Crop-scientfic studies on the yield-forecast of lowland rice (Peliminary report) : VIII. Effects of shading at diffent growing stages of rice upon the yield and yield-constitutional factors

Abstract

In order to make clear how and when the yield of rice and its constitutional factors are determined, experiments on shading had been conducted at an interval of lO days after transplanting, using linen blothes to cover the plants. The experiments were carried out in 1951 and 1952, in paddy field, according to 4-blocks randomized method, using a medium maturing variety, Norin-No.25 (Each plot was 8×8 feet). The results of two years of experiments are in close agreement with each other on main points, and they are summerized as follows : (l) So far as the present experiments are concerned, effects of shading on numder of heads per unit area were hardly found.(2) The most striking effect of shading on number of spikelets per head was found at the period from 15 to 16 days prior to heading, and at the succeeding period no effects were observed The reason why the spikelet number per head was remarkably reduced by shading in the above period may be explained by the results of our anatomical investigation done in the past, which revealed the fact that a large majority of spikelets in a given hill differentiates in the period from 15 to 16 days prior to heading. (3) Highly significant differences in fruit bearing percentage were found among the plots. No effects of shading were found in all the plots treated during the period until 6 days prior to heading, but marked effects appeared in those treated in the period from 5 days prior to heading to 10 or 20 days prior to maturity. And a most remarkable reduction in fruit bearing percentage occurs in the period for 23 days immediately after heading. It my be recognized that in the period, when the development of caryopsis takes place more actively, the effect of shading comes out more strongly.(4) Remarkable effects on weight of 1, 000 grains were found in the two sparate periods, the first was 15∼6 days prior to heading and the seeond was 14∼23 days after heading. The effect at the former was more intensive than that at the latter. Strange as it may seem that most intensive effect were found in the period before heading, it had, however, been proved by no means unreasonable. The results of other experiments conducted by the authors showed clearly that the size of lemma and palea was determined during the above metioned period, and when they once are formed into a small size, the caryopsis which develops in them, however vigorous its development may be, can never become large because of mechanical restraint. The latter case of strong effect observed after heading might largely be due to retardation in the fruiting prseess. (5) Remarkable effect on yield were found in the two periods, 15∼6 days prior to heading and 14∼23 days after heading. The reduction in grain yield in the former period is brought about by a reduction in number of spikelets per head and in weight of 1, 000 grains, and that of the latter by a reduction in fruit bearing percentage and in weight of 1, 000 grains. But, among these factors which determine the grain of yield, the weight of 1, 000 grains proved itself the least effective.