Yoshida - 1981 - Fundamentals of Rice Crop Science

CLIMATIC ENVIRONMENT AND ITS INFLUENCE 89
are to be achieved. It is, however, inadequate if we refer to the current yield levels
when the national average yield is still about 2 t/ha in most Southeast Asian
countries.
2.4.3. Solar energy use
When solar radiation is incident on a well-developed rice crop, about 20% of the
total radiation is reflected into the sky (Murata et al 1968, Kishida 1973). Of the
reflected radiation, only about 10% is photosynthetically active radiation (PAR);
the rest is in the near far red region. In other words, about 3–5% of PAR is lost by
reflection at the canopy surface (Fig. 2.16) (Kanda 1975). Taking 5% for the loss
attributed to reflection, about 90% of PAR will be absorbed by the leaves and about
5% will be transmitted to the ground.
Photosynthetic efficiency of a rice crop can be estimated by:
Photosynthetic
=
efficiency
chemical energy captured by a crop
solar energy received
(2.7)
In practice, however, there are several ways of computing the photosynthetic
efficiency. First, chemical energy can be estimated in terms of net gains or gross.
Second, solar radiation may be expressed in terms of total incident radiation, PAR,
or absorbed PAR. The simplest and most widely used measurement uses net gains
for the numerator and total incident radiation for the denominator:
The measured E µ values vary with growth stages, ranging from 0.52% at a LAI of
0.36 to 2.88% at a LAI of 4.10, for example. The measured maximum E µ value
for rice is 3.7% (Murata et al 1968). In the more comprehensive 5-year
experiment of the Japanese International Biological Program (IBP), however, the
highest E µ for each year ranged from 2.83 to 3.32% with a mean value of 3.00%
(Kanda 1975). For the entire growing period, the 5-year mean E µ is 1.25% for
total dry matter. If grains alone are considered, E µ becomes 0.48% (Table 2.12).
The theoretical maximum E µ is estimated at 5.3% (Loomis and Williams 1963).
In the IBP experiment. the highest E µ (4.53%) was recorded for maize (Kanda
1975).
When 1 is substituted for T in the equation 2.8, D W becomes dry matter increase
per square meter per day, which is crop growth rate. Thus, crop growth rate