The purpose of this study was to develop a novel method to dramatically improve the production efficiency of sweet potatoes (Ipomoea batatas (L.) Lam.) by elucidating the effect of solar radiation stress on the growth of sweet potato in a multilayer cultivation system. Twenty-five pots planted with sweet potato vine seedlings were arranged in three layers and cultivated for 160 days while supplying liquid fertilizer to the root zone. While solar radiation in the middle and lower layers decreased to 69% and 45% of that in the upper layer, respectively, the yield of tuberous roots was 0.89 kg/pot in the upper layer, 0.79 kg/pot in the middle layer, and 0.66 kg/pot in the lower layer. As a result, the productivity of tuberous roots reached 10.5 kg/m2, 4.4 times that of conventional farming. On the other hand, the amounts of leaves and stems increased in the lower layer than in the upper layer, and the biomass energy yield (photosynthetic efficiency) was 2.8% in the upper layer, 3.7% in the middle layer, and 5.1% in the lower layer. Leaves in the lower layer with less solar radiation had a lower polyphenol content and increased the amounts of low-brightness leaves. In contrast, the upper leaves were found to contain more polyphenols and have brighter, smaller leaves. These results suggest that the yield can be further increased by optimizing solar radiation stress by using the multilayer cultivation method.

To develop a way to mass-produce sweet potatoes (Ipomoea batatas (L.) Lam.) as an energy crop to replace fossil fuels, the effects of using a sewage supply as a fertilizer and heat source were investigated. When 25 pots planted with sweet potato vine seedlings were arranged in three layers and cultivated for 160 days from June to November by supplying treated sewage to the root zone, the yield of tuberous roots reached 19.5 kg m−2 due to the massive growth of leaves. In addition, when sweet potato seedlings were replanted in December and treated sewage was supplied to maintain the irrigation water temperature above 15 °C even in winter, overwintering cultivation was successful and 8.4 kg m−2 of tuberous roots were harvested in July. As a result, the annual production rate for 12 months increased to 25.3 kg m−2, about 10 times the national average of 2.4 kg m−2 for open-field cultivation. The results far exceed previously reported maximum production of resource crops, such as sugarcane and eucalyptus, suggesting that the mass production of sweet potatoes by supplying treated sewage could provide an alternative to fossil fuels on a large scale.