| dc.description.abstract | 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
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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
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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. | en_US |
