Yoshida - 1981 - Fundamentals of Rice Crop Science

132 FUNDAMENTALS OF RICE CROP SCIENCE
where N is number of tillers, t is time, and r is a constant. The solution for the
equation 3.14 is:
r = log e N 2 – log e N 1 2.303 (log 10 N 2 – log 10 N 1 )
=
t 2 – t 1 t 2 – t 1
(3.15)
The constant r is called the relative tillering rate (RTR) on the analogy of the
relative growth rate. Equation 3.14 can be rewritten as:
dN 1
RTR = × (3.16)
dt N
Thus, RTR can be considered the mean tillering rate per tiller and used as a
quantitative measure for the relationship between mineral nutrition and tillering.
As shown in Figure 3.8a and b, the RTR is closely correlated with nitrogen,
phosphorus, and potassium content in the leaf blades. Tillering stops when
nitrogen content in the blade becomes 2.0%, phosphorus 0.03%, and potassium
0.5%.
The tillering rate increases linearly with an increasing nitrogen content of up to
5%. With phosphorus, the tillering rate increases up to about 0.2%, above which
an increase in phosphorus has no effect on tillering. Similarly, potassium content
as high as 1.5% increases the tillering rate.
The critical phosphorus concentration for tillering appears to be affected by
temperature. Tiller number per square meter increases with an increasing phosphorus
content of up to 0.35% in rice crops grown in Hokkaido, Japan, where low
temperatures prevail (Shiga et al 1976). Such a high phosphorus requirement can
be a reason why phosphate applications are particularly beneficial in cool years in
Hokkaido.
An examination of tillering is one useful way to examine the growth status of a
rice crop. When tillering is retarded by a nutrient shortage, growth parameters
such as leaf area and dry weight also decrease (Table 3.13). The table also
illustrates that any nutrient whose content is below optimum level limits overall
growth even though other nutrients are present in sufficient quantities.
3.6 MINERAL NUTRITION AND PHOTOSYNTHESIS
Nutrient content is related to the photosynthetic activity of leaves because essential
nutrients are directly or indirectly involved in photosynthesis and respiration. For
example, nitrogen is a constituent of proteins which, in turn, are constituents of
protoplasm, chloroplasts, and enzymes. Phosphorus as inorganic phosphate, an
energy-rich phosphate compound, and a coenzyme, is directly involved in photosynthesis.
Potassium is involved in opening and closing the stomata that control
carbon dioxide diffusion into green tissues, a first step in photosynthesis (Fujino
1967, Fischer and Hsiao 1968). Potassium is also essential in activating enzymes
such as starch synthetase (Nitsos and Evans 1969).
The relative photosynthetic rate of leaf blades is positively correlated with their
nutrient content (Fig. 3.9). The critical nutrient contents for a high leaf photo-