Native starch evaluation and genetic distance analysis using AFLP of elite cassava (Manihot esculenta Crantz) genotypes from Malawi

[摘要] English:The starchy tuberous roots of cassava (Manihot esculenta Crantz) provide more thanhalf of the calories consumed by more than 800 million people in the South America,Asia and Africa. Cassava is a staple for more than 30 % of the Malawi population,while in the rest of the country it is grown for food security, as a snack, and a cashcrop. A prerequisite for any genetic improvement programme of cassava is knowledgeof the extent of genetic variation present between cultivars and genetic distancesbetween them, and between cassava and closely related species with which hybridscould be produced. This can be achieved through characterisation of germplasm eitherusing morphological, biochemical or DNA markers. Industries that use starch inMalawi have not been willing to use cassava starch because the powder sold by somesuppliers as 'cassava starch' was inferior. This study was therefore initiated with thefollowing objectives: (1) To determine the genetic distances/relatedness ofcommercial Malawi cassava varieties and promising clones using AFLP andmorphology descriptors; (2) To evaluate the Malawi commercial cassava varieties andpromising clones for starch extraction and quality; (3) To assess the feasibility ofusing native cassava starch in the industrial sector in Malawi; and (4) To evaluate thecommercial cassava varieties in Malawi and promising clones for dry matter content.Trials with 20 cassava genotypes were planted at Chitedze and Makoka in Malawi inNovember 2000, and were harvested in December 2001. Morphologicalcharacterisation was done during plant growth and at harvest. The quality parametersincluded: protein, moisture and ash content, pH and whiteness. Feasibility studies inthe use of cassava starch in the pharmaceutical, textile, battery making and packagingindustries started in December 2001.106The results show that all the cassava genotypes produced starch containing nodetectable protein, similar to the starch used in the pharmaceutical industry. Themoisture content ranged from 11.85 to 13.65 %, which is lower than therecommended maximum of 14 %. The recommended maximum ofO.5 % for ash wasmuch higher than the values of cassava starch, which ranged from 0.10 to 0.20 %. The recommended pH for starch is between 4.5 and 7.0, and cassava starch was withinthese limits, ranging from 5.0 to 5.9. The cassava starch was as white as the cornstarch currently being used. Trials have shown that cassava starch can successfully beused in the making of tablets, batteries, packaging material and textile manufacture.Thus, native cassava starch is suitable for use in various industries. This study hasalso dispelled fears that cassava starch is of low quality and that it is not effective insome industries. Hence, the ideal quality of starch was confirmed in this study. Whatis required is strict quality control for cassava starch so that industry receives whatthey pay for. MBS needs to develop national standards for starch, which will help inthe enforcement of the quality control for any starch produced in Malawi. This taskwill be facilitated by the recommendations made in this thesis.Additive main effects and multiplicative interaction (AMMI) was strongly correlatedwith other stability parameters like Wi-ecovalence, stability variance - no covariateand cultivar superiority measure. AMMI is therefore recommended for use in thestability analysis of starch quality parameters since it provides additional informationon the allocation of unstable genotypes into their appropriate environments.This study has demonstrated that genotype has a greater influence on root dry matterthan the environment. This agrees with the hypothesis that one or a few major genescontrol root dry matter in cassava. This study has also revealed that the genetic distances for the local cultivars, locallybred clones and introduced genotypes are equally divergent. This is supported by thedistribution of the genotypes in different clusters despite their origin Genetic distancesdetermined by morphological characterisation correlates to similar values using AFLPfingerprinting (r=0.98). It is however, a prerequisite that morphologicalcharacterisation be based on constant traits, and the conversion of the morphologicdata into binary characters needs careful consideration to meaningful results.

[发布机构] University of the Free State