Sweetpotato (I. batatas) is one of the world’s most important and widely grown starch crops (FAOSTAT, 2014). Sweetpotato is grown primarily for direct human consumption, or as a processed food product, but sweetpotato biomass can be readily converted to simple sugars, which then have industrial end uses such as for the production of fuels (e.g., ethanol or butanol) or bioplastics (United States Sweet Potato Council, 2011; Ziska et al., 2009).
Sweetpotato has traits that make it attractive as an alternative carbohydrate crop to corn in the eastern United States—ease of propagation, tolerance of different soil types, fast growth rates, and wide harvest windows. The eastern United States also has a pre-existing sweetpotato industry with readily available production equipment, university-based research support, and experienced extension agents, which would facilitate the adoption of sweetpotato as an industrial crop.
In the eastern United States, it has been shown that the carbohydrate yield of sweetpotato can exceed that of corn, 11,000 kg·ha−1 vs. 5,900 kg·ha−1, respectively (Ziska et al., 2009). Historically, sweetpotato breeding programs have focused on developing cultivars for human consumption, but desirable quality traits for food markets differ from those for industrial end uses. In the United States, the dry matter content of sweetpotato roots—which is generally accepted to be closely related to carbohydrate content—is ≈20% for most commercial clones. It should be possible to select for new clones with similar yields to conventional clones but dry matter contents of around 30%, thereby increasing total carbohydrate yield. In addition, by removing the two constraints to breeding sweetpotato for food markets—appearance and size distribution—faster progress can be made toward high carbohydrate production.
The production of sweetpotato is relatively expensive and if it is to be a viable industrial carbohydrate crop it will be necessary to lower production costs. High costs can be attributed in part to the need for manual labor at planting. Sweetpotato is generally propagated using unrooted stem cuttings, called “slips,” and in the U.S. manual labor associated with planting slips is estimated to be between 15% and 20% of total production costs (Estes et al., 2002; Hinson and Boudreaux, 2007; Martin et al., 2000; MSU, 2007). An alternative propagation method is to use root pieces, similar to the system used for potatoes (Solanum tuberosum), which uses cut tubers as “seed” and can be mechanized and therefore reduces labor demands. The yield of existing commercial sweetpotato cultivars planted as root pieces tends to be lower than for slips (George et al., 2011).
It has been found that sweetpotato grown from both slips and root pieces can display significant genotype by environment interaction (G × E), and yield instability (Collins et al., 1987; George et al., 2014; Grüneberg et al., 2005; Kanua and Floyd, 1988; Manrique and Hermann, 2002; Ngeve, 1993), making it difficult to predict the performance of clones across sites and years, and complicating cultivar development.
In our study, the sweetpotato cultivar Beauregard was grown in a multienvironment trial with other sweetpotato clones specifically selected for high dry matter production. All clones were grown from both slips and root pieces. The objectives of the research were: 1) to estimate the carbohydrate yield of all the sweetpotato clones under commercial-like production conditions, 2) to compare the carbohydrate yield of Beauregard with new clones selected for higher carbohydrate content, 3) to directly compare carbohydrate production from slip and root piece planting methods, and 4) to test the significance of G × E effects on the carbohydrate yield of clones grown from both slips and root pieces.
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