Yoshida - 1981 - Fundamentals of Rice Crop Science

236 FUNDAMENTALS OF RICE CROP SCIENCE
7.3. Relationship between total number of spikelets per square meter. grain yield. filled-spikelet
percentage, and grain weight of variety IR8 (Yoshida et al 1972).
number per square meter was the most important component limiting yield in this
experiment.
A similar conclusion was obtained from IR8 crops grown at Los Baños (Fig.
7.3). The grain yield increased linearly with increasing spikelet number per square
meter while filled-spikelet percentage and 1,000-grain weight remained fairly
constant regardless of spikelet number. At some locations and in some weather
conditions, filled-spikelet percentage may be more limiting to yield than spikelet
number per square meter (Yoshida and Parao 1976). The 1,000-grain weight may
affect yield to some extent but rarely becomes limiting under most conditions.
Therefore, under given conditions it is important to examine the causes of yield
variation in components, particularly spikelet number per square meter and filledspikelet
percentage.
7.3.2. Spikelet number per unit land area
Spikelet number per unit land area is affected by:
• Cultural practices — spacing (seeding density) and nitrogen application.
• Growth characteristics — tillering (seedling emergence).
• Climatic conditions — solar radiation and temperature.
Under subnormal cultural practices, the spikelet number per unit land area can
be increased by increasing plant density and nitrogen application. Optimum
spacing varies with tillering capacity and growth duration of a variety. A higher
plant density is required for low-tillering, early-maturing varieties, and low
nitrogen levels. The relationships between spikelet number per unit land area and
amount of nitrogen absorbed by the crop have been discussed in Chapter 2 (Fig.
2.9).