Yoshida - 1981 - Fundamentals of Rice Crop Science

242 FUNDAMENTALS OF RICE CROP SCIENCE
will then examine the so-called high yielding cultivation technology.
7.5.1. Physiological requirements for high yields
Present knowledge of crop photosynthesis, mineral nutrition, and yield components
indicates several requirements for high rice yields:
• The variety must have a short, stiff culm.
• The most desirable leaf arrangement is that in which the erect upper leaves
graduate down to droopy leaves at low canopy levels.
• A LAI of 5–6 is necessary to achieve maximum crop photosynthesis during
the reproductive stage.
• During ripening, the LAI will decrease as grain filling proceeds and leaf
senescence occurs. While this is a normal event, it is important to maintain as
many active, green leaves as possible until the linear phase of grain growth is
completed.
• Planting time should be chosen so that the crop is exposed to high solar
radiation during the reproductive or ripening stages, whichever is more
critical to yield.
• Yield components must meet the requirements for a target yield.
• All the essential nutrients must be supplied to meet the crop’s requirements.
Nitrogen is the most important. The amount of nitrogen required to produce 1
t rough rice is about 20 kg and it is independent of yield levels. Thus, the
amount of nitrogen needed will increase as yield increases. A similar consideration
should apply to other nutrients.
• Nitrogen absorption after heading will assume an increased importance when
high yields are achieved by increasing the harvest index.
During the ripening, about 70% of the nitrogen absorbed by the straw will
be translocated to the grain. Nitrogen content of the grain tends to be
maintained at a certain percentage. When a larger number of grains relative to
the size of vegetative parts is produced, more nitrogen will be needed to
support grain growth, and there will be a sharper drain in leaf nitrogen
content. Some grains may suffer from nitrogen shortages.
To produce high yields, it is essential to maintain the level of leaf nitrogen
required for high photosynthetic activity. This requirement can be met when
nitrogen absorption by a crop is continued after heading, or when the higher
nitrogen content of the vegetative parts is attained before heading so that the
nitrogen absorbed by heading is sufficient for ripening. The Japanese experience
suggests that continuous absorption of nitrogen after heading is important
for high yielding crops (Matsushima 1969, Murayama 1967).
7.5.2. Soil culture vs water culture
Water culture has been extensively used in studies of the mineral nutrition of
plants. Compared with soil culture as a rooting medium, water culture provides the
following features:
• Amounts of all the essential nutrients and the time of their supply can be
controlled.