Studies on the dry matter production of sweet potato X. An explanation of the determination of dry matter percentage in tuber with the viewpoint of nutrient conditions and dry matter production

It is particularly desirable that it has get both the high tuber yiled and the high starch content of tuber in the actual cultivation of sweet potato. Dry matter percentage of tuber(y) highly relating with the starch content can be calculated as the quotient of the nitrogen concentration of tuber on the fresh weight bassis(z) divided by the nitrogen concentration of tuber on the dry weight bassis(x), i. e. y=z/x×100. Although we would substitute the concentration of phosphorus, potassium or other optional nutrient elements for nitrogen content in the above formula, nitrogen concentration was considered to be most suitable because of the intimate relationship (table 1) between the nitrogen concentration of tuber on the fresh weight bassis and the K2O/N ratio in tuber closely rerating with the tuber bulking. The values of z and x were contrlled by each of the factors. The values of z varied amomng the varieties. The higher the dry matter percentage of tuber in the varieties, showed larger the value of z (table 2). Moreover the value of z was controlled by the K2O/N ratio of tuber in a variety (fig. 1). The value of x was controlled by the amount of nitrogen absorbed and the amount of dry matter produced. As shown in the previous paper, the K2O/N ratio in tuber should be maintained on a high level to promote the tuber bulking. The high K2O/N ratio, however, will tend to the value of y by making the value of z decreased (fig. 1). Terefore, it is very effective that the value of x keeps small in order to prevent lowering the value of y. It would be necessary that the overgrowth of top is prevented by the adequate absorption of nitrogen and the light-receiving system is favourably kept up and also that the photosynthetic activity of leaves is highly maintained by manuring much potassium to nitrogen, in order to produce the tuber with both of high yield and high starch content in the actual cultivation of sweet potato.