IMPROVING BETA-CAROTENE CONTENT IN FARMERS’ PREFERRED CASSAVA CULTIVARS IN NIGERIA, ABSTRACT
The study was conducted to assess the potential to improve farmers’ preferred cassava cultivars with high carotene content in Nigeria. A participatory rural appraisal (PRA) and sensory evaluation was conducted in Abia, Imo and Ebonyi states of Nigeria to identify farmers’ and consumers’ adoption challenges and preferences for cassava with yellow storage roots.
High production cost, low yield, scarcity of planting materials, high cost of fertilizers and agro-inputs, drudgery in peeling and processing, and pests and diseases were identified as major challenges to yellow root cassava adoption. Cultivars TMS 01-1368, TMS 05-1636 and TMS 05-0473 (yellow fleshed root, high carotene and low dry matter) were crossed with farmers’ cultivars TMS 98-0002, TMS 97-2205 and TMS 98-0505 with white fleshed root, high dry matter content, high yield and resistance to pests and diseases. Using 36 SSR markers, the genetic distance between the 6 cassava cultivars used in the crosses were found to be high, ranging from 0.55 to 0.77 for the TMS 98/0505 by TMS 98/0002, and TMS 98/0505 by TMS 05/1636 crosses respectively. One yellow root cultivar clustered with a white cultivar, indicating a close genetic relationship. In screening of the 6 parents and selected 147 genotypes which was got from 464 F1 progenies using SSR markers linked to beta-carotene genes, three yellow root parents and 65 F1 progenies had marker alleles 206 and 181 from SSR markers NS 717 and SSRY 240 which are putative markers associated with the presence of beta-carotene gene. Though there was relatively high significant correlation (r =0.58; P<0.001) between the spectrophotometric quantification and standard colour chart values among the 147 F1 genotypes, there were significant but low correlation between SSR markers associated with beta-carotene genes and the yellow roots assessed with stardard color chart (17- 20%) and spectrophotometer values of total carotene content (14- 20%). The six parental cultivars in a 3 x 3 topcross mating design generated one thousand, one hundred and ten (1, 110) seeds from nine crosses. Each cross produced from 3 to 385 seeds to give a total of 1,110 seeds.Four hundred and sixty-four F1 progenies survived and were evaluated at harvest (12 months after planting, MAP) for total carotene content and yield parameters. Genotypic variations in dry matter content, total carotene content, cassava mosaic disease reaction and storage fresh root yield of the families were recorded. The dry matter content of the white fleshed root parents ranged from 30 to 39 %, while the yellow fleshed parents ranged from 26 to 32 %. Also, the carotene content of the white fleshed parents’ ranged from 0.89 to 1.12, and yellow fleshed parents ranged from 4.5µg/g to 6.5µg/g respectively. Among the parents, TMS 01/1368 and population B4 had the highest mean total carotene content across two locations with 8.5µg/g and 7.9 µg/g at Umudike, and 5.6µg/g and 8.7µg/g at Otobi, respectively. Narrow sense heritability by midparent-offspring regression analysis and genetic gain were estimated for carotene content, dry matter content, storage fresh root yield and cassava mosaic disease. Carotene content, dry matter content and cassava mosaic disease gave high heritability estimate of 0.73, 0.83 and 0.84, suggesting that genetic factors played a more important role than environmental factors in expression. Correlation studies showed that carotene content had negative correlation with dry matter content across the two evaluation stages (seedling and clonal) and two locations (Umudike and Otobi), respectively. Four genotypes were identified as having potential for high carotene content, dry matter content and storage flesh root yield, and were therefore selected for further evaluations in Nigeria.