Analysis of Yield-Determination Process and Its "Application to Yield-Prediction and Culture Improvement of Lowland Rice : LXXVII. Relation between nitrogen content and starch content in rice plants, with special reference to a critical evaluation of the diagnosis by iodine reaction on nitrogen requirement at the panicle initiation stage

Abstract

On the basis of the fact that the nitrogen content was negatively correlated with the starch content in rice plants, some trials had hitherto been conducted which had intended to diagnose the nitrogen requirement of rice plants at the panicle initiation stage by knowing the relative starch content which was easily known by the iodine reaction in leaf-sheaths. The authors, therefore, examined from 1958 to 1965 the relation between the nitrogen content and the starch content at the neck-node initiation stage (the initial stage of panicle differentiation) as well as the heading stage with a special view to evaluating the diagnosis of nitrogen requirement by iodine reaction as above mentioned. The results may be summarized as follows. 1. A high and positive correlation coefficient was always found between the starch content in the rice plant and the staining degree of a leaf-sheath by iodine solution. In particular, the strongest correlation was obtained when the third leaf-sheath counted from the uppermost developing leaf was used. 2. In cases of an identical variety being used and transplanted on an identical date under an identical season with different amounts of basic ferfilizers without top dressing, a strong and negative correlation coefficient was always found between the nitrogen content and the starch content in the rice plant. However, in case of varieties, transplanting dates, seasons and topi dressings being different, the correlation coefficients markedly decreased in most cases. 3. The nitrogen content seemed to be negatively correlated with the starch content in the rice plant by nature. The correlation coefficient between them, however, decreased markedly when the rice plant had been subjected to abnormal weather conditions or top-dressed with nitrogenous fertilizers just before the plant was sampled, presumably because the nitrogen content had not yet come to be balanced with the starch content in the plant. 4. The effect on yield of top-dressing at the panicle initiation stage had no connection with the content of nitrogen or starch in the plant at the neck-node initiation stage, but it was closely connected with the number of spikelets per unit area, i. e. the effect was progressively remarkable with the decrease in the number of spikelets. 5. Only in the following case one might decide, from the diagnosis by iodine reaction, the top-dressing at the panicle initiation stage to be necessary for the rice plant, i. e. only whenth e rice plant showed not only a severe iodine reaction but also a small amount, of nitrogen absorbed at the neck-node initiation stage and the plant could also be estimated to absorb a small amount of nitrogen during the period from the neck-node initiation stage to the final stage of spikelet initiation : in other words, only when the plant might be expected to have a small number of spikelets per unit area : and moreover the plant could be expected to be subjected to fine weather conditions after heading. Accordingly, the diagnosis by iodine reaction in the rice plant to judge the necessity of the top-dressing to the plant at the panicle initiation stage may be said to be of little utility value.