2054 AJCS 7(13):2054-2061 (2013) ISSN:1835-2707 Storage root formation, dry matter synthesis, accumulation and genetics in sweetpotato Placide Rukundo 1,2,* , Hussein Shimelis 1 , Mark Laing 1 , Daphrose Gahakwa 2 1 African Centre for Crop Improvement, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, Pietermaritzburg, South Africa 2 Rwanda Agriculture Board (RAB), P.O. Box 5016, Kigali-Rwanda *Corresponding author: rukundoplacide@yahoo.fr Abstract Sweetpotato is a relatively drought tolerant crop providing the highest dry matter content for human consumption. High dry matter content is the main characteristic preferred by consumers and processors of sweetpotato. There is a continued need to develop and release new and high yielding sweetpotato varieties possessing high dry matter content. The objective of this paper is to review important aspects in the breeding of the crop to achieve high storage root yield and increased dry matter content. The paper highlights development and synthesis processes of dry matter of sweetpotato storage root, gene actions and correlation between traits associated with dry matter accumulation, breeding of sweetpotato for high dry matter content, approaches to screening of clones with high dry matter content and effects of genotype by environment interaction on yield and dry matter content. Keywords: Dry matter, storage root, sweetpotato. Abbreviations: AMMI -Additive main effects and multiplicative interactions, ASV-AMMI stability value, CV-Coefficient of variation, ESTs-Expressed sequence tags, G×E-Genotype by environment interactions, GGE-Genotype main effect plus genotype by environment interactions, SSR-Simple sequence repeat, QTL-Quantitative trait loci. Introduction Sweetpotato (Ipomoea batatas [L.] Lam.); 2n=6x=90) is one of the valuable crops producing the highest root dry matter content for human consumption. It provides comparatively high calorie at 152 MJ ha -1 day -1 . Other crops such as cassava, wheat, rice and maize provide 121, 135, 151, and 159 MJ ha -1 day -1 calories, respectively (Horton and Fano, 1985; Scott et al., 2000). Starch is the main component constituting 70% of the dry weight of sweetpotato (Woolfe, 1992). Slafer and Savin (1994) and Mwanga et al. (2007) reported high dry matter content as an important characteristic of a good sweetpotato variety. Storage roots with high starch and low hexoses contents are important characteristics preferred by the sweetpotato industry (Slafer and Savin, 1994). High starch and low soluble sugar contents decrease the cost of sweetpotato processing due to the absence of oxidation reactions (McKibbin et al., 2006). Oxidation reaction is mainly favoured by high content of hexoses such as glucose and fructose. This reaction leads to the development of brown and dark colours and bitter taste after drying or frying. Such change of colour is associated with Maillard reaction (Dale and Bradshaw, 2003). Li, (2008) reported that the high dry matter content of the root significantly reduces industrial processing cost because of low oil absorption. Sweetpotato is an important raw material to manufacture different products such as noodles, vermicelli, jell, amylophosphate, soluble and refined starch, and alcohol drinks (Loebenstein and Thottappilly, 2009; Woolfe, 1992). Currently the crop is targeted as an important source for biofuel production because of its ability to provide high amount of starch biomass which can be fermented and converted into ethanol (Loebenstein and Thottappilly, 2009; Cervantes-Flores et al., 2010). The use of sweetpotato as a raw material for the biofuel and processing industries requires varieties with a dry matter content that is above 35% of the flesh weight (Gruneberg et al., 2009). High dry matter content is the main characteristic preferred by consumers and processors of sweetpotato. For instance in sub-Sahara Africa, small-scale farmers prefer sweetpotato varieties that have a high dry matter content (Mwanga et al., 2007; Cervantes- Flores et al., 2010). Also high dry matter content, low fibre, and good taste are the most preferred traits of the crop by women farmers’ (Gruneberg et al., 2009; Mwanga et al., 2010). A dry matter content >25% is an important component for acceptability of a new sweetpotato variety by farmers (Shumbusha et al., 2010). Recently the orange fleshed sweetpotato varieties with high β-carotene are being promoted in the sub-Sahara Africa to improve vitamin A nutrition of the poor. However, these varieties are reportedly possessing low dry matter content, a challenge towards their adoption and wide-scale production by farmers. Therefore, these varieties should carry increased β-carotene and dry matter to promote their adoption and large-scale production (Cervantes-Flores et al., 2010; Mwanga et al., 2010). The sustainability and expansion of sweetpotato production depend on the availability of varieties that meet end-users preferences. Consequently, a sweetpotato breeding programme should incorporate valuable traits such as high dry matter content and farmers-preferred traits before the release of elite clones. The development of a new variety of sweetpotato with high dry matter content requires efficient methods of crossing, selection of clones from recombined parents and evaluation of the effects of genetic by environment interactions. This permits the release of end- users preferred varieties at the target production environment.