Inheritannce and genotype by environment interaction of high Iron and zinc content in Rwandan and Ugandan common bean seed.
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Bio-fortification of beans can sustainably and substantially increase iron (Fe) and zinc (Zn) in the diet, and so reduce ailments associated with their deficiencies. Despite the large consumption of beans in Rwanda and Uganda occurrence of these ailments remains high. There is therefore a need to increase consumption of high Fe and Zn beans in these countries. This however requires that varieties rich in these micronutrients be developed. For this to be done, the genetics of micronutrient accumulation in beans must be studied. Stability of these nutrients across environments and the reaction of the potential varieties to common biotic stresses also need to be known. These aspects made up the objective of this study. In this study, 153 Uganda bean landraces, 17 released varieties, 15 pre-released varieties and two checks (high Fe and low Fe) were grown at National Agricultural Research Laboratories (NARL) Kawanda and analyzed for seed Fe and Zn content. Six high Fe and Zn parental lines were used in the mode of inheritance of high iron and zinc study using a complete diallel mating design. A G X E study was carried out using 16 varieties including 14 high iron and zinc content varieties and 2 low Fe standard checks planted at Kachwekano and Kawanda in Uganda in two seasons. In addition, 57 bean genotypes were screened for bean angular leaf spot (Phaeoisariopsis griseola) and bean root rot (Pythium ultimum and Fusarium solani fsp. phaseoli) diseases. Among the Uganda bean germplasm, there were eleven genotypes (UGK116, UGK4, UGK103, UGK149, UGK95, UGK111, UGK72, UGK117, UGK39, UGK85 and UGK68) with significantly higher than average (>75 ppm of iron and >35ppm of Zinc) levels of Fe and Zn content. These lines should be promoted as high iron beans (HIB) in Uganda. There was also a strong positive correlation between iron and zinc content. In contrast, iron/ zinc content and seed size based on 100 seed weight were moderately negatively correlated. Lines from the mesoamerican gene pool were found to have higher iron and zinc content compared to the lines from the Andean gene pool at the ratio of 9: 1 xvii respectively confirming the negative relationship between iron/ zinc and seed size observed. The inheritance study showed that additive gene effects were the most important in determining the expression of zinc and iron seed content; 82% for zinc and 64% for iron while non-additive gene effects contributed 18 % for zinc and 36 % for iron respectively. Narrow sense heritability of iron and zinc seed content level was moderate and high respectively and was estimated at 55% for iron and 71 % for zinc. The significant maternal and reciprocal effects suggested that cytoplasmic inheritance is involved in zinc and iron content. A strong positive correlation between iron and zinc content (r=0.75) was observed suggesting that these two micronutrients have a similar inheritance pattern and are possibly not independently inherited. The negative correlation between zinc and seed size suggest that micronutrient concentration can be improved in the Mesoamerican gene pool. The variability in seed micronutrient content among crosses was larger for iron (47-77ppm) than for zinc (28-38ppm). The consistence and stability of zinc content than iron content suggests that it is probably more efficient to select for zinc content when selecting for Fe content since the same locus contributes to both minerals. Transgressive segregation in iron and zinc content was observed. Parents KAB06F2.8-27 and RWR2076 showed good general combining ability and should be used to maximize opportunities for transgressive segregation for iron and zinc content. The study revealed strong G x E interaction on iron and zinc content at P=0.001. Despite these effects, random error effects contributed more on iron content followed by G X E effects and lastly by genotype effects at 38%, 32 % and 30 % respectively. In contrast, the largest contribution to zinc content was due to genotypic effects followed by random error effects and G X E effects at 54%, 24 and 22 % respectively. Genotypes performed differently for iron and zinc in each season and in each location. The across environment mean of 69 ppm for iron content and 35 ppm for zinc content was observed in this study. The within location means showed that genotypes in Kachwekano performed better than in Kawanda with 71 and 67 ppm respectively for iron and 38 and 31 ppm respectively for xviii zinc. Differences of iron and zinc content in different environment might have been due to the soil characteristics of the sites and also deep weather differences. Therefore variations in iron and zinc content is attributed to the genotype background and the environment in which they are grown. Stability analysis in this study allowed identification of promising varieties with wide and specific adaptations for iron and zinc content. Varieties Ndimirakaguja volubile and Garukurare had consistently high iron and zinc content. The study showed that some bean entries selected for high iron and zinc content have other beneficial traits such as good resistance levels to important bean diseases; angular leaf spot (ALS), Pythium root rot and Fusarium root rot. The different levels of resistance to different pathogens among different bean varieties suggests the existence of varying numbers of resistance genes that can be pyramided into appropriate backgrounds to provide durable resistance to these pathogens in genotypes with high Fe and Zn content. Our results suggest that selecting biofortified beans as parents in plant breeding programs could result in significant increases in genetic resources for multiple purposes with minimal resources.