Effects of Soil Temperature on the Thickening Growth and the Quality of Sweetpotatoes during the Latter Part of Their Growth.

We carried out investigations on the effects of different soil temperatures caused by artificial heating and cooling on the thickening growth and the quality, such as factors that determine the market value as skin color, dry matter percentage, starch and sugar of the tuberous root of sweetpotatoes (Ipomoea batatas (L.) Lam.) during the latter period of their growth. The soil temperatures of the heating and the cooling treatments were within the range of 20 and 36°C, and 13 and 31°C, respectively. The leaf and stem weight was the smallest in the cooling treatment. The number of roots increased in each plant when the soil was heated and decreased when cooled. There was an inverse relationship between the average root weight and the number of roots. Dry matter percentage, starch and total sugar contents were the highest in the cooling treatment and the lowest in the heating treatment. The skin color of the root was brilliant red in the cooling treatment, but almost white in the heating treatment. From these results, we can surmise that starch and total sugar contents increase in the root of sweetpotatoes cultivated in cooler soil temperature, with a vivid red skin color.