Analysis of Yield-Determining Process and Its Application to Yield-Prediction and Culture Improvement of Lowland Rice. : LXXX. An experiment on maximizing yield by controlling the plant growth to form an 'ideal plant type' by means of the application of nitrate nitrogen in an ordinary paddy field

Abstract

In the previous report the authors reported that one could easily obtain high yields by using a circulating irrigating method combined with the application of nitrate nitrogen. By this method one could exactly take the nitrogen off the soil at a given growth stage for controlling the growth of rice plants. In the present experiment, furthermore, the authors tried to apply the same principle as the above to an ordinary paddy field without the circulating irrigation method. The main points in conducting the experiment were the following three points: (1) In order to ensure the necessary and sufficient number of bearing-tillers for obtaining a terget yield by the time of 70 in 'leaf-number-index', and to take nitrogen off the soil without difficulty during the critical period in which the plant type after heading is definitely predetermined, much amount of nitrate nitrogen in stead of ammonium nitrogen was given at the early growth stage. (2) In order to take nitrogen off the soil immediately during the critical period, logged water which contained nitrate nitrogen in the paddy field was completely drained out, and fresh water which contained no nitrogen was again irrigated. (3) In order to increase the rate of carbon assimilation after the critical period, nitrogen was again top-dressed at the end of the critical period (about 20 days before heading) and at the full heading time. Water permeability in the experimental field was at a rate of 10∼15 mm per day. The results might be summarized as follows. 1. Nitrate nitrogen applied at the early stage of growth was quite effective for increasing the number of tillers, and it could be taken off the soil by exchanging logged water in the field for fresh water at the begining of the critical period. The success in restricting the nitrogen supply in the paddy field mentioned above could be recognized directly by the remarkable decrease in the nitrogen content of leaf-blades, and be checked indirectly by the discoloration of the plant and the retardation in increasing rate of the number of tillers and the dry-matter during the period, and furthermore by shortening of leaf-blades, culms and internodes in length at maturity. 2. Accordingly, the type of plants was improved to a near type by the above-mentioned treatment, and, as a result of it, treated plots were higher than the control in the percentage of ripened grains and in the grain yield as seen in table 3. 3. It was quite noticeable that the treated plants (1) had gained a high lodging resistance which was mainly ascribed to the shortening of lower internodes in length, and (2) has yielded high percentages of ripened grains which was largely ascribed to abundant accumulation of starch in culms and leaf-sheaths before heading. 4. From the results of the present experiments it was strongly proved that if one would utilize nitrate nitrogen at the early stage of growth, one could make the plant type with ease by taking nitrogen off the soil at the critical period, and could obtain high grain yield through the ideal plant.