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Sweetpotato [Ipomoea batatas (L.) Lam.] storage root formation is a complex developmental process. Little quantitative information is available on storage root initiation in response to a wide range of soil moisture levels. This study aimed to quantify the effects of different levels of soil moisture on sweetpotato storage root initiation and to develop functional relationships for crop modeling. Five levels of soil moisture, 0.256, 0.216, 0.164, 0.107, and 0.058 m³·m⁻³ soil, were maintained using sensor-based soil moisture monitoring and semiautomated programmed irrigation. Two commercial sweetpotato cultivars, Beauregard and Evangeline, were grown in pots under greenhouse conditions and treatments were imposed from transplanting to 50 days. Identification of storage roots was based on anatomical, using cross-sections of adventitious roots, and visual features harvested at 5-day intervals from 14 to 50 days after transplanting (DAT). Recorded time-series storage root numbers exhibited sigmoidal responses at all soil moisture levels in both cultivars. Time to 50% storage root initiation and maximum storage root numbers were estimated from those curves. Rate of storage root development was determined as a reciprocal of time to 50% storage root formation data. Time to 50% storage root initiation declined quadratically from 0.05 to 0.15 m³·m⁻³ soil moisture and increased slightly at the higher soil moisture levels in both the cultivars. Cultivars differed in time to 50% storage root initiation and the storage root developmental rate. Soil moisture optima for storage root initiation were 0.168 and 0.199 m³·m⁻³ soil, equivalent to 63% and 75% field capacity for cultivars Beauregard and Evangeline, respectively. The data and the inferences derived from the functional algorithms developed in this study could be used to advise growers to schedule irrigation more precisely, make planting decisions based on available soil moisture, and to develop sweetpotato crop models for field applications.

Skinning or surface abrasion in sweetpotato [Ipomoea batatas (L.) Lam.] roots during harvest causes a substantial loss of marketable products in storage as a result of rots, loss of moisture, and simply unattractive marketable appearance. In 2008, 2010, and 2011, changes in skinning incidence/severity and skin lignin/suberin content in response to preharvest foliar applications of ethephon or defoliation/devining were investigated. Field-grown ‘Beauregard’ (B-14) sweetpotato plots were treated with ethephon at 0.84, 1.68, and 2.52 kg·ha⁻¹ (based on the recommendations for tobacco) applied at 1, 3, and 7 days before harvest (DBH). Defoliated/devined treatments were applied at 0, 1, 3, and 7 DBH. Skinning incidence and severity were reduced with ethephon when applied 3 and 7 DBH in 2 of 3 years compared with 1 DBH. The force required to skin the storage root was measured at harvest in 2011 and it increased with defoliation/devining and ethephon applications at 3 and 7 DBH. Skin lignin/suberin was higher in roots from ethephon-treated plants but was weakly correlated (r = 0.51) with the force required to peel the skin. Ethephon applications also increased cortex phenolic content and either decreased or maintained skin phenolic content in storage roots compared with defoliated/devined treatments. These results suggest that skin set and/or skinning resistance in sweetpotato appears to be influenced by other factors in addition to skin lignification/suberization.

Various workers have attempted to develop a root piece planting system for sweetpotato, similar to the system used commercially for potato, but attempts to select and breed sweetpotato clones adapted to root piece planting have met with mixed success. It has been hypothesized this is the result of significant genotype × environment effects, which are complicating phenotype screening. The aim of this work was to investigate genotype × environment interactions and yield stability of sweetpotato grown from cut root pieces. Ten sweetpotato clones were grown from cut root pieces in three locations over three seasons at sites in North Carolina and Mississippi. The study found sweetpotato clones grown from root pieces were influenced by both genetic and environmental factors and that the interaction was often complicated and dependent on the trait being measured. A significant genotype × environment interaction and yield instability were found to be present. Further work will be required to understand the nature of the genotype × environment effects; however, the results suggest programs aiming to develop sweetpotato clones adapted to root piece planting will need to use appropriate multienvironment screening so as to account for genotype × environment effects.