Yoshida - 1981 - Fundamentals of Rice Crop Science

62 FUNDAMENTALS OF RICE CROP SCIENCE
Table 1.15. The model yield components of IR8 in the cloudy
wet season and in the sunny dry season. a
Component Wet season Dry season
Panicle number per m 2 250 375
Spikelet number per panicle 100 100
Total number of spikelets per m 2 25,000 37,500
Filled spikelets (%) 85 85
1,000-grain wt (g) 29 29
Expected grain yield (t/ha) 6.16 9.24
a Chandler (1969).
The model yield components of transplanted IR8 are shown in Table 1.15. To
produce a yield of 6 t/ha in the cloudy wet season, the required number of spikelets
per square meter could be obtained if plants are spaced at 20 × 20 cm (i.e., 25
plants/m 2 ), and each plant produces 10 panicles with 100 spikelets/ panicle. Under
most weather conditions, 85% filled spikelets is normally expected and the
1,000-grain weight is usually a stable varietal character.
Since equation 1.20 is a kind of arithmetic computation and since each component,
particularly the first three, is dependent on others. one cannot increase each
yield component at his discretion. For instance, when the number of panicles per
square meter is increased by using a high plant density, the number of spikelets per
panicle is decreased. A comparison of direct-seeded rice and transplanted rice
gives a good example of such a relationship (Yoshida and Parao 1972). It is
relatively easy to obtain from direct-seeded rice 600 panicles/m 2 , which is twice
the number normally obtained from good transplanted rice. The number of
spikelets per panicle, however, is much smaller for the direct-seeded rice. As a
result, the total number of spikelets per square meter may be about the same for
rice grown under the two cultivation systems.
Maximum yield is predetermined by the potential of a variety and the environment.
A computation of yield components would be meaningful for designing a
blueprint of the target yield and examining the defects of a given crop if a
comparison is made with a crop that has already achieved a good yield under a
similar environment.
Each yield component is determined at a particular stage in the plant's life
(Matsushima 1970). In the transplanting rice cultivation, the number of panicles
per square meter is largely dependent on tillering performance, which is largely
determined by 10 days after the maximum tiller number stage. In the directseeding
system, however, the number of panicles per square meter is largely
dependent on seeding rate and percentage of emergence. The number of spikelets
per panicle, on the other hand, is determined during the reproductive growth stage.
Early in the reproductive growth, the maximum number of spikelets is determined
by the differentiation of branches and spikelets. After spikelet differentiation,
some spikelets may degenerate. The number of spikelets observed at heading or at