ACCUMULATION OF CARBON AND NITROGEN-COMPOUNDS IN SWEET-POTATO PLANTS GROWN UNDER DIFFERENT NITROGEN APPLICATION RATES

Sweet potato (Ipomoea batatas L. var. Beniazuma) plants were grown in a cold region at 4 nitrogen application rates. Results are as follows. 1) Total dry weight and tuberous root dry weight decreased at a high nitrogen application rate (30 g N m(-2)). The growth at 30 N was restr icted from the early growth stage and the nitrogen content was higher at 30 N. As the plant architecture at 30 N did not show a large differ ence compared to the low nitrogen application rate, it is assumed that the carbon metabolism of sweet potato was disturbed by the high rate of nitrogen application at which level other held crops grew well, but not by mutual shading. 2) Harvest index of dry weight and nitrogen fo r sweet potato ranged from 0.6 to 0.7, and 0.2 to 0.5, respectively. T hus, the distribution of nitrogen to tuberous roots was lower than tha t of carbon (dry weight). Therefore, sweet potato has an ability to at tain a high potential productivity at low nitrogen application rates b ecause a high photosynthetic rate can be maintained due to the high di stribution ratio of nitrogen to leaves compared to potato or grain cro ps. 3) Ratio of sporamine and beta-amylase to total nitrogen in tubero us roots increased with growth until 105 d after transplanting, then d ecreased. 4) Main amino acids consisted of alanine, aspartic acid, asp aragine, and glutamic acid in leaves, asparagine in stems, asparagine and glutamic acid in tuberous roots.