaris L.) production worldwide is root rot, one form of which is caused by Fusarium solani f. sp. phaseoli (Burk.) Snyd. & Hans (FSP). Sources of resistance to this pathogen exist in P. vulgaris, and, in the current paper, we studied the inheritance of one such source, FR266, using two recombinant inbred populations, MF and IF, derived from crosses of susceptible cultivars Montcalm (M) and Isles (I) with FR266 (F). Random amplified polymorphic DNA (RAPD) markers, associated with quantitative trait loci (QTL) controlling resistance to Fusarium root rot, also were identified. Genetic resistance to FSP, originally derived from PI 203958, was polygenically controlled and strongly influenced by environmental factors. Heritability estimates (h2) were moderate and ranged from 0.48 to 0.71 for MF population. Several RAPD markers were identified that demonstrated significant associations with resistance to FSP determined from both greenhouse and field evaluations. Markers associated with field ratings tended not to be associated with greenhouse ratings and vice versa, except for the P700 marker which was significantly associated with both greenhouse and field data. Individual markers identified in this study did not explain more than 15% of the phenotypic variation for root rot resistance, whereas a combination of four markers explained 29% of the phenotypic variation for root rot ratings in the field. The two regions of the bean genome associated with root rot resistance corresponded to loci controlling the Pv pathogenesis‐related proteins (PvPR). Mechanisms associated with host defense responses may be involved in resistance to FSP and selection directed towards enhancing these traits may allow for rapid improvement of resistance to Fusarium root rot in bean.
Breeding for a quantitative trait like drought resistance would be facilitated by the development of a method of marker-assisted selection (MAS) that is capable of identifying high performing genotypes in early generations. Two recombinant inbred populations were grown from 1990 to 1994 at eight locations in Michigan and Mexico under stress and nonstress conditions to identify random amplified polymorphic DNA (RAPD) markers associated with drought resistance in common bean (Phaseolus vuigaris L). Six-hundred random decamer primers were screened against the parents of the two populations, 50% of which were polymorphic. Seventy of these polymorphic primers were screened against each population. Using one-way analysis of variance and multiple regression, four RAPD markers were identified in one population and five in another that were consistently and significantly associated with yield under stress, yield under nonstress, and/or geometric mean yield across a broad range of environments. To test the effectiveness of MAS for drought resistance, markers were used to select genotypes from either extreme. Yield data from three locations were examined to evaluate the effectiveness of these markerbased selections. Marker-assisted selection in the Sierra/AC1028 population was found to be effective in Michigan under severe stress and ineffective in Mexico under moderate stress. The five RAPD markers used for MAS in the Sierra/Lef-2RB population improved performance 11% under stress and 8% under nonstress, whereas conventional selection based on yield performance failed to increase performance. Response to conventional selection was three times greater in the Sierra/ AC1028 population, supporting the conclusion that the effectiveness of MAS is inversely proportional to the heritability of the trait under examination.
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