Analysis of Yield-Determining Process and Its Application to Yield-Prediction and Culture Improvement of Lowland Rice : LXXII. Effects of top-dressing with ammonium sulfate on the grain yield and its components of directly sown rice plants at critical stages relating to determination of each yield-component.

Abstract

With a view to making clear the difference in yield-determining process between transplanted rice plants and directly sown ones, the authors carried out nitrogen top-dressing experiments on directly sown plants in 1962 and '63 along the same line as in the previous experiments (LI) in which transplanted plants were dealt, obtaining the following results. 1. The most effective time for top-dressing to increase the magnitude of each yield-component was found as follows: (1) The number of panicles per unit area is most effectively increased at the most active tillering stage, (2) the number of spikelets per panicle is at the neck-node differentiation stage, (3) the percentage of ripened grains is at the full heading stage and (4) the weight of 1, 000 grains is just before the reduction division stage. 2. On directly sown rice plants the number of degenerated spikelets was much less than that on transplanted ones, so the nitrogen top-dressing just before the reduction division stage was hardly effective for decreasing the number of degenerated spikelets. (On transplanted plants the nitrogen top-dressing at this time is quite effective for decreasing the number of degenerated spikelets.) The reason why the number of degenerated spikelets on directly sown plants was much less than that on transplanted could be ascribed to the fact that the number of differentiated spikelets was much less on account of much more number of panicles per unit area being produced on directly sown plants than that on transplanted ones, respectively. 3. There was no top-dressing method which consistently gave either the highest yield or the lowest yield under all conditions, which suggested that the best top-dressing method varies with the cultural conditions and seasons. The four components of yield have their individual best time for top-dressing a time when each component is most easily increased, while the yield which is a product of these four components has no one definite top-dressing time at which it is most easily increased. 4. Examining the results, the authors reached the following conclusion which was the same as that obtained from the previous experiments on transplanted plants. The most suitable method of top-dressing differs with conditions under which the plants grow. Under conditions which produce plants with a high percentage of ripened grains and a small number of spikelets, a top-dressing which makes the plants increase its number of spikelets is suitable, while under conditions which produce the reverse effect, a top-dressing which makes the plants increase its percentage of ripened grains is suitable. Accordingly, it is necessary for the growers to adopt the most suitable method of top-dressing by examining the rice plants in their own paddy fields. However, in case of the percentage of ripened grains and the number of spikelets per unit area being both moderate in magnitude or both components being well balanced (and such cases seem to be common), the methods of top-dressings which increase the number of spikelets per unit area as well as the percentage of ripened grains (such as Treatment No. 7 and 10) would be the most suitable.