GENETIC IMPROVEMENT OF COMMON BEAN (Phaseolus vulgaris L.) FOR TOLERANCE TO LOW SOIL MOISTURE AND PHOSPHORUS
Common bean (Phaseolus vulgaris L.) is a cheap source of proteins, carbohydrates, amino acids and vitamins in diets of people and livestock. High seed yield productivity under rain-fed is mostly constrained by drought and low soil fertility in particular low soil phosphorus. Understanding genetic control of root traits will enhance genetic improvement for tolerance to drought and low soil-P. The objectives of this study were to; (i) assess farmers’ perceptions on drought and low soil fertility, coping strategies and preferred traits of common bean; (ii) determine variability of root traits, seed weight and genetic inheritance of root traits in common bean for tolerance to low soil moisture and phosphorus. Questionnaires were administered to 139 household representatives in Balaka district on demographic characteristics of farmers, constraints to high seed yield productivity, strategies towards adaptation, perceptions on drought and low soil fertility, and preferred traits. The findings revealed that farmers (24.5 %) cultivated beans in the rainy season and would therefore benefit from improved varieties if adopted. However, drought under rain-fed production and low soil fertility were major constraints (31.7 %) to high seed yield productivity and the impact was rated high (74.8 %). Deforestation (81.3 %) was one of the major causes of drought and soil erosion, as such, farmers implemented afforestation (44.6 %), practiced conservation agriculture and cultivated improved common bean varieties (32.4 %). However, farmers (91.4 %) were not involved in variety development but they preferred good (creamy) taste (61.2 %), short cooking time (25.2 %), red mottled bean grain colour (13.6 %) and determinate plant type (95.7 %) for further crop improvement. Forty-three common bean genotypes were evaluated in a split-plot design with three replicates under low and optimum soil moisture. High root traits and seed weight genotypic variability was observed and the first two Principal Component (PC) axes explained 95.44 % of the total variability. The most potential tolerant genotypes under low soil moisture were CER-78 and SER-125 based on drought tolerance indices. Genotypes SAB-560 (P1) and Kalima-PVA-692 (P2) were crossed to generate the F1, F2, BC1.1 and BC1.2 populations. The generations were evaluated in a Completely Randomised Design (CRD) with three replicates under low soil moisture. Generation Mean Analysis (GMA) revealed that monogenic and epistasis genetic effects were important in the inheritance of root traits and duplicate type of epistasis was observed for hypocotyl roots number, hypocotyl root length and basal root number. Cumulative epistasis was higher than main gene effects, additive gene effect was predominant than dominance. The additive × dominance and dominance × dominance epistatic gene effects were more important in controlling inheritance of root traits under low soil moisture. Fourteen genotypes were evaluated in a split-plot design with two replicates under low and optimum soil phosphorus. High root traits and seed weight variability was observed and cumulatively the first two components of the PC analysis accounted for 99.31 % of total variability. Potential tolerant genotypes to low soil-P were BFS-29, USRM-20 and SEF-15 based on percent reduction in seed weight and low fertility susceptibility index. Genotypes BFS-95 (P1) and Kabalabala-UBR(92)25 (P2) were crossed to generate the F1, F2, BC1.1 and BC1.2. The generations were evaluated in a CRD with two replicates under low soil-P. GMA revealed that both allelic and non-allelic genetic interactions controlled inheritance of root traits studied. Cumulative main gene effect was higher than epistasis effects for hypocotyl root number and hypocotyl root length. Additive genetic effects were more predominant than dominance effects and the additive and additive × dominance epistatic gene effects were more important in controlling inheritance of root traits under low soil-P as revealed by the magnitudes of gene effects. The potential tolerant genotypes identified, genetic variability and gene effects observed can be utilised to develop high seed yielding genotypes with root traits for tolerance to drought and low soil-P through recombination crossing followed by screening and selection in later generations for high seed yield, root and other preferred traits.