Selection for drought resistance in common beans has improved yields under both drought stress and favorable conditions for both Mesoamerican and Andean beans. Drought-selected lines of Mesoamerican beans significantly outyielded commercial checks under drought stress and also yielded significantly higher than checks under irrigated conditions. Similarly, drought-selected lines of Andean beans yielded significantly more than checks under drought stress in multiple environments and also yielded well under irrigation with no stress. Selection for drought resistance has provided genetic gains for improved yield potential and stress tolerance in common beans.

1. Breeding for Drought Resistance Improves Yield Potential in both Mesoamerican and Andean beans Yield of Andean beans: Y ields of SAB lines in four environments are summarized in Figure 1. Each lattice is represented as a column of data points with the check genotypes highlighted as diamonds. For most of the trials (columns A through E), some of the advanced lines were significantly superior to the controls (AFR298, CALIMA, COS16 and ABA36) at p=0.05. For example, in Darién SAB 663 with an average yield of 2047 kg/ha was significantly higher yielding than the ‘Calima’ check with 1472 kg/ha (see column B). Results confirm the genetic gain for drought resistance and yield potential in the SAB lines compared to both their drought-resistant and susceptible parents. In addition, certain SAB lines can be selected with greater stability across mid-elevation and lower-elevation sites based on this analysis. The most promising series of lines appear to be those of the large-red and cream-mottled commercial classes. Seed size is commercial and most advanced lines are early maturing. Conclusions: Selection for drought resistance has resulted in improved yield both under drought stress and in favorable conditions. This was first observed in Mesoamerican beans, and now this result can be extended to Andean types. We are investigating the physiological basis of this yield advantage, but we speculate that it is associated with improved photosynthate remobilization to grain under different environmental conditions. The prospect of improving yield potential of bean is exciting; especially in the Andean types in which yield improvement has been a major challenge. Populations of Mesoamerican and Andean beans were created from drought resistant parents and were selected in F2 and F5 generations in the drought nurseries in CIAT, Palmira, Colombia (996 masl; 26 ° C average; Mollisol soil with estimated storage capacity of 130 mm available water). Many Mesoamerican populations included SEA 15, a Durango derivative, in their pedigree. Triple crosses of Andean types were developed between: drought-resistant ‘ICA Quimbaya’; commercial genotypes ABA36, ABA58 and COS16; and drought sources (SAB258 and SAB259) that were derived from multiple crosses including a Durango source. Irrigations were timed to submit the crop to terminal drought initiating at about 25-30 days after planting, but intermittent rainfall at times interrupted the drought stress. Drought evaluation in advanced lines was carried out under a similar moisture regime. A total of 362 F6.8 and F5.7 Mesoamerican lines were evaluated under drought in 2005 to confirm drought resistance. Lines were organized by color class (red, black and beige) into eleven yield trials in lattice design (5x5 or 7x7) with 3 replications. Commercial checks by color class were the cultivars ‘Tio Canela’ (red seeded), ‘DOR 390’ (black seeded), and ‘Pérola’ (beige). Andean SAB lines were derived in the red mottled, cream mottled, large-red and large white commercial classes. These were tested in CIAT-Palmira for drought in 2007 and 2008, and in Darién (Valle del Cauca, Colombia; 1500 masl; 19°C average) during the July-August 2008 dry season. Yield trials were organized by color class as 5 lattice design experiments (6x6 or 5x5) with 3 replications each. Checks included: AFR298 (=ICA Quimbaya; large-red), COS16 (cream mottled), ABA36 (large-white), SAB560 (large red) and the local commercial variety Calima (red mottled). Additionally, we wished to know if drought selected lines would respond to favorable environments. The same Andean trials were planted in Palmira in 2008 under irrigation, with no significant stress. Ninety-four elite Mesoamerican drought lines from the 2005 trials were organized in three trials in 6 x 6 lattice design and planted in Palmira (Mollisol), Popayán (Andisol) and Santander de Quilichao (Oxisol) in the 2005 September-October planting season without stress. Common checks were included in all three trials to facilitate comparison across trials. Materials and Methods Results and Conclusions Yield of Mesoamerican beans: Table 1 presents absolute yields with and without stress and also as a percent of check yields. Yield under drought improved significantly compared to commercial checks in three color classes of Mesoamerican bean. In favored conditions several lines significantly out-yielded the respective checks in the red and Brazilian grain classes (Table 1). Yield gains were registered in materials with similar or even earlier maturity, resulting in greater yield/day. The fact that yield increased in earlier materials suggests that selection for drought has resulted in greater plant efficiency. Pedigrees of many advanced lines included breeding line SEA 15 which was derived from races Durango and Mesoamerica of the Middle American gene pool. Thus, interracial combinations continue to be productive in breeding small seeded common bean for drought stress tolerance. Table 1. Yields of drought-selected Mesoamerican lines under drought in 2005, and in three favorable environments in 2006. 1 *, **: Significantly higher yielding, or significantly earlier to mature, than the respective check variety at p=0.05 or p=0.01 level of significance. Checks are ‘Tio Canela’(red), DOR 390 (black), and ‘Pérola’ (cream). Grain filling differences between drought susceptible (left) and resistant (right) lines Fig. 1. Yield of drought-selected Andean beans in 4 trials. Darién: Moderate drought, 2008 Palmira: Intermittent Drought, 2008 Palmira: Intermittent Drought, 2007 Palmira: Irrigated, 2008 Acosta-Gallegos, J.A., E. Acosta, S. Padilla, M.A. Goytia, R. Rosales, and E. López. 1999. Mejoramiento de la resistencia a la sequía del frijol común en México. Agron. Mesoam. 10:83-90. Bänziger, M., G. O. Edmeades, and H. R. Lafitte. 1999. Selection for drought tolerance increases maize yields across a range of nitrogen levels. Crop Sci 39: 1035-1040. Beebe, S., P.W. Skroch, J. Tohme, M.C. Duque, F. Pedraza, and J. Nienhuis. 2000. Structure of genetic diversity among common bean landraces of Mesoamerican origin based on Correspondence Analysis of RAPD. Crop Sci. 40:264-273. Blum, A. 2005. Drought resistance, water use efficiency, and yield potential –are they compatible, dissonant, or mutually exclusive? Aust. J. Agric. Res. 56:1159-1168. Singh S.P., P. Gepts, and D.G. Debouck, 1991. Races of common bean ( Phaseolus vulgaris , Fabaceae). Econ. Bot. 45:379-396. Specht, J.E., D.J. Hume and S.V. Kumudini. 1999. Soybean yield potential – a genetic and physiological perspective. Crop Sci. 39:1560-1570. Teran, H., and S.P. Singh. 2002. Comparison of sources and lines selected for drought resistance in common bean. Crop Sci. 42: 64-70. Thung, M., and I. M. Rao. 1999. Integrated management of abiotic stresses. In: S.P. Singh, ed. Common bean improvement in the twenty-first century. Dordrecht, The Netherlands: Kluwer Academic Publishers. Pp 331-370. Tollenaar, M., and J. Wu. 1999. Yield improvement in temperate maize is attributable to greater stress tolerance. Crop Sci. 39:1597-1604. Wortmann, C. S., R. A. Kirkby, C. A. Eledu and D. J. Allen. 1998. Atlas of common bean ( Phaseolus vulgaris L.) production in Africa. pp 133. CIAT, Cali, Colombia. References S. Beebe, I. Rao, M. W. Blair, M. Grajales, C. Cajiao and F. Monserrate Improved Beans for the Developing World Acknowledgements The authors gratefully acknowledge the partial financial support of Bundesministerium für Wirtschaftliche Zusammenarbeit und Entwicklung (BMZ) of the German Federal Government, and the Bill and Melinda Gates Foundation through the Tropical Legume I and II projects that made possible the research reported here. Drought stressed Unstressed (ave. 3 sites) Color Line Yield, kg ha -1 (% check) 1 Yield / day Yield, kg ha -1 (% check) Yield / day Days to maturity Red SER 43 1589** (250) 24.9** 2127** (123) 28.1* 76 SER 48 1607** (253) 26.8** 2220** (128) 31.9** 70** SER 51 946** (210) 15.1** 2082* (120) 29.8** 72** SER 94 1196** (230) 19.9** 2893** (119) 41.0** 72** SER 113 1025** (295) 15.9** 3009** (124) 42.3** 73** SER 118 1534** (241) 24.6** 2824** (116) 39.8** 74** Black NCB 226 1240** (244) 18.7** 2265 (105) 30.0 75** SEN 36 712* (384) 11.1* 2805* (115) 38.7* 76** SEN 38 1055** (570) 17.2** 2477 (102) 36.5 72** SEN 43 974** (526) 16.3** 2537 (104) 37.8* 71** SEN 52 908** (490) 15.5** 2606 (107) 39.6** 70** SEN 56 1191** (643) 20.3** 2618 (107) 38.9** 71** Beige SXB 398 1022** (549) 15.0** 2606* (119) 37.2** 73** SXB 403 1217** (654) 18.5** 2720** (124) 37.3** 74** SXB 410 937** (503) 14.3** 2738** (125) 36.4** 78 SXB 412 1283** (689) 20.2** 2471 (112) 35.5* 75* SXB 416 1125** (604) 17.5** 2587* (118) 35.1* 76 SXB 418 1121** (602) 17.3** 2675** (122) 37.0** 75* Drought is the single most important factor affecting food security in tropical developing countries. As much as 60% of the bean crop is cultivated under the risk of drought (Thung and Rao, 1999). Drought is endemic in bean producing areas of highland Mexico, Central America, northeast Brazil, and much of eastern and southern Africa. In Africa as much as 300,000 MT of beans are lost to drought annually (Wortmann et al ., 1998). Another question is the relationship of stress resistance to yield potential under favored conditions (Blum, 2005). Bänziger et al (1999) found that selection for tolerance to midseason drought stress increased maize yields in four lowland tropical maize populations. Yields of corn and soybeans in the United States have increased as a result of tolerance to high plant densities (Specht, 1999; Tollenaar and Wu, 1999), which is an expression of stress tolerance. Thus, yield potential in stressed and in non-stressed environments may not be mutually exclusive. Cultivated common bean has two major gene pools and several races within pools (Beebe et al, 2000; Singh et al, 1991). Mesoamerican race Durango from dryland Mexico has been an important source of useful drought resistance genes (Acosta et al, 1999; Terán and Singh, 2002b). This presentation reports on the results of breeding for drought resistance in both Mesoamerican and Andean beans, under drought and favorable conditions. Introduction