2002 Volume 71 Issue 6 Pages 747-751
Sweet potatoes (Ipomoea batatas Poir.) were field-grown or grown in boxes in which the soil temperature was kept at 20°C (cool), 30°C (heat), or ambient (control). Field-grown plants were dug at 5 : 00, 11 : 00, 17 : 00 and 23 : 00, weighed, then dried and then re-weighed. The total fresh weights per plant, sampled at the respective times, were : 7250g, 7800g, 10650g, 7666g. The dry samples of stems and roots were analyzed for soluble sugars and starch. Sucrose content in the stem that was 44.5mg·g-1 DW at 5 : 00 increased gradually to 72.0mg·g-1 DW by 23 : 00, whereas that in the roots increased from 60mg·g-1 DW at 5 : 00 and 11 : 00 to 70.5mg·g-1 DW and 82.2mg·g-1 DW at 17 : 00 and 23 : 00, respectively. Sucrose content in stems of plants grown at the cool plot at night was lower than that in the heat plot. The skin of the root was brilliant red in the control and cool plots, whereas it was almost pink in the heat plot. Root sucrose content was higher in the heat plot than in the cool plot ; the starch level was inverse of that of sucrose. These results indicate that sucrose transported into the root is converted to starch mainly at night. As the conversion of sucrose to starch by low night soil-temperature is not suppressed, the excess is diverted to root growth and/or metabolized into anthocyanin resulting in highly colored red roots with high sugar and starch contents.