Genetic diversity, starch physicochemical properties and cyanide levels of farmer preferred cassava landraces in the Eastern zone of Tanzania

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.