Genetic diversity and carotenoid degradation in yellow root cassava in Uganda.

Abstract

Yellow root cassava (Manihot esculenta Cranz) has high levels of pro-vitamin A carotenoid and its consumption has been perceived as a sustainable approach for addressing Vitamin A deficiencies. β–carotene, the main pro-vitamin A carotenoid in yellow root cassava, has been reported to have negative correlation with postharvest physiological deterioration (PPD). These two characteristics constrain people whose livelihoods heavily rely on cassava. Subsequent efforts have been made towards development of varieties that are enriched with β-carotene. In this study, distribution of yellow root cassava landraces in Uganda was mapped to assess farmer awareness, distribution of the landraces, genetic diversity and relationship between carotenoid degradation and postharvest physiological deterioration (PPD). There was limited distribution of yellow root cassava with 20.7% of the farmers interviewed showing awareness, and only 4.8% had yellow root cassava in their fields basically for human consumption. Farmers attributed the limited occurrence of yellow root cassava to its susceptibility to pests and diseases. High morphological diversity was observed in the landraces. To elucidate the genetic diversity of Uganda’s core collection of yellow root cassava, SSR marker variation was assessed at 26 loci in 64 cassava genotypes randomly selected from accessions from Colombia, Nigeria and Uganda. Average genetic diversity was high in all accession groups with an average heterozygosity of 0.5583 ± 0.0182. Despite the low level of differentiation (FST) found among accession groups, sufficient genetic distance existed between individual accessions to separate them according to their accession groups. Phenetic analyses using Unweighted Pair Group Method with Arithmetic Mean (UPGMA) and Principle Coordinate Analysis (PCoA) revealed that Ugandan yellow root cassava is closely related to Nigerian genotypes suggesting common origin, but quite distinct from Colombian accessions, suggesting limited gene flow between the two groups and an opportunity for broadening the genetic base through hybridization by exploiting the heterotic pool between African Colombian genotypes. In order to assess the relationship between carotenoid degradation and PPD in roots under storage, total carotenoid content and PPD were respectively analyzed using UV/visible spectrophotometry visual inspection at two-day intervals during a thirteen-day storage period. Levels of beta carotene and trans/cis-isomers differed significantly (P < 0.05) across genotypes. Low PPD was significantly (P < 0.05) associated with high levels of beta carotene in the roots. There was significant variation in PPD along storage period and among genotypes. PPD was strongly and positively associated with total carotenoid degraded during storage (R2 = 0.893). Variation in the levels of beta carotene and PPD due to genotypic differences suggested opportunity for improvement in these cassava characteristics. The implication is that an opportunity exists for development of pro-vitamin A cassava varieties, a sustainable approach to addressing vitamin A malnutrition in communities heavily dependent on cassava for food and increase of storage period of cassava by reduction of PPD.