Genetic analysis of common bean (Phaseolus vulgaris L.) genotypes for tolerance to drought and heat stress in Zambia.
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Common bean (Phaseolus vulgaris L.) is widely grown and consumed in Zambia but its production is limited by drought and high temperature stresses. In the country there is limited information on farmers’ preferences and the genetics of drought and heat tolerance of common bean for breeding to enhance its production. Therefore, the objectives of this study were to: 1) determine farmers’ selection criteria and preferences for common bean varieties, 2) evaluate the Zambian bean germplasm for drought and heat tolerance, and 3) determine the genetic effects and inheritance of drought and heat stress tolerance in Zambian common bean germplasm. A participatory rural appraisal and survey studies established that the preferred bean varieties by the farmers in Siavonga (Lusitu) and Gwembe districts were of determinate bush type growth habit, red speckled seeds, large and elongated seed shape, early maturing, and prolific with high numbers of pods per plant. Lyambai, a red speckled seed variety, was chosen as their most preferred type meeting most of their selection criteria. It was also established that women were better able to distinguish between common bean varieties in terms of taste and cooking time than men. It was further established that the educated farmers based their variety selections on a background understanding of varietal characteristics. A screening study involving 120 common bean genotypes identified LY4-4-4-B as the most drought tolerant genotype followed by LY1-2-B, ZM 3831, KAL–ZA, SCCI 13, ZM 4512-5 and LYA–ZA based on yield and yield related traits. Two genotypes, a mutant, LY4-4-4-B and a landrace, ZM 3831 were selected among the most drought tolerant genotypes for developing F1 populations used in the genetic study. This study also established that 100-seed weight was not affected by drought stress, probably due to the compensatory effects of reduced numbers of pods per plant and number of seeds per pod. The evaluation of the 120 genotypes under elevated temperatures (>33oC) established that ZM 4143, ZM 4497, SCCI 4, KE 1, and ZM 07, were more tolerant to heat stress. ZM 4143 and ZM 4497 were further selected among the most heat tolerant genotypes for developing F1 populations used in the genetic study. The significant (P≤0.05) GCA effects for 14 parental lines for yield, number of seeds pod-1, and number of pods plant-1 indicated that additive gene effects were important in the inheritance of these traits under heat stress. The significant (P≤0.05) and positive SCA effects for the F2 families of Kapisha X SEN 39, Kapisha X ZM 4497, Kalungu X SEN 39, and Lyambai X ZM 4143 were generated from parental lines with high and positive GCA values indicating their potential for further selection for high temperature tolerance from these populations. Further genetic studies on drought tolerance for the 14 parents and the 48 F2 populations established that ZM 4143 and ZM3831 were drought tolerant male parents with significant (P≤0.05) and high positive GCA effects. The crosses with high SCA values for the F2 population emanating from Chambeshi X ZM 4143, Pan 148 X ZM 4143, Lyambai X SER 124, Chambeshi X ZM 3831, SCCI 2 X Ly 4 -4-4-B, ZM 05 X SER 124 and Lyambai and ZM 3831 had parents with high and positive GCA effects indicating potential for making further selections for drought tolerant genotypes. The high heritability estimate for yield of 60% found in drought stressed conditions also indicated that breeders can make progress in breeding for drought tolerance.