Genetic diversity, starch physicochemical properties and cyanide levels of farmer preferred cassava landraces in the Eastern zone of Tanzania
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Date
2015
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Sokoine University of Agriculture
Abstract
Cassava is an important staple crop and its starch is an important source of biomaterial
for food and non-food industries. Processing of cassava is very important due to its rapid
postharvest deterioration and cyanide content, but it can also add value and provides
employment opportunities. There is a correlation between cyanogenic poison and
poverty in communities where cassava is a staple food. Therefore development of
starches occupies a central position in the quest for cassava commercialization, thus
stimulating increased cassava production. Moreover,the farmer will realize profit from
cassava production and hence improved food security at farmer‟s level. To meet such a
high demand of cassava in Tanzania, cultivar selection, production and processing need
to be improved. A study was therefore conducted to understand farmer preference for
cassava landraces, cassava genetic diversity present in the farmer‟s field and to assess
suitability of commonly grown cultivars for commercialized starch production for
different applications and cyanogenic potential.
The specific objectives were:
i) To gather farmer‟s indigenous knowledge on cassava variety selection and
conservation in the Eastern zone of Tanzania.
ii) To characterize farmers preferred cassava landraces in Eastern zone of Tanzania using
morphological and molecular techniques.
iii) To assess variation in physicochemical characteristics and functional properties of
selected cassava starch.
iv) To determine the effect of genotype, location and harvesting time on fresh root yield,
starch yield and root cyanide content of selected cassava landraces.
Participatory rural appraisal approaches were used to gather farmers‟ knowledge on
conservation of cassava genetic resources. During the survey 52 cassava landraces wereiii
collected and characterized. Farmer‟s decision on landrace to be grown was the main
factor which was found to influence cassava diversity in the fields. Farmer‟s decision
was based on of diverse factors from food security, market forces and culinary attributes.
Seed flow occurs as farmers exchange or buy from local market also contributed to the
diversity found in the farmer‟s fields. Fresh root yield followed by early maturity were
the most preferred attributes. The genetic relationship and diversity of 52 farmers
preferred cassava landraces were successfully characterised using morphological and
Single Nucleotide Polymorphisms (SNPs) data. The results of analysis showed a
substantial diversity in cassava germplasm found in farmer‟s field. Both morphological
(genetic distance of 1.18 to 0.15) and SNPs (genetic distance of 0.076 to 0.002) analysis
revealed considerable variability among cassava landraces and cluster analysis did not
segregate landraces according to geographic location. In general, the internal branches of
the dendogram from SNPs analysis were short while external branches were long,
indicating that within group variability was higher than between groups. The most
divergent cultivars revealed by morphological analysis were Kichooko, Mbega,
Shibatumbo and Pusuu, and SNPs analysis revealed Mbega and Mzungu Mweupe to be
highly diverse.
Of the 52 collected cassava landraces, six commonly grown were identified and further
analysis was done to assess variation in physicochemical characteristics, starch yield and
cyanogenic potentials. Moreover,analysis has shown that there is a difference in
physicochemical characteristics between landraces (p≤0.05), and can be targeted to
different industrial applications. The study also illustrated genotypic difference in starch
yield and cyanogenic potential as previously described by other authors. Among
landraces, Kiroba showed potential for maximum starch yield (12.8 t ha -1 ) followed by
Msenene (12.3 t ha -1 ) and third was Kilusungu (10.2 t ha -1 ). The optimal harvest time foriv
maximum starch yield was found to be 12 months after planting for most cultivars. The
cyanide content of cassava landraces was between 15 and 800 ppm across all trial sites.
Moreover Kilusungu displayed high cyanide (400 ppm) levels compared with cultivars
across all trial site. This cultivar exhibited a high potential for starch production as
displayed by near average starch yield compared with other cultivars.
Partial least square discriminant analysis (PLS-DA) was done to distinguish among
cassava starches based on the physicochemical and functional properties. When only
starch functionality properties were considered, landrace Nyamkagile was the most
divergent among landrace, followed by Kalolo and Msenene, with setback viscosity,
solubility at 90 °C and syneresis at -20 °C underpinning this differentiation. Msenene
and Kilusungu had high swelling power, which makes them potentially suitable for use
as thickeners and binding agents for food and non-food uses. Msenene also had a
relatively low setback viscosity after cooling, and low syneresis, (p>0.05), desirable
properties in starches for gelling agents and thickeners in refrigerated and frozen food
products. Kibandameno starch had the highest enzyme digestibility and lowest particle
size distribution (p<0.05) compared with starches. This makes the cultivar suitable for
making glucose syrup, adjuncts in breweries (fermentation stock), low fibre feed and
sweeteners.Nyamkagile(p<0.05) had the lowest digestibility and may find application in
food for individuals wishing to manage their glycemic index such as diabetic and
overweight patients. Based on this study, farmer‟s knowledge was documented and
diversity found in farmer‟s field was confirmed by morphological descriptors and SNP
analysis. SNP markers were able to discriminate morphologically similar landraces
(Kasunga and Nyamkagile) and morphologically different landraces, Pusuu and Pushuli
were found by SNPs analysis to be genetically near identical. The advantage of SNP to
discriminate closely related individuals has been shown by this study. This collectionv
revealed a wide range of genetic diversity and represents a valuable resource for trait
improvement enabling capture of farmer preferred traits in future cassava breeding
programmes. Other desirable traits can be exploited and incorporated during breeding.
Data generated from this study will help the breeders to devise more appropriate and cost
effective breeding strategies and will aid in deciding which germplasm to conserve.It is
recommended thatappropriate policies need to be put in place in favour of development
of starch industries. It is is also argued to devise germplasm conservation strategy to
prevent loss of germplasm and ensure conservation of desirable traits.
Description
Keywords
Genetic diversity, Cassava landraces, Starch physicochemical, Tanzania, Cyanide levels