Abstract
Sweetpotato is an important crop in many parts of the world especially in developing countries. It is used for both human consumption as well as livestock feed. It is an important source of carbohydrates, vitamin C, fibre, iron, potassium, protein and β-carotene. Its production is, however, constrained by several biotic and abiotic factors, including pests and diseases, low soil fertility, drought, cold and salinity. Breeding is one of the ways to overcome some of these constraints and in sweetpotato the polycross or controlled cross methods can be used. To determine which of the two methods was more efficient, genotypes generated by both methods were evaluated over two seasons at Namulonge and Kachwekano. The type of cross (polycross or controlled) was significantly (P ≤ 0.05) different for storage root yield, response to sweetpotato virus disease, Alternaria blight, and harvest index (HI). The controlled cross families had a significantly higher mean HI of 43.2% than the polycross families with a mean HI of 31.8%. Therefore, controlled crosses could be deployed to systematically increase the HI in sweetpotato breeding populations. Significant (P ≤ 0.05) differences were observed among families for all traits. This stresses that the parents used in a cross are very important in generating genotypes with desired attributes. It was apparent that both the polycross and controlled crosses are good methods for generating new sweetpotato genotypes in a sweetpotato breeding program. Where aggregate performance was considered (selection index) the controlled crosses method produced more (75% of the top 20 desirable genotypes) than the polycross method across the two sites. However, the best three genotypes over the two sites were from the polycross family of Ejumula. Therefore, sweetpotato controlled crosses could be very useful for population improvement using recurrent selection while polycrosses could be suitable for variety development. Both hybridization methods require cautious selection of parents to match the breeding objectives.
Highlights
Sweetpotato is the seventh most important food crop in the world (Loebnstein, 2016; FAOSTAT, 2016)
Progenies derived from polycross and controlled crosses evaluated at Namulonge and Kachwekano during the 2013A and 2013B seasons had an average storage root yield of 9.1 t/ha, average biomass of 24.7 t/ha, average harvest index (HI) of 39%, and average β-carotene content of 0.8 mg/100 g on a fresh weight basis and average sweetpotato virus disease (SPVD), Alternaria blight and weevil damage scores of 2.2, 1.9 and 1.9, respectively (Table 1)
Biomass yield was highest in the controlled cross family of ‘Wagabolige × NASPOT 1’ of 25.4 t/ha and lowest in the controlled cross family of ‘Ejumula × New Kawogo’ at 21.0 t/ha
Summary
Sweetpotato is the seventh most important food crop in the world (Loebnstein, 2016; FAOSTAT, 2016). Two hybridisation methods are used, namely, the polycross or open pollination and the controlled cross method (Wilson et al, 1989). A polycross is the natural inter-crossing of a group of plants in an isolated crossing block (Stuber, 1980; Nyquist & Santini, 2007) In this design, only the female parent of each family is known, and the progeny are half-sibs (Stuber, 1980). For genetic studies the controlled cross method is used where both the female and male parents are known (Wilson et al, 1989) Both the controlled and polycross methods generate viable botanical seed and this study wanted to determine which of the two methods is more efficient in generating genotypes with desired attributes under Ugandan conditions
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