Inheritance of tolerance to intermittent drought stress in selected common bean genotypes.
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In Uganda, common bean (Phaseolus vulgaris L.) is an important source of food and income generation for all categories of people. It also forms the main source of dietary protein for the poor. Bean is also very essential for feeding by relief agencies and in institutions. Despite its importance, however, bean production in Uganda is being affected by drought which has resulted from recent variability in climate. The effects are especially profound among small scale producers living in drought-prone areas, who cannot afford to apply irrigation. Developing high-yielding and drought-tolerant bean cultivars would, thus, significantly contribute towards increased and stable yields in drought-prone environments. Breeding for drought tolerance in bean in Uganda is still in its infancy. There was, therefore, need to generate knowledge on inheritance of drought tolerance, in the genotypes relevant to Uganda, in order to enhance further breeding efforts. As a result, this study was conducted to determine the level of drought tolerance in selected bean genotypes and to establish the mechanism of inheritance of drought tolerance in crosses of selected Ugandan bean genotypes with drought-tolerant genotypes from the International Center for Tropical Agriculture (CIAT). The study was conducted in Uganda at the National Crops Resources Research Institute (NaCR-RI) in Namulonge. Eight bean genotypes were used, 5 previously characterized by CIAT as tolerant to drought (SEN 98, SEN 99, SCR 48, SCN 6 and SCN 9) and 3 genotypes that are sensitive to drought but preferred by the market in Uganda (NABE 15, NABE 4, and K132). Parental genotypes were screened for two seasons under screen house conditions in a split plot design with 4 replications in each of the three screen houses used. Four watering regimes includ-ing: daily watering, 6 days between watering (DBW), 9DBW, and 12DBW for season one; and daily watering, 3DBW, 5DBW and 7DBW for season two were used. In each season, the three water-stressed treatments were fully watered until 14 days after planting, and thereafter were supplied with a pre-determined amount of water in late morning on the appropriate days. The non water-stressed treatment was irrigated daily throughout the growth cycle. After screening, F1 hybrids were obtained from a 3 x 5 North Carolina II mating design adapted to include reciproc-als. These hybrids were advanced to F2 and phenotyped alongside parents under well watered and water stress conditions in the screen house. Data were collected on yield and potential physiological indicators of drought stress that were used to give insights on response of geno-types to drought stress. These included: primary leaf lamina drooping, leaf roll, relative leaf water content, leaf chlorophyll content, dry seed weight and seed number per pod, dry pod weight and pod number per plant, total shoot biomass and root spread among others. Data were subjected to analyses of variance and correlation analyses using the GenStat computer package. Combining ability and various measures of ratios were also calculated to establish the mechanism of inher-itance of drought tolerance in the crosses. Three of the five drought tolerant lines obtained from CIAT, namely; SEN 98, SCR48 and SEN 99 performed best in pod partitioning index and pod harvest index. These lines thus expressed greater ability to mobilize photosynthates to grain under drought stress and recorded high geometric mean seed yields. Of the local genotypes, NABE 15 performed best in most measured variables, emerging similar to SEN 98, SCR48 and SEN 99 in pod partitioning index, pod harvest index and harvest index. In addition, from the evaluation of F2 hybrid individuals alongside parents, the roles of both additive and non-additive gene effects were apparent in most measured parameters. However, additive genes seem to play a predominant role for most yield and associated variables implying that selection could be done in early generation for seed yield, dry pod weight, seed number per pod, pod number per plant, number of trifoliate leaves and primary leaf lamina drooping. Leaf rolling and leaf chlorophyll content in which high non-additive gene effects were recorded would require late selection. From the findings, it is also possible to conclude that SEN 98, SEN 99 and SCR 48 are good donors for drought tolerance genes and could be used in the breeding program. In addition, the findings revealed that the genotypes SEN 99 and NABE 15 are good combiners.